Marine life#Marine protists

File:Earth elevation histogram 2.svg

{{See also|Hydrosphere}}

There is no life without water.{{cite book |vauthors=Xiao-Feng P |date=2014 |url=https://books.google.com/books?id=O_26CgAAQBAJ&q=%22Water%3A+Molecular+Structure+And+Properties%22 |title=Water: Molecular Structure And Properties |chapter=Chapter 5 |pages=390–461 |publisher=World Scientific |isbn=9789814440448 }} It has been described as the universal solvent for its ability to dissolve many substances,{{Greenwood&Earnshaw2nd|page=620|name-list-style=vanc}}{{cite web |title=Water, the Universal Solvent |url=http://water.usgs.gov/edu/solvent.html |website=USGS |access-date=27 June 2017 |archive-url=https://web.archive.org/web/20170709141251/https://water.usgs.gov/edu/solvent.html |archive-date=9 July 2017 |url-status=live |df=dmy-all }} and as the solvent of life.{{Cite book |title=Campbell Biology |vauthors=Reece JB |date=31 October 2013 |publisher=Pearson |isbn=9780321775658 |edition=10 |page=48 }} Water is the only common substance to exist as a solid, liquid, and gas under conditions normal to life on Earth.{{Cite book |title=Campbell Biology |vauthors=Reece JB |date=31 October 2013 |publisher=Pearson |isbn=9780321775658 |edition=10 |page=44 }} The Nobel Prize winner Albert Szent-Györgyi referred to water as the mater und matrix: the mother and womb of life.{{cite book |vauthors=Collins JC |date=1991 |url=https://books.google.com/books?id=Rm_wAAAAMAAJ&q=%22materund+matrix%22 |title=The Matrix of Life: A View of Natural Molecules from the Perspective of Environmental Water |publisher=Molecular Presentations |isbn=978-0-9629719-0-7 }}

File:Composition of seawater.jpg

The abundance of surface water on Earth is a unique feature in the Solar System. Earth's hydrosphere consists chiefly of the oceans but technically includes all water surfaces in the world, including inland seas, lakes, rivers, and underground waters down to a depth of {{convert|2000|m|ft}}. The deepest underwater location is Challenger Deep of the Mariana Trench in the Pacific Ocean, having a depth of {{convert|10900|m|mi}}.

Conventionally, the planet is divided into five separate oceans, but these oceans all connect into a single world ocean.{{cite web |url=https://oceanservice.noaa.gov/facts/howmanyoceans.html |title=How many oceans are there? |work=NOAA |date=9 April 2020 }} The mass of this world ocean is 1.35{{e|18}} metric tons or about 1/4400 of Earth's total mass. The world ocean covers an area of {{val|3.618|e=8|u=km2}} with a mean depth of {{val|3682|u=m}}, resulting in an estimated volume of {{val|1.332|e=9|u=km3}}. If all of Earth's crustal surface was at the same elevation as a smooth sphere, the depth of the resulting world ocean would be about {{convert|2.7|km|mi}}.{{cite encyclopedia |vauthors=Duxbury AC, Cenedese C |date=7 May 2021 |url=https://www.britannica.com/EBchecked/topic/559627/sphere-depth-of-the-ocean |title=Sphere depth of the ocean – hydrology |encyclopedia=Encyclopædia Britannica |access-date=12 April 2015}}{{cite web |url=https://ase.tufts.edu/cosmos/print_chapter.asp?id=4 |title=Third rock from the Sun – restless Earth |work=NASA's Cosmos |access-date=12 April 2015}}

File:Diagram of the Water Cycle.jpg]]

About 97.5% of the water on Earth is saline; the remaining 2.5% is fresh water. Most fresh water – about 69% – is present as ice in ice caps and glaciers.{{cite web |url=https://water.usgs.gov/edu/earthwherewater.html |title=The World's Water |vauthors=Perlman H |date=17 March 2014 |access-date=12 April 2015 |work=USGS Water-Science School }} The average salinity of Earth's oceans is about {{convert|35|g|oz}} of salt per kilogram of seawater (3.5% salt). Most of the salt in the ocean comes from the weathering and erosion of rocks on land.{{Cite web |url=https://oceanservice.noaa.gov/facts/whysalty.html |title=Why is the ocean salty? }} Some salts are released from volcanic activity or extracted from cool igneous rocks.

The oceans are also a reservoir of dissolved atmospheric gases, which are essential for the survival of many aquatic life forms. Sea water has an important influence on the world's climate, with the oceans acting as a large heat reservoir. Shifts in the oceanic temperature distribution can cause significant weather shifts, such as the El Niño-Southern Oscillation.

Altogether the ocean occupies 71 percent of the world surface,{{cite web |title=National Oceanic and Atmospheric Administration – Ocean |url=http://www.noaa.gov/ocean.html |publisher=NOAA |access-date=20 Feb 2019 }} averaging nearly {{convert|3.7|km|mi}} in depth.{{cite web |title=Volumes of the World's Oceans from ETOPO1 |url=https://www.ncei.noaa.gov/products/etopo-global-relief-model |publisher=NOAA |access-date=20 Feb 2019 |url-status=live |archive-url=https://web.archive.org/web/20150311032757/http://ngdc.noaa.gov/mgg/global/etopo1_ocean_volumes.html |archive-date=2015-03-11 }} By volume, the ocean provides about 90 percent of the living space on the planet. The science fiction writer Arthur C. Clarke has pointed out it would be more appropriate to refer to planet Earth as planet Ocean.{{cite web |url=https://quoteinvestigator.com/2017/01/25/water-planet/ |title=Planet "Earth": We Should Have Called It "Sea" |work=Quote Invertigator |date=25 January 2017 }}{{cite web |url=https://science.nasa.gov/science-news/science-at-nasa/2002/14mar_grace_oceans |title=Unveiling Planet Ocean |work=pNASA Science |date=14 March 2002 |access-date=19 February 2019 |archive-date=8 October 2022 |archive-url=https://web.archive.org/web/20221008093758/https://science.nasa.gov/science-news/science-at-nasa/2002/14mar_grace_oceans |url-status=dead }}

However, water is found elsewhere in the Solar System. Europa, one of the moons orbiting Jupiter, is slightly smaller than the Earth's Moon. There is a strong possibility a large saltwater ocean exists beneath its ice surface.{{cite web |vauthors=Dyches P, Brown D |title=NASA Research Reveals Europa's Mystery Dark Material Could Be Sea Salt |url=http://www.jpl.nasa.gov/news/news.php?feature=4586 |date=12 May 2015 |work=NASA |access-date=12 May 2015 }} It has been estimated the outer crust of solid ice is about 10–30 km (6–19 mi) thick and the liquid ocean underneath is about 100 km (60 mi) deep.{{Cite web |vauthors=Adamu Z |date=1 October 2012 |work=CNN Light Years |url=http://lightyears.blogs.cnn.com/2012/10/01/a-moon-of-jupiter-may-have-water-temporarily/?hpt=us_bn4 |title=Water near surface of a Jupiter moon only temporary |access-date=24 April 2019 |archive-date=5 October 2012 |archive-url=https://web.archive.org/web/20121005011205/http://lightyears.blogs.cnn.com/2012/10/01/a-moon-of-jupiter-may-have-water-temporarily/?hpt=us_bn4 |url-status=dead }} This would make Europa's ocean over twice the volume of the Earth's ocean. There has been speculation Europa's ocean could support life,{{cite web |url=http://people.msoe.edu/~tritt/sf/europa.life.html |title=Possibility of Life on Europa |vauthors=Tritt CS |access-date=10 August 2007 |publisher=Milwaukee School of Engineering |date=2002 |url-status=dead |archive-url=https://web.archive.org/web/20070609150109/http://people.msoe.edu/~tritt/sf/europa.life.html |archive-date=9 June 2007 }}{{cite web |title=Alternative Energy Sources Could Support Life on Europa |url=http://www.geo.utep.edu/pub/dirksm/geobiowater/pdf/EOS27March2001.pdf |vauthors=Schulze-Makuch D, Irwin LN |work=Departments of Geological and Biological Sciences, University of Texas at El Paso |date=2001 |access-date=21 December 2007 |archive-url=https://web.archive.org/web/20060703033956/http://www.geo.utep.edu/pub/dirksm/geobiowater/pdf/EOS27March2001.pdf |archive-date=3 July 2006 }} and could be capable of supporting multicellular microorganisms if hydrothermal vents are active on the ocean floor.{{Cite web |url=http://www.planetary.org/programs/projects/explore_europa/update_12142005.html |title=Projects: Europa Mission Campaign |publisher=The Planetary Society |vauthors=Friedman L |date=14 December 2005 |access-date=8 August 2011 |url-status=dead |archive-url=https://web.archive.org/web/20110811002508/http://www.planetary.org/programs/projects/explore_europa/update_12142005.html |archive-date=11 August 2011 }} Enceladus, a small icy moon of Saturn, also has what appears to be an underground ocean which actively vents warm water from the moon's surface.{{cite web |url=http://astrobiology.com/2015/03/ocean-within-enceladus-may-harbor-hydrothermal-activity.html |title=Ocean Within Enceladus May Harbor Hydrothermal Activity |work=NASA Press Release |date=11 March 2015 }}

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{{Life timeline}}

{{Further|Evolutionary history of life|Timeline of evolutionary history of life}}

The Earth is about 4.54 billion years old.{{cite web |url=http://pubs.usgs.gov/gip/geotime/age.html |title=Age of the Earth |date=9 July 2007 |publisher=United States Geological Survey |access-date=2015-05-31 }}{{cite journal |vauthors=Dalrymple GB |title=The age of the Earth in the twentieth century: a problem (mostly) solved. |publisher=Geological Society |location=London |journal=Special Publications |date=January 2001 |volume=190 |issue=1 |pages=205–21 |doi=10.1144/GSL.SP.2001.190.01.14 |bibcode=2001GSLSP.190..205D |s2cid=130092094 }}{{cite journal |vauthors=Manhes G, Allègre CJ, Dupré B, Hamelin B |author-link2=Claude Allègre |date=May 1980 |title=Lead isotope study of basic-ultrabasic layered complexes: Speculations about the age of the earth and primitive mantle characteristics |journal=Earth and Planetary Science Letters |volume=47 |issue=3 |pages=370–382 |bibcode=1980E&PSL..47..370M |doi=10.1016/0012-821X(80)90024-2 |issn=0012-821X }} The earliest undisputed evidence of life on Earth dates from at least 3.5 billion years ago,{{cite journal |vauthors=Schopf JW, Kudryavtsev AB, Czaja AD, Tripathi AB |author-link1=J. William Schopf |date=5 October 2007 |title=Evidence of Archean life: Stromatolites and microfossils |journal=Precambrian Research |volume=158 |pages=141–155 |issue=3–4 |doi=10.1016/j.precamres.2007.04.009 |issn=0301-9268 |bibcode=2007PreR..158..141S }}{{cite book |vauthors=Raven PH, Johnson GB |title=Biology |date=2002 |publisher=McGraw-Hill |location=Boston |isbn=978-0-07-112261-0 |edition=6th |page=68 }} during the Eoarchean era after a geological crust started to solidify following the earlier molten Hadean Eon. Microbial mat fossils have been found in 3.48 billion-year-old sandstone in Western Australia.{{cite journal |vauthors=Baumgartner RJ, Van Kranendonk MJ, Wacey D, Fiorentini ML, Saunders M, Caruso S, Pages A, Homann M, Guagliardo P |display-authors=6 |year=2019 |title=Nano−porous pyrite and organic matter in 3.5-billion-year-old stromatolites record primordial life |url=https://discovery.ucl.ac.uk/id/eprint/10087275/1/Baumgartner%20et%20al%202019%20accepted.pdf|journal=Geology |volume=47 |issue=11 |pages=1039–1043 |doi=10.1130/G46365.1 |bibcode=2019Geo....47.1039B |s2cid=204258554 }}{{cite web |url=https://phys.org/news/2019-09-earliest-life-scientists-microbial-ancient.html |title=Earliest signs of life: Scientists find microbial remains in ancient rocks |work=Phys.org |date=26 September 2019 }} Other early physical evidence of a biogenic substance is graphite in 3.7 billion-year-old metasedimentary rocks discovered in Western Greenland{{cite journal | vauthors = Ohtomo Y, Kakegawa T, Ishida A, Nagase T, Rosing MT |date=January 2014 |title=Evidence for biogenic graphite in early Archaean Isua metasedimentary rocks |journal=Nature Geoscience |volume=7 |issue=1 |pages=25–28 |bibcode=2014NatGe...7...25O |doi=10.1038/ngeo2025 |issn=1752-0894}} as well as "remains of biotic life" found in 4.1 billion-year-old rocks in Western Australia.{{cite news | vauthors = Borenstein S |title=Hints of life on what was thought to be desolate early Earth |url=https://apnews.com/e6be2537b4cd46ffb9c0585bae2b2e51 |date=19 October 2015 |work=Associated Press |access-date=2018-10-09}}{{cite journal | vauthors = Bell EA, Boehnke P, Harrison TM, Mao WL | title = Potentially biogenic carbon preserved in a 4.1 billion-year-old zircon | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 112 | issue = 47 | pages = 14518–21 | date = November 2015 | pmid = 26483481 | pmc = 4664351 | doi = 10.1073/pnas.1517557112 | bibcode = 2015PNAS..11214518B | doi-access = free }} According to one of the researchers, "If life arose relatively quickly on Earth ... then it could be common in the universe."

All organisms on Earth are descended from a common ancestor or ancestral gene pool.{{cite journal | vauthors = Penny D, Poole A | title = The nature of the last universal common ancestor | journal = Current Opinion in Genetics & Development | volume = 9 | issue = 6 | pages = 672–7 | date = December 1999 | pmid = 10607605 | doi = 10.1016/S0959-437X(99)00020-9 }}{{cite journal | vauthors = Theobald DL | title = A formal test of the theory of universal common ancestry | journal = Nature | volume = 465 | issue = 7295 | pages = 219–22 | date = May 2010 | pmid = 20463738 | doi = 10.1038/nature09014 | bibcode = 2010Natur.465..219T | s2cid = 4422345 }}

Highly energetic chemistry is thought to have produced a self-replicating molecule around 4 billion years ago, and half a billion years later the last common ancestor of all life existed.{{cite journal | vauthors = Doolittle WF | title = Uprooting the tree of life | journal = Scientific American | volume = 282 | issue = 2 | pages = 90–5 | date = February 2000 | pmid = 10710791 | doi = 10.1038/scientificamerican0200-90 | url = http://shiva.msu.montana.edu/courses/mb437_537_2004_fall/docs/uprooting.pdf | access-date = 2015-04-05 | bibcode = 2000SciAm.282b..90D | archive-url = https://web.archive.org/web/20060907081933/http://shiva.msu.montana.edu/courses/mb437_537_2004_fall/docs/uprooting.pdf | archive-date = 2006-09-07 | author-link = Ford Doolittle }} The current scientific consensus is that the complex biochemistry that makes up life came from simpler chemical reactions.{{cite journal | vauthors = Peretó J | title = Controversies on the origin of life | journal = International Microbiology | volume = 8 | issue = 1 | pages = 23–31 | date = March 2005 | pmid = 15906258 | url = http://www.im.microbios.org/0801/0801023.pdf | url-status = dead | archive-url = https://web.archive.org/web/20150824074726/http://www.im.microbios.org/0801/0801023.pdf | archive-date = 24 August 2015 }} The beginning of life may have included self-replicating molecules such as RNA{{cite journal | vauthors = Joyce GF | title = The antiquity of RNA-based evolution | journal = Nature | volume = 418 | issue = 6894 | pages = 214–21 | date = July 2002 | pmid = 12110897 | doi = 10.1038/418214a | bibcode = 2002Natur.418..214J | s2cid = 4331004 | author-link = Gerald Joyce }} and the assembly of simple cells.{{cite journal | vauthors = Trevors JT, Psenner R | title = From self-assembly of life to present-day bacteria: a possible role for nanocells | journal = FEMS Microbiology Reviews | volume = 25 | issue = 5 | pages = 573–82 | date = December 2001 | pmid = 11742692 | doi = 10.1111/j.1574-6976.2001.tb00592.x | doi-access = free }} In 2016 scientists reported a set of 355 genes from the last universal common ancestor (LUCA) of all life, including microorganisms, living on Earth.{{cite news | vauthors = Wade N |author-link=Nicholas Wade |title=Meet Luca, the Ancestor of All Living Things |url=https://www.nytimes.com/2016/07/26/science/last-universal-ancestor.html |date=25 July 2016 |work=New York Times |access-date=25 July 2016 }}

Current species are a stage in the process of evolution, with their diversity the product of a long series of speciation and extinction events.{{cite journal | vauthors = Bapteste E, Walsh DA | title = Does the 'Ring of Life' ring true? | journal = Trends in Microbiology | volume = 13 | issue = 6 | pages = 256–61 | date = June 2005 | pmid = 15936656 | doi = 10.1016/j.tim.2005.03.012 }} The common descent of organisms was first deduced from four simple facts about organisms: First, they have geographic distributions that cannot be explained by local adaptation. Second, the diversity of life is not a set of unique organisms, but organisms that share morphological similarities. Third, vestigial traits with no clear purpose resemble functional ancestral traits and finally, that organisms can be classified using these similarities into a hierarchy of nested groups—similar to a family tree.{{cite web | vauthors = Darwin C | title = On The Origin of Species | location = London | publisher = John Murray | date = 1859 | url = http://darwin-online.org.uk/content/frameset?itemID=F373&viewtype=text&pageseq=16 }} However, modern research has suggested that, due to horizontal gene transfer, this "tree of life" may be more complicated than a simple branching tree since some genes have spread independently between distantly related species.{{cite journal | vauthors = Doolittle WF, Bapteste E | title = Pattern pluralism and the Tree of Life hypothesis | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 104 | issue = 7 | pages = 2043–9 | date = February 2007 | pmid = 17261804 | pmc = 1892968 | doi = 10.1073/pnas.0610699104 | bibcode = 2007PNAS..104.2043D | doi-access = free }}{{cite journal | vauthors = Kunin V, Goldovsky L, Darzentas N, Ouzounis CA | title = The net of life: reconstructing the microbial phylogenetic network | journal = Genome Research | volume = 15 | issue = 7 | pages = 954–9 | date = July 2005 | pmid = 15965028 | pmc = 1172039 | doi = 10.1101/gr.3666505 }}

Past species have also left records of their evolutionary history. Fossils, along with the comparative anatomy of present-day organisms, constitute the morphological, or anatomical, record.{{cite journal | vauthors = Jablonski D | title = The future of the fossil record | journal = Science | volume = 284 | issue = 5423 | pages = 2114–6 | date = June 1999 | pmid = 10381868 | doi = 10.1126/science.284.5423.2114 | s2cid = 43388925 }} By comparing the anatomies of both modern and extinct species, paleontologists can infer the lineages of those species. However, this approach is most successful for organisms that had hard body parts, such as shells, bones or teeth. Further, as prokaryotes such as bacteria and archaea share a limited set of common morphologies, their fossils do not provide information on their ancestry.

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{{PhylomapA|size=320px|align=left|caption=Evolutionary tree showing the divergence of modern species from their common ancestor in the centre.{{cite journal | vauthors = Ciccarelli FD, Doerks T, von Mering C, Creevey CJ, Snel B, Bork P | title = Toward automatic reconstruction of a highly resolved tree of life | journal = Science | volume = 311 | issue = 5765 | pages = 1283–7 | date = March 2006 | pmid = 16513982 | doi = 10.1126/science.1123061 | bibcode = 2006Sci...311.1283C | s2cid = 1615592 | citeseerx = 10.1.1.381.9514 | author-link6 = Peer Bork }} The three domains are coloured, with bacteria blue, archaea green and eukaryotes red.}}

More recently, evidence for common descent has come from the study of biochemical similarities between organisms. For example, all living cells use the same basic set of nucleotides and amino acids.{{cite journal | vauthors = Mason SF | title = Origins of biomolecular handedness | journal = Nature | volume = 311 | issue = 5981 | pages = 19–23 | date = 6 September 1984 | pmid = 6472461 | doi = 10.1038/311019a0 | bibcode = 1984Natur.311...19M | s2cid = 103653 }} The development of molecular genetics has revealed the record of evolution left in organisms' genomes: dating when species diverged through the molecular clock produced by mutations.{{cite journal | vauthors = Wolf YI, Rogozin IB, Grishin NV, Koonin EV | title = Genome trees and the tree of life | journal = Trends in Genetics | volume = 18 | issue = 9 | pages = 472–9 | date = September 2002 | pmid = 12175808 | doi = 10.1016/S0168-9525(02)02744-0 | author-link4 = Eugene Koonin }} For example, these DNA sequence comparisons have revealed that humans and chimpanzees share 98% of their genomes and analyzing the few areas where they differ helps shed light on when the common ancestor of these species existed.{{cite journal | vauthors = Varki A, Altheide TK | title = Comparing the human and chimpanzee genomes: searching for needles in a haystack | journal = Genome Research | volume = 15 | issue = 12 | pages = 1746–58 | date = December 2005 | pmid = 16339373 | doi = 10.1101/gr.3737405 | doi-access = free | author-link1 = Ajit Varki }}

Prokaryotes inhabited the Earth from approximately 3–4 billion years ago.{{cite journal | vauthors = Cavalier-Smith T | title = Cell evolution and Earth history: stasis and revolution | journal = Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences | volume = 361 | issue = 1470 | pages = 969–1006 | date = June 2006 | pmid = 16754610 | pmc = 1578732 | doi = 10.1098/rstb.2006.1842 | author-link = Thomas Cavalier-Smith }}{{cite journal | vauthors = Schopf JW | title = Fossil evidence of Archaean life | journal = Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences | volume = 361 | issue = 1470 | pages = 869–85 | date = June 2006 | pmid = 16754604 | pmc = 1578735 | doi = 10.1098/rstb.2006.1834 | author-link = J. William Schopf }}

• {{cite journal | vauthors = Altermann W, Kazmierczak J | title = Archean microfossils: a reappraisal of early life on Earth | journal = Research in Microbiology | volume = 154 | issue = 9 | pages = 611–7 | date = November 2003 | pmid = 14596897 | doi = 10.1016/j.resmic.2003.08.006 | doi-access = free }} No obvious changes in morphology or cellular organization occurred in these organisms over the next few billion years.{{cite journal | vauthors = Schopf JW | title = Disparate rates, differing fates: tempo and mode of evolution changed from the Precambrian to the Phanerozoic | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 91 | issue = 15 | pages = 6735–42 | date = July 1994 | pmid = 8041691 | pmc = 44277 | doi = 10.1073/pnas.91.15.6735 | bibcode = 1994PNAS...91.6735S | doi-access = free }} The eukaryotic cells emerged between 1.6 and 2.7 billion years ago. The next major change in cell structure came when bacteria were engulfed by eukaryotic cells, in a cooperative association called endosymbiosis.{{cite journal | vauthors = Poole AM, Penny D | title = Evaluating hypotheses for the origin of eukaryotes | journal = BioEssays | volume = 29 | issue = 1 | pages = 74–84 | date = January 2007 | pmid = 17187354 | doi = 10.1002/bies.20516 }}{{cite journal | vauthors = Dyall SD, Brown MT, Johnson PJ | title = Ancient invasions: from endosymbionts to organelles | journal = Science | volume = 304 | issue = 5668 | pages = 253–7 | date = April 2004 | pmid = 15073369 | doi = 10.1126/science.1094884 | bibcode = 2004Sci...304..253D | s2cid = 19424594 | author-link3 = Patricia J. Johnson }} The engulfed bacteria and the host cell then underwent coevolution, with the bacteria evolving into either mitochondria or hydrogenosomes.{{cite journal | vauthors = Martin W | title = The missing link between hydrogenosomes and mitochondria | journal = Trends in Microbiology | volume = 13 | issue = 10 | pages = 457–9 | date = October 2005 | pmid = 16109488 | doi = 10.1016/j.tim.2005.08.005 }} Another engulfment of cyanobacterial-like organisms led to the formation of chloroplasts in algae and plants.{{cite journal | vauthors = Lang BF, Gray MW, Burger G | title = Mitochondrial genome evolution and the origin of eukaryotes | journal = Annual Review of Genetics | volume = 33 | pages = 351–97 | date = December 1999 | pmid = 10690412 | doi = 10.1146/annurev.genet.33.1.351 }}
• {{cite journal | vauthors = McFadden GI | title = Endosymbiosis and evolution of the plant cell | journal = Current Opinion in Plant Biology | volume = 2 | issue = 6 | pages = 513–9 | date = December 1999 | pmid = 10607659 | doi = 10.1016/S1369-5266(99)00025-4 | bibcode = 1999COPB....2..513M }}

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File:Tree of Living Organisms 2.png

The history of life was that of the unicellular eukaryotes, prokaryotes and archaea until about 610 million years ago when multicellular organisms began to appear in the oceans in the Ediacaran period.{{cite journal | vauthors = DeLong EF, Pace NR | title = Environmental diversity of bacteria and archaea | journal = Systematic Biology | volume = 50 | issue = 4 | pages = 470–8 | date = August 2001 | pmid = 12116647 | doi = 10.1080/106351501750435040 | citeseerx = 10.1.1.321.8828 | author-link2 = Norman R. Pace | author-link1 = Edward DeLong }} The evolution of multicellularity occurred in multiple independent events, in organisms as diverse as sponges, brown algae, cyanobacteria, slime moulds and myxobacteria.{{cite journal | vauthors = Kaiser D | title = Building a multicellular organism | journal = Annual Review of Genetics | volume = 35 | pages = 103–23 | date = December 2001 | pmid = 11700279 | doi = 10.1146/annurev.genet.35.102401.090145 | s2cid = 18276422 | author-link = A. Dale Kaiser }} In 2016 scientists reported that, about 800 million years ago, a minor genetic change in a single molecule called GK-PID may have allowed organisms to go from a single cell organism to one of many cells.{{cite news | vauthors = Zimmer C |author-link=Carl Zimmer |title=Genetic Flip Helped Organisms Go From One Cell to Many |url=https://www.nytimes.com/2016/01/12/science/genetic-flip-helped-organisms-go-from-one-cell-to-many.html |date=7 January 2016 |work=The New York Times |access-date=7 January 2016 }}

Soon after the emergence of these first multicellular organisms, a remarkable amount of biological diversity appeared over a span of about 10 million years, in an event called the Cambrian explosion. Here, the majority of types of modern animals appeared in the fossil record, as well as unique lineages that subsequently became extinct.{{cite journal | vauthors = Valentine JW, Jablonski D, Erwin DH | title = Fossils, molecules and embryos: new perspectives on the Cambrian explosion | journal = Development | volume = 126 | issue = 5 | pages = 851–9 | date = February 1999 | pmid = 9927587 | doi = 10.1242/dev.126.5.851 | author-link3 = Douglas Erwin | author-link1 = James W. Valentine }} Various triggers for the Cambrian explosion have been proposed, including the accumulation of oxygen in the atmosphere from photosynthesis.{{cite journal | vauthors = Ohno S | title = The reason for as well as the consequence of the Cambrian explosion in animal evolution | journal = Journal of Molecular Evolution | volume = 44 | issue = Suppl. 1 | pages = S23-7 | date = January 1997 | pmid = 9071008 | doi = 10.1007/PL00000055 | s2cid = 21879320 | bibcode = 1997JMolE..44S..23O }}

• {{cite journal | vauthors = Valentine JW, Jablonski D | title = Morphological and developmental macroevolution: a paleontological perspective | journal = The International Journal of Developmental Biology | volume = 47 | issue = 7–8 | pages = 517–22 | year = 2003 | pmid = 14756327 | url = http://www.ijdb.ehu.es/web/paper.php?doi=14756327 | access-date = 2014-12-30 }}

About 500 million years ago, plants and fungi started colonizing the land. Evidence for the appearance of the first land plants occurs in the Ordovician, around {{Ma|450}}, in the form of fossil spores.{{cite journal | vauthors = Wellman CH, Osterloff PL, Mohiuddin U | title = Fragments of the earliest land plants | journal = Nature | volume = 425 | issue = 6955 | pages = 282–5 | date = September 2003 | pmid = 13679913 | doi = 10.1038/nature01884 | s2cid = 4383813 | bibcode = 2003Natur.425..282W | url = http://eprints.whiterose.ac.uk/106/1/wellmanch1.pdf }} Land plants began to diversify in the Late Silurian, from around {{Ma|430}}.{{cite book | vauthors = Barton N |author-link=Nick Barton |year=2007 |title=Evolution |pages=273–274|publisher=CSHL Press |url=https://books.google.com/books?id=mMDFQ32oMI8C |access-date=30 September 2012|isbn=9780199226320}} The colonization of the land by plants was soon followed by arthropods and other animals.{{cite journal | vauthors = Waters ER | title = Molecular adaptation and the origin of land plants | journal = Molecular Phylogenetics and Evolution | volume = 29 | issue = 3 | pages = 456–63 | date = December 2003 | pmid = 14615186 | doi = 10.1016/j.ympev.2003.07.018 | bibcode = 2003MolPE..29..456W }} Insects were particularly successful and even today make up the majority of animal species.{{cite journal | vauthors = Mayhew PJ | title = Why are there so many insect species? Perspectives from fossils and phylogenies | journal = Biological Reviews of the Cambridge Philosophical Society | volume = 82 | issue = 3 | pages = 425–54 | date = August 2007 | pmid = 17624962 | doi = 10.1111/j.1469-185X.2007.00018.x | s2cid = 9356614 | author-link = Peter Mayhew (biologist) }} Amphibians first appeared around 364 million years ago, followed by early amniotes and birds around 155 million years ago (both from "reptile"-like lineages), mammals around 129 million years ago, homininae around 10 million years ago and modern humans around 250,000 years ago.{{cite journal | vauthors = Carroll RL |author-link=Robert L. Carroll |date=May 2007 |title=The Palaeozoic Ancestry of Salamanders, Frogs and Caecilians |journal=Zoological Journal of the Linnean Society |volume=150 |issue=Supplement s1 |pages=1–140 |doi=10.1111/j.1096-3642.2007.00246.x |issn=1096-3642 |doi-access=free }}{{cite journal | vauthors = Wible JR, Rougier GW, Novacek MJ, Asher RJ | title = Cretaceous eutherians and Laurasian origin for placental mammals near the K/T boundary | journal = Nature | volume = 447 | issue = 7147 | pages = 1003–6 | date = June 2007 | pmid = 17581585 | doi = 10.1038/nature05854 | bibcode = 2007Natur.447.1003W | s2cid = 4334424 }}{{cite journal | vauthors = Witmer LM | title = Palaeontology: An icon knocked from its perch | journal = Nature | volume = 475 | issue = 7357 | pages = 458–9 | date = July 2011 | pmid = 21796198 | doi = 10.1038/475458a | s2cid = 205066360 | author-link = Lawrence Witmer }} However, despite the evolution of these large animals, smaller organisms similar to the types that evolved early in this process continue to be highly successful and dominate the Earth, with the majority of both biomass and species being prokaryotes.{{cite journal | vauthors = Schloss PD, Handelsman J | title = Status of the microbial census | journal = Microbiology and Molecular Biology Reviews | volume = 68 | issue = 4 | pages = 686–91 | date = December 2004 | pmid = 15590780 | pmc = 539005 | doi = 10.1128/MMBR.68.4.686-691.2004 | author-link2 = Jo Handelsman }}

Estimates on the number of Earth's current species range from 10 million to 14 million,{{cite book | vauthors = Miller GT, Spoolman S| title = Environmental Science | publisher = Cengage Learning | date = January 2012 | isbn = 978-1-133-70787-5 | chapter = Chapter 4.1: What is Biodiversity and Why is it Important? | page = 62 | chapter-url = https://books.google.com/books?id=NYEJAAAAQBAJ&pg=PA62 }} of which about 1.2 million have been documented and over 86 percent have not yet been described.{{cite journal | vauthors = Mora C, Tittensor DP, Adl S, Simpson AG, Worm B | title = How many species are there on Earth and in the ocean? | journal = PLOS Biology | volume = 9 | issue = 8 | pages = e1001127 | date = August 2011 | pmid = 21886479 | pmc = 3160336 | doi = 10.1371/journal.pbio.1001127 | author-link5 = Boris Worm | doi-access = free }}

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{{multiple image

| align = right

| direction = horizontal

| width = 220

| header = microbial mats

| header_align = center

| image1 = Cyanobacterial-algal mat.jpg

| caption1 = Microbial mats are the earliest form of life on Earth for which there is good fossil evidence. The image shows a cyanobacterial-algal mat.

| image2 = Stromatolites in Sharkbay.jpg

| caption2 = Stromatolites are formed from microbial mats as microbes slowly move upwards to avoid being smothered by sediment.

}}

{{main|Marine microorganism}}

Microorganisms make up about 70% of the marine biomass. A microorganism, or microbe, is a microscopic organism too small to be recognized with the naked eye. It can be single-celled{{cite book |veditors=Madigan M, Martinko J |title=Brock Biology of Microorganisms |edition=13th |publisher=Pearson Education |year=2006 |isbn=978-0-321-73551-5 |page=1096 }} or multicellular. Microorganisms are diverse and include all bacteria and archaea, most protozoa such as algae, fungi, and certain microscopic animals such as rotifers.

Many macroscopic animals and plants have microscopic juvenile stages. Some microbiologists also classify viruses (and viroids) as microorganisms, but others consider these as nonliving.{{Cite journal |vauthors=Rybicki EP |title=The classification of organisms at the edge of life, or problems with virus systematics |journal=South African Journal of Science |volume=86 |pages=182–6 |year=1990 |issn=0038-2353}}{{cite journal |vauthors=Lwoff A |title=The concept of virus |journal=Journal of General Microbiology |volume=17 |issue=2 |pages=239–53 |date=October 1957 |pmid=13481308 |doi=10.1099/00221287-17-2-239 |doi-access=free }}

Microorganisms are crucial to nutrient recycling in ecosystems as they act as decomposers. Some microorganisms are pathogenic, causing disease and even death in plants and animals.{{cite web |url=https://www.who.int/healthinfo/bodgbd2002revised/en/index.html |archive-url=https://web.archive.org/web/20060819224428/http://www.who.int/healthinfo/bodgbd2002revised/en/index.html |url-status=dead |archive-date=19 August 2006 |title=2002 WHO mortality data |access-date=20 January 2007 }} As inhabitants of the largest environment on Earth, microbial marine systems drive changes in every global system. Microbes are responsible for virtually all the photosynthesis that occurs in the ocean, as well as the cycling of carbon, nitrogen, phosphorus, other nutrients and trace elements.{{cite web |url=https://www.sciencedaily.com/releases/2015/12/151210181647.htm |title=Functions of global ocean microbiome key to understanding environmental changes |date=10 December 2015 |website=www.sciencedaily.com |publisher=University of Georgia |access-date=11 December 2015}}

File:Relative scale.svgs (bacteria and archaea) and viruses relative to those of other organisms and biomolecules]]

{{clade

|label1=Marine microorganisms

|1={{clade

|1 = Viruses 45px

|label2=Cellular life

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|label1=Prokaryotes

|1={{clade

|1 = Bacteria 45px

|2 = Archaea 25px

}}

|label2=Eukaryotes

|2={{clade

|1 = Protists 55px

|2 = Microfungi 25px

|4 = Microanimals 40px

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Microscopic life undersea is diverse and still poorly understood, such as for the role of viruses in marine ecosystems.{{cite journal |vauthors=Suttle CA |title=Viruses in the sea |journal=Nature |volume=437 |issue=7057 |pages=356–61 |date=September 2005 |pmid=16163346 |doi=10.1038/nature04160 |s2cid=4370363 |bibcode=2005Natur.437..356S }} Most marine viruses are bacteriophages, which are harmless to plants and animals, but are essential to the regulation of saltwater and freshwater ecosystems.{{cite book |vauthors=Shors T |date=2017 |title=Understanding Viruses |edition=3rd |publisher=Jones and Bartlett Publishers |isbn=978-1-284-02592-7}}{{rp|5}} They infect and destroy bacteria in aquatic microbial communities, and are the most important mechanism of recycling carbon in the marine environment. The organic molecules released from the dead bacterial cells stimulate fresh bacterial and algal growth.{{rp|593}} Viral activity may also contribute to the biological pump, the process whereby carbon is sequestered in the deep ocean.{{cite journal |vauthors=Suttle CA |title=Marine viruses--major players in the global ecosystem |journal=Nature Reviews. Microbiology |volume=5 |issue=10 |pages=801–12 |date=October 2007 |pmid=17853907 |doi=10.1038/nrmicro1750 |s2cid=4658457 }}

File:Ocean mist and spray 2.jpg, and can travel the globe before falling back to earth.]]A stream of airborne microorganisms circles the planet above weather systems but below commercial air lanes.{{cite web | vauthors = Morrison J | date = 11 January 2016 | url = https://www.smithsonianmag.com/science-nature/living-bacteria-are-riding-earths-air-currents-180957734/ | title = Living Bacteria Are Riding Earth's Air Currents | work = Smithsonian Magazine }} Some peripatetic microorganisms are swept up from terrestrial dust storms, but most originate from marine microorganisms in sea spray. In 2018, scientists reported that hundreds of millions of viruses and tens of millions of bacteria are deposited daily on every square meter around the planet.{{cite news | vauthors = Robbins J |title=Trillions Upon Trillions of Viruses Fall From the Sky Each Day |url=https://www.nytimes.com/2018/04/13/science/virosphere-evolution.html |date=13 April 2018 |work=The New York Times |access-date=14 April 2018 }}{{cite journal | vauthors = Reche I, D'Orta G, Mladenov N, Winget DM, Suttle CA | title = Deposition rates of viruses and bacteria above the atmospheric boundary layer | journal = The ISME Journal | volume = 12 | issue = 4 | pages = 1154–1162 | date = April 2018 | pmid = 29379178 | pmc = 5864199 | doi = 10.1038/s41396-017-0042-4 | bibcode = 2018ISMEJ..12.1154R }}

Microscopic organisms live throughout the biosphere. The mass of prokaryote microorganisms — which includes bacteria and archaea, but not the nucleated eukaryote microorganisms — may be as much as 0.8 trillion tons of carbon (of the total biosphere mass, estimated at between 1 and 4 trillion tons).{{cite web |author=Staff |title=The Biosphere |url=http://www.agci.org/classroom/biosphere/index.php |date=2014 |work=Aspen Global Change Institute |access-date=10 November 2014 |archive-date=2 September 2010 |archive-url=https://web.archive.org/web/20100902111048/http://www.agci.org/classroom/biosphere/index.php |url-status=dead }} Single-celled barophilic marine microbes have been found at a depth of {{convert|10900|m|ft|abbr=on}} in the Mariana Trench, the deepest spot in the Earth's oceans.{{cite web | vauthors = Choi CQ | title=Microbes Thrive in Deepest Spot on Earth |url=http://www.livescience.com/27954-microbes-mariana-trench.html |date=17 March 2013 |publisher=LiveScience |access-date=17 March 2013 }}{{cite journal | vauthors = Glud RN, Wenzhöfer F, Middelboe M, Oguri K, Turnewitsch R, Canfield DE, Kitazato H |title=High rates of microbial carbon turnover in sediments in the deepest oceanic trench on Earth |doi=10.1038/ngeo1773 |date=17 March 2013 |journal=Nature Geoscience |volume=6 |issue=4 |pages=284–288 |bibcode = 2013NatGe...6..284G }} Microorganisms live inside rocks {{convert|580|m|ft|abbr=on}} below the sea floor under {{convert|2590|m|ft|abbr=on}} of ocean off the coast of the northwestern United States,{{cite web | vauthors = Oskin B |title=Intraterrestrials: Life Thrives in Ocean Floor |url= http://www.livescience.com/27899-ocean-subsurface-ecosystem-found.html |date=14 March 2013 |publisher=LiveScience |access-date=17 March 2013 }} as well as {{convert|2400|m|ft mi|abbr=on}} beneath the seabed off Japan.{{cite news | last = Morelle | first=Rebecca | author-link=Rebecca Morelle | name-list-style=vanc |title=Microbes discovered by deepest marine drill analysed |url=https://www.bbc.com/news/science-environment-30489814 |date=15 December 2014 |work=BBC News |access-date=15 December 2014 }} The greatest known temperature at which microbial life can exist is {{convert|122|°C|°F|abbr=on}} (Methanopyrus kandleri).{{cite journal | vauthors = Takai K, Nakamura K, Toki T, Tsunogai U, Miyazaki M, Miyazaki J, Hirayama H, Nakagawa S, Nunoura T, Horikoshi K | display-authors = 6 | title = Cell proliferation at 122 degrees C and isotopically heavy CH4 production by a hyperthermophilic methanogen under high-pressure cultivation | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 105 | issue = 31 | pages = 10949–54 | date = August 2008 | pmid = 18664583 | pmc = 2490668 | doi = 10.1073/pnas.0712334105 | bibcode = 2008PNAS..10510949T | doi-access = free }} In 2014, scientists confirmed the existence of microorganisms living {{convert|800|m|ft|abbr=on}} below the ice of Antarctica.{{cite journal | vauthors = Fox D | title = Lakes under the ice: Antarctica's secret garden | journal = Nature | volume = 512 | issue = 7514 | pages = 244–6 | date = August 2014 | pmid = 25143097 | doi = 10.1038/512244a | doi-access = free | bibcode = 2014Natur.512..244F }}{{cite web | vauthors = Mack E |title=Life Confirmed Under Antarctic Ice; Is Space Next? |url=https://www.forbes.com/sites/ericmack/2014/08/20/life-confirmed-under-antarctic-ice-is-space-next/ |date=20 August 2014 |work=Forbes |access-date=21 August 2014 }} According to one researcher, "You can find microbes everywhere — they're extremely adaptable to conditions, and survive wherever they are."

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{{main|Marine viruses}}

Viruses are small infectious agents that do not have their own metabolism and can replicate only inside the living cells of other organisms.{{cite journal | vauthors = Wimmer E, Mueller S, Tumpey TM, Taubenberger JK | title = Synthetic viruses: a new opportunity to understand and prevent viral disease | journal = Nature Biotechnology | volume = 27 | issue = 12 | pages = 1163–72 | date = December 2009 | pmid = 20010599 | pmc = 2819212 | doi = 10.1038/nbt.1593 }} Viruses can infect all types of life forms, from animals and plants to microorganisms, including bacteria and archaea.{{cite journal | vauthors = Koonin EV, Senkevich TG, Dolja VV | title = The ancient Virus World and evolution of cells | journal = Biology Direct | volume = 1 | pages = 29 | date = September 2006 | pmid = 16984643 | pmc = 1594570 | doi = 10.1186/1745-6150-1-29 | doi-access = free }} The linear size of the average virus is about one one-hundredth that of the average bacterium. Most viruses cannot be seen with an optical microscope so electron microscopes are used instead.{{cite book | vauthors = Topley WW, Wilson GS, Collier LH, Balows A, Sussman M |title=Topley and Wilson's Microbiology and Microbial Infections. |date=1998 |publisher=Arnold |location=London |isbn=978-0-340-66316-5 |edition=9th | volume = 1 | pages = 33–37 | veditors = Mahy BW, Collier L }}

Viruses are found wherever there is life and have probably existed since living cells first evolved.{{cite journal | vauthors = Iyer LM, Balaji S, Koonin EV, Aravind L | title = Evolutionary genomics of nucleo-cytoplasmic large DNA viruses | journal = Virus Research | volume = 117 | issue = 1 | pages = 156–84 | date = April 2006 | pmid = 16494962 | doi = 10.1016/j.virusres.2006.01.009 | url = https://zenodo.org/record/1259447 }} The origin of viruses is unclear because they do not form fossils, so molecular techniques have been used to compare the DNA or RNA of viruses and are a useful means of investigating how they arise.{{cite journal | vauthors = Sanjuán R, Nebot MR, Chirico N, Mansky LM, Belshaw R | title = Viral mutation rates | journal = Journal of Virology | volume = 84 | issue = 19 | pages = 9733–48 | date = October 2010 | pmid = 20660197 | pmc = 2937809 | doi = 10.1128/JVI.00694-10 }}

Viruses are now recognized as ancient and as having origins that pre-date the divergence of life into the three domains.{{cite book | veditors = Mahy WJ, Van Regenmortel MH |title=Desk Encyclopedia of General Virology |publisher=Academic Press |location=Oxford |year=2009 |pages=28 |isbn=978-0-12-375146-1}} But the origins of viruses in the evolutionary history of life are unclear: some may have evolved from plasmids—pieces of DNA that can move between cells—while others may have evolved from bacteria. In evolution, viruses are an important means of horizontal gene transfer, which increases genetic diversity.

{{multiple image

| align = left

| direction = horizontal

| header = Bacteriophages (phages)

| image1 = Phage.jpg

| width1 = 160

| caption1 = Multiple phages attached to a bacterial cell wall at 200,000× magnification

| image2 = Tailed phage.png

| width2 = 166

| caption2 = Diagram of a typical tailed phage

}}

File:Cyanophages.pngs, viruses that infect cyanobacteria (scale bars indicate 100 nm)]]

Opinions differ on whether viruses are a form of life or organic structures that interact with living organisms.{{cite journal | vauthors = Koonin EV, Starokadomskyy P | title = Are viruses alive? The replicator paradigm sheds decisive light on an old but misguided question | journal = Studies in History and Philosophy of Biological and Biomedical Sciences | volume = 59 | pages = 125–34 | date = October 2016 | pmid = 26965225 | pmc = 5406846 | doi = 10.1016/j.shpsc.2016.02.016 }} They are considered by some to be a life form, because they carry genetic material, reproduce by creating multiple copies of themselves through self-assembly, and evolve through natural selection. However they lack key characteristics such as a cellular structure generally considered necessary to count as life. Because they possess some but not all such qualities, viruses have been described as replicators and as "organisms at the edge of life".{{cite journal| vauthors = Rybicki EP |year = 1990|title = The classification of organisms at the edge of life, or problems with virus systematics|journal = South African Journal of Science |volume = 86|pages = 182–186}}

Bacteriophages, often just called phages, are viruses that parasite bacteria and archaea. Marine phages parasite marine bacteria and archaea, such as cyanobacteria.{{cite journal | vauthors = Mann NH | title = The third age of phage | journal = PLOS Biology | volume = 3 | issue = 5 | pages = e182 | date = May 2005 | pmid = 15884981 | pmc = 1110918 | doi = 10.1371/journal.pbio.0030182 | doi-access = free }} They are a common and diverse group of viruses and are the most abundant biological entity in marine environments, because their hosts, bacteria, are typically the numerically dominant cellular life in the sea. Generally there are about 1 million to 10 million viruses in each mL of seawater, or about ten times more double-stranded DNA viruses than there are cellular organisms,{{cite journal | vauthors = Wommack KE, Colwell RR | title = Virioplankton: viruses in aquatic ecosystems | journal = Microbiology and Molecular Biology Reviews | volume = 64 | issue = 1 | pages = 69–114 | date = March 2000 | pmid = 10704475 | pmc = 98987 | doi = 10.1128/MMBR.64.1.69-114.2000 }}{{cite journal | vauthors = Suttle CA | title = Viruses in the sea | journal = Nature | volume = 437 | issue = 7057 | pages = 356–61 | date = September 2005 | pmid = 16163346 | doi = 10.1038/nature04160 | bibcode = 2005Natur.437..356S | s2cid = 4370363 }} although estimates of viral abundance in seawater can vary over a wide range.{{cite journal | vauthors = Bergh O, Børsheim KY, Bratbak G, Heldal M | title = High abundance of viruses found in aquatic environments | journal = Nature | volume = 340 | issue = 6233 | pages = 467–8 | date = August 1989 | pmid = 2755508 | doi = 10.1038/340467a0 | bibcode = 1989Natur.340..467B | s2cid = 4271861 }}{{cite journal | vauthors = Wigington CH, Sonderegger D, Brussaard CP, Buchan A, Finke JF, Fuhrman JA, Lennon JT, Middelboe M, Suttle CA, Stock C, Wilson WH, Wommack KE, Wilhelm SW, Weitz JS | display-authors = 6 | title = Re-examination of the relationship between marine virus and microbial cell abundances | journal = Nature Microbiology | volume = 1 | pages = 15024 | date = January 2016 | issue = 3 | pmid = 27572161 | doi = 10.1038/nmicrobiol.2015.24 | s2cid = 52829633 | url = http://www.vliz.be/imisdocs/publications/23/301523.pdf }} Tailed bacteriophages appear to dominate marine ecosystems in number and diversity of organisms. Bacteriophages belonging to the families Corticoviridae,{{cite journal | vauthors = Krupovic M, Bamford DH | title = Putative prophages related to lytic tailless marine dsDNA phage PM2 are widespread in the genomes of aquatic bacteria | journal = BMC Genomics | volume = 8 | pages = 236 | date = July 2007 | pmid = 17634101 | pmc = 1950889 | doi = 10.1186/1471-2164-8-236 | doi-access = free }} Inoviridae{{cite journal | vauthors = Xue H, Xu Y, Boucher Y, Polz MF | title = High frequency of a novel filamentous phage, VCY φ, within an environmental Vibrio cholerae population | journal = Applied and Environmental Microbiology | volume = 78 | issue = 1 | pages = 28–33 | date = January 2012 | pmid = 22020507 | pmc = 3255608 | doi = 10.1128/AEM.06297-11 | bibcode = 2012ApEnM..78...28X }} and Microviridae{{cite journal | vauthors = Roux S, Krupovic M, Poulet A, Debroas D, Enault F | title = Evolution and diversity of the Microviridae viral family through a collection of 81 new complete genomes assembled from virome reads | journal = PLOS ONE | volume = 7 | issue = 7 | pages = e40418 | year = 2012 | pmid = 22808158 | pmc = 3394797 | doi = 10.1371/journal.pone.0040418 | bibcode = 2012PLoSO...740418R | doi-access = free }} are also known to infect diverse marine bacteria.

Microorganisms make up about 70% of the marine biomass. It is estimated viruses kill 20% of this biomass each day and that there are 15 times as many viruses in the oceans as there are bacteria and archaea. Viruses are the main agents responsible for the rapid destruction of harmful algal blooms, which often kill other marine life.{{cite web

|url=https://www.cdc.gov/hab/redtide/|title=Harmful Algal Blooms: Red Tide: Home {{!}}CDC HSB|publisher=www.cdc.gov|access-date=2014-12-19}}

The number of viruses in the oceans decreases further offshore and deeper into the water, where there are fewer host organisms.

There are also archaeal viruses which replicate within archaea: these are double-stranded DNA viruses with unusual and sometimes unique shapes.{{cite journal | vauthors = Lawrence CM, Menon S, Eilers BJ, Bothner B, Khayat R, Douglas T, Young MJ | title = Structural and functional studies of archaeal viruses | journal = The Journal of Biological Chemistry | volume = 284 | issue = 19 | pages = 12599–603 | date = May 2009 | pmid = 19158076 | pmc = 2675988 | doi = 10.1074/jbc.R800078200 | doi-access = free }}{{cite journal | vauthors = Prangishvili D, Forterre P, Garrett RA | title = Viruses of the Archaea: a unifying view | journal = Nature Reviews. Microbiology | volume = 4 | issue = 11 | pages = 837–48 | date = November 2006 | pmid = 17041631 | doi = 10.1038/nrmicro1527 | s2cid = 9915859 }} These viruses have been studied in most detail in the thermophilic archaea, particularly the orders Sulfolobales and Thermoproteales.{{cite journal | vauthors = Prangishvili D, Garrett RA | title = Exceptionally diverse morphotypes and genomes of crenarchaeal hyperthermophilic viruses | journal = Biochemical Society Transactions | volume = 32 | issue = Pt 2 | pages = 204–8 | date = April 2004 | pmid = 15046572 | doi = 10.1042/BST0320204 | s2cid = 20018642 | url = https://curis.ku.dk/ws/files/51497971/0320204.pdf }}

Viruses are an important natural means of transferring genes between different species, which increases genetic diversity and drives evolution.{{cite journal | vauthors = Canchaya C, Fournous G, Chibani-Chennoufi S, Dillmann ML, Brüssow H | title = Phage as agents of lateral gene transfer | journal = Current Opinion in Microbiology | volume = 6 | issue = 4 | pages = 417–24 | date = August 2003 | pmid = 12941415 | doi = 10.1016/S1369-5274(03)00086-9 }} It is thought that viruses played a central role in the early evolution, before the diversification of bacteria, archaea and eukaryotes, at the time of the last universal common ancestor of life on Earth.{{cite journal | vauthors = Forterre P, Philippe H | title = The last universal common ancestor (LUCA), simple or complex? | journal = The Biological Bulletin | volume = 196 | issue = 3 | pages = 373–5; discussion 375–7 | date = June 1999 | pmid = 11536914 | doi = 10.2307/1542973 | jstor = 1542973 }} Viruses are still one of the largest reservoirs of unexplored genetic diversity on Earth.

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File:Vibrio vulnificus 01.png, a virulent bacterium found in estuaries and along coastal areas]]

{{Further|Marine prokaryotes|Bacterioplankton}}

Bacteria constitute a large domain of prokaryotic microorganisms. Typically a few micrometers in length, bacteria have a number of shapes, ranging from spheres to rods and spirals. Bacteria were among the first life forms to appear on Earth, and are present in most of its habitats. Bacteria inhabit soil, water, acidic hot springs, radioactive waste,{{cite journal | vauthors = Fredrickson JK, Zachara JM, Balkwill DL, Kennedy D, Li SM, Kostandarithes HM, Daly MJ, Romine MF, Brockman FJ | display-authors = 6 | title = Geomicrobiology of high-level nuclear waste-contaminated vadose sediments at the hanford site, washington state | journal = Applied and Environmental Microbiology | volume = 70 | issue = 7 | pages = 4230–41 | date = July 2004 | pmid = 15240306 | pmc = 444790 | doi = 10.1128/AEM.70.7.4230-4241.2004 | bibcode = 2004ApEnM..70.4230F }} and the deep portions of Earth's crust. Bacteria also live in symbiotic and parasitic relationships with plants and animals.

Once regarded as plants constituting the class Schizomycetes, bacteria are now classified as prokaryotes. Unlike cells of animals and other eukaryotes, bacterial cells do not contain a nucleus and rarely harbor membrane-bound organelles. Although the term bacteria traditionally included all prokaryotes, the scientific classification changed after the discovery in the 1990s that prokaryotes consist of two very different groups of organisms that evolved from an ancient common ancestor. These evolutionary domains are called Bacteria and Archaea.{{cite journal | vauthors = Woese CR, Kandler O, Wheelis ML | title = Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and Eucarya | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 87 | issue = 12 | pages = 4576–9 | date = June 1990 | pmid = 2112744 | pmc = 54159 | doi = 10.1073/pnas.87.12.4576 | bibcode = 1990PNAS...87.4576W | doi-access = free }}

The ancestors of modern bacteria were unicellular microorganisms that were the first forms of life to appear on Earth, about 4 billion years ago. For about 3 billion years, most organisms were microscopic, and bacteria and archaea were the dominant forms of life.{{cite journal | vauthors = DeLong EF, Pace NR | title = Environmental diversity of bacteria and archaea | journal = Systematic Biology | volume = 50 | issue = 4 | pages = 470–8 | date = August 2001 | pmid = 12116647 | doi = 10.1080/106351501750435040 | citeseerx = 10.1.1.321.8828 }} Although bacterial fossils exist, such as stromatolites, their lack of distinctive morphology prevents them from being used to examine the history of bacterial evolution, or to date the time of origin of a particular bacterial species. However, gene sequences can be used to reconstruct the bacterial phylogeny, and these studies indicate that bacteria diverged first from the archaeal/eukaryotic lineage.{{cite journal | vauthors = Brown JR, Doolittle WF | title = Archaea and the prokaryote-to-eukaryote transition | journal = Microbiology and Molecular Biology Reviews | volume = 61 | issue = 4 | pages = 456–502 | date = December 1997 | doi = 10.1128/mmbr.61.4.456-502.1997 | pmid = 9409149 | pmc = 232621 }}

Bacteria were also involved in the second great evolutionary divergence, that of the archaea and eukaryotes. Here, eukaryotes resulted from the entering of ancient bacteria into endosymbiotic associations with the ancestors of eukaryotic cells, which were themselves possibly related to the Archaea. This involved the engulfment by proto-eukaryotic cells of alphaproteobacterial symbionts to form either mitochondria or hydrogenosomes, which are still found in all known Eukarya. Later on, some eukaryotes that already contained mitochondria also engulfed cyanobacterial-like organisms. This led to the formation of chloroplasts in algae and plants. There are also some algae that originated from even later endosymbiotic events. Here, eukaryotes engulfed a eukaryotic algae that developed into a "second-generation" plastid.{{cite journal | vauthors = Lang BF, Gray MW, Burger G | title = Mitochondrial genome evolution and the origin of eukaryotes | journal = Annual Review of Genetics | volume = 33 | pages = 351–97 | year = 1999 | pmid = 10690412 | doi = 10.1146/annurev.genet.33.1.351 }}{{cite journal | vauthors = McFadden GI | title = Endosymbiosis and evolution of the plant cell | journal = Current Opinion in Plant Biology | volume = 2 | issue = 6 | pages = 513–9 | date = December 1999 | pmid = 10607659 | doi = 10.1016/S1369-5266(99)00025-4 | bibcode = 1999COPB....2..513M }} This is known as secondary endosymbiosis.

File:Sulphide bacteria crop2.jpg|The marine Thiomargarita namibiensis, the largest known bacterium

File:Potomac river eutro.jpg|Cyanobacteria blooms can contain lethal cyanotoxins.

File:Glaucocystis sp.jpg|The chloroplasts of glaucophytes have a peptidoglycan layer, evidence suggesting their endosymbiotic origin from cyanobacteria.{{cite journal|journal = American Journal of Botany|year = 2004|volume = 91|pages = 1481–1493|title = Diversity and evolutionary history of plastids and their hosts|vauthors = Keeling PJ|doi = 10.3732/ajb.91.10.1481|issue = 10|pmid = 21652304| bibcode=2004AmJB...91.1481K |s2cid = 17522125}}

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The largest known bacterium, the marine Thiomargarita namibiensis, can be visible to the naked eye and sometimes attains {{convert|0.75|mm|μm|abbr=on}}.{{Citation|work=Max Planck Institute for Marine Microbiology |date=8 April 1999 |title=The largest Bacterium: Scientist discovers new bacterial life form off the African coast |url=http://www.mpg.de/english/illustrationsDocumentation/documentation/pressReleases/1999/news17_99.htm |url-status=dead |archive-url=https://web.archive.org/web/20100120043846/http://www.mpg.de/english/illustrationsDocumentation/documentation/pressReleases/1999/news17_99.htm |archive-date=20 January 2010 }}{{Citation |title=List of Prokaryotic names with Standing in Nomenclature - Genus Thiomargarita |url=https://lpsn.dsmz.de/genus/thiomargarita |df=dmy-all }}

{{Further|Marine prokaryotes}}

The archaea (Greek for ancient{{cite web | url = http://www.etymonline.com/index.php?l=a&p=41 | title = Archaea | work = Online Etymology Dictionary | access-date = 17 August 2016 }}) constitute a domain and kingdom of single-celled microorganisms. These microbes are prokaryotes, meaning they have no cell nucleus or any other membrane-bound organelles in their cells.

Archaea were initially classified as bacteria, but this classification is outdated.{{cite journal | vauthors = Pace NR | title = Time for a change | journal = Nature | volume = 441 | issue = 7091 | pages = 289 | date = May 2006 | pmid = 16710401 | doi = 10.1038/441289a | s2cid = 4431143 | bibcode = 2006Natur.441..289P | doi-access = free }} Archaeal cells have unique properties separating them from the other two domains of life, Bacteria and Eukaryota. The Archaea are further divided into multiple recognized phyla. Classification is difficult because the majority have not been isolated in the laboratory and have only been detected by analysis of their nucleic acids in samples from their environment.

Archaea and bacteria are generally similar in size and shape, although a few archaea have very strange shapes, such as the flat and square-shaped cells of Haloquadratum walsbyi.{{cite journal | vauthors = Stoeckenius W | title = Walsby's square bacterium: fine structure of an orthogonal procaryote | journal = Journal of Bacteriology | volume = 148 | issue = 1 | pages = 352–60 | date = October 1981 | pmid = 7287626 | pmc = 216199 | doi = 10.1128/JB.148.1.352-360.1981 }} Despite this morphological similarity to bacteria, archaea possess genes and several metabolic pathways that are more closely related to those of eukaryotes, notably the enzymes involved in transcription and translation. Other aspects of archaeal biochemistry are unique, such as their reliance on ether lipids in their cell membranes, such as archaeols. Archaea use more energy sources than eukaryotes: these range from organic compounds, such as sugars, to ammonia, metal ions or even hydrogen gas. Salt-tolerant archaea (the Haloarchaea) use sunlight as an energy source, and other species of archaea fix carbon; however, unlike plants and cyanobacteria, no known species of archaea does both. Archaea reproduce asexually by binary fission, fragmentation, or budding; unlike bacteria and eukaryotes, no known species forms spores.

Archaea are particularly numerous in the oceans, and the archaea in plankton may be one of the most abundant groups of organisms on the planet. Archaea are a major part of Earth's life and may play roles in both the carbon cycle and the nitrogen cycle.

File:Halobacteria with scale.jpg|Halobacteria, found in water near saturated with salt, are now recognized as archaea.

File:Methanosarcina barkeri fusaro.gif|Methanosarcina barkeri, a marine archaea that produces methane

File:Thermophile bacteria2.jpg|Thermophiles, such as Pyrolobus fumarii, survive well over 100 °C.

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{{main|Marine protists}}

Protists are eukaryotes that cannot be classified as plants, fungi or animals. They are usually single-celled and microscopic. Life originated as single-celled prokaryotes (bacteria and archaea) and later evolved into more complex eukaryotes. Eukaryotes are the more developed life forms known as plants, animals, fungi and protists. The term protist came into use historically as a term of convenience for eukaryotes that cannot be strictly classified as plants, animals or fungi. They are not a part of modern cladistics, because they are paraphyletic (lacking a common ancestor). Protists can be broadly divided into four groups depending on whether their nutrition is plant-like, animal-like, fungus-like,{{cite journal | vauthors = Whittaker RH, Margulis L | title = Protist classification and the kingdoms of organisms | journal = Bio Systems | volume = 10 | issue = 1–2 | pages = 3–18 | date = April 1978 | pmid = 418827 | doi = 10.1016/0303-2647(78)90023-0 | bibcode = 1978BiSys..10....3W }} or a mixture of these.{{cite journal | vauthors = Faure E, Not F, Benoiston AS, Labadie K, Bittner L, Ayata SD | title = Mixotrophic protists display contrasted biogeographies in the global ocean | journal = The ISME Journal | volume = 13 | issue = 4 | pages = 1072–1083 | date = April 2019 | pmid = 30643201 | pmc = 6461780 | doi = 10.1038/s41396-018-0340-5 | bibcode = 2019ISMEJ..13.1072F }}

class="wikitable" ! colspan=8 |{{centre|Protists according to how they get food}}
colspan=2 |Type of protist ! Description ! colspan=2 | Example ! Other examples
width=90px | Plant-like | width=90px | {{center|Algae (see below)}} | Autotrophic protists that make their own food without needing to consume other organisms, usually by using photosynthesis | 100px | Red algae, Cyanidium sp. | Green algae, brown algae, diatoms and some dinoflagellates. Plant-like protists are important components of phytoplankton discussed below.
Animal-like | {{center|Protozoans}} | Heterotrophic protists that get their food consuming other organisms | 100px | Radiolarian protist as drawn by Haeckel | Foraminiferans, and some marine amoebae, ciliates and flagellates.
Fungus-like | {{center|Slime moulds and slime nets}} | Saprotrophic protists that get their food from the remains of organisms that have broken down and decayed | 100px | Marine slime nets form labyrinthine networks of tubes in which amoeba without pseudopods can travel | Marine lichen
Mixotropes | {{center|Various}} | Mixotrophic and osmotrophic protists that get their food from a combination of the above | 100px | Euglena mutabilis, a photosynthetic flagellate | Many marine mixotropes are found among protists, including among ciliates, Rhizaria and dinoflagellates

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16px [https://www.youtube.com/watch?v=9rtUEPuzhbE&ab_channel=JourneytotheMicrocosmos Getting to know our single-celled ancestors] - MicroCosmos

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Protists are highly diverse organisms currently organized into 18 phyla, but are not easy to classify.{{cite journal | vauthors = Cavalier-Smith T | title = Kingdom protozoa and its 18 phyla | journal = Microbiological Reviews | volume = 57 | issue = 4 | pages = 953–94 | date = December 1993 | pmid = 8302218 | pmc = 372943 | doi = 10.1128/MMBR.57.4.953-994.1993 }}{{cite journal | vauthors = Corliss JO | title = Should there be a separate code of nomenclature for the protists? | journal = Bio Systems | volume = 28 | issue = 1–3 | pages = 1–14 | year = 1992 | pmid = 1292654 | doi = 10.1016/0303-2647(92)90003-H | bibcode = 1992BiSys..28....1C }} Studies have shown high protist diversity exists in oceans, deep sea-vents and river sediments, suggesting a large number of eukaryotic microbial communities have yet to be discovered.{{cite journal | vauthors = Slapeta J, Moreira D, López-García P | title = The extent of protist diversity: insights from molecular ecology of freshwater eukaryotes | journal = Proceedings. Biological Sciences | volume = 272 | issue = 1576 | pages = 2073–81 | date = October 2005 | pmid = 16191619 | pmc = 1559898 | doi = 10.1098/rspb.2005.3195 }}{{cite journal | vauthors = Moreira D, López-García P | title = The molecular ecology of microbial eukaryotes unveils a hidden world | journal = Trends in Microbiology | volume = 10 | issue = 1 | pages = 31–8 | date = January 2002 | pmid = 11755083 | doi = 10.1016/S0966-842X(01)02257-0 }} There has been little research on mixotrophic protists, but recent studies in marine environments found mixotrophic protests contribute a significant part of the protist biomass.{{cite journal | vauthors = Leles SG, Mitra A, Flynn KJ, Stoecker DK, Hansen PJ, Calbet A, McManus GB, Sanders RW, Caron DA, Not F, Hallegraeff GM, Pitta P, Raven JA, Johnson MD, Glibert PM, Våge S | display-authors = 6 | title = Oceanic protists with different forms of acquired phototrophy display contrasting biogeographies and abundance | journal = Proceedings. Biological Sciences | volume = 284 | issue = 1860 | page = 20170664 | date = August 2017 | pmid = 28768886 | pmc = 5563798 | doi = 10.1098/rspb.2017.0664 }}

File:Diatoms through the microscope.jpg|Diatoms are a major algae group generating about 20% of world oxygen production.{{cite web | vauthors = Alverson A | date = 11 June 2014 | work = Live Science | url = https://www.livescience.com/46250-teasing-apart-the-diatom-genome.html | title = The Air You're Breathing? A Diatom Made That }}

File:Diatom algae Amphora sp.jpg|Diatoms have glass like cell walls made of silica and called frustules.{{cite web | title=More on Diatoms | website=University of California Museum of Paleontology | url=http://www.ucmp.berkeley.edu/chromista/diatoms/diatommm.html | access-date=27 June 2019 | archive-url=https://web.archive.org/web/20121004130024/http://www.ucmp.berkeley.edu/chromista/diatoms/diatommm.html | archive-date=4 October 2012 | url-status=dead }}

File:Podocyrtis papalis Ehrenberg - Radiolarian (30448963206).jpg|Radiolarian

File:Gephyrocapsa oceanica color (lightened).jpg|Single-celled alga, Gephyrocapsa oceanica

File:CSIRO ScienceImage 7609 SEM dinoflagellate.jpg|Two dinoflagellates

File:Zooxanthellae.jpg|Zooxanthellae is a photosynthetic algae that lives inside hosts like coral.

File:Paramecium bursaria.jpg|A single-celled ciliate with green zoochlorellae living inside endosymbiotically.

File:Euglenoid movement.jpg|Euglenoid

File:The ciliate Frontonia sp.jpg|This ciliate is digesting cyanobacteria. The cytostome or mouth is at the bottom right.

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In contrast to the cells of prokaryotes, the cells of eukaryotes are highly organized. Plants, animals and fungi are usually multi-celled and are typically macroscopic. Most protists are single-celled and microscopic. But there are exceptions. Some single-celled marine protists are macroscopic. Some marine slime molds have unique life cycles that involve switching between unicellular, colonial, and multicellular forms.{{cite journal | vauthors = Devreotes P | title = Dictyostelium discoideum: a model system for cell-cell interactions in development | journal = Science | volume = 245 | issue = 4922 | pages = 1054–8 | date = September 1989 | pmid = 2672337 | doi = 10.1126/science.2672337 | bibcode = 1989Sci...245.1054D }} Other marine protist are neither single-celled nor microscopic, such as seaweed.

{{anchor|Macroscopic protists}}

File:Chaos carolinensis Wilson 1900.jpg|The single-celled giant amoeba has up to 1000 nuclei and reaches lengths of {{cvt|5|mm|in}}.

File:Xenophyophore.jpg|The xenophyophore, another single-celled foraminiferan, lives in abyssal zones. It has a giant shell up to {{cvt|20|cm|in}} across.{{Cite journal| vauthors = Gooday AJ, Da Silva AA, Pawlowski J |date=2011-12-01|title=Xenophyophores (Rhizaria, Foraminifera) from the Nazaré Canyon (Portuguese margin, NE Atlantic)|journal=Deep-Sea Research Part II: Topical Studies in Oceanography|series=The Geology, Geochemistry, and Biology of Submarine Canyons West of Portugal|volume=58|issue=23–24|pages=2401–2419|doi=10.1016/j.dsr2.2011.04.005|bibcode=2011DSRII..58.2401G}}

File:Giant Kelp.jpg|Giant kelp, a brown algae, is not a true plant, yet it is multicellular and can grow to 50m.

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Protists have been described as a taxonomic grab bag where anything that doesn't fit into one of the main biological kingdoms can be placed.{{cite book | vauthors = Neil AC, Reece JB, Simon EJ | date = 2004 | url = https://books.google.com/books?id=lRhFAQAAIAAJ&q=protists+%22taxonomic+grab+bag%22 | title = Essential biology with physiology | publisher = Pearson/Benjamin Cummings | page = 291 | isbn = 978-0-8053-7503-9 }} Some modern authors prefer to exclude multicellular organisms from the traditional definition of a protist, restricting protists to unicellular organisms.{{Cite journal | doi = 10.1007/s10539-012-9354-y| title = The other eukaryotes in light of evolutionary protistology| journal = Biology & Philosophy| volume = 28| issue = 2| pages = 299–330| year = 2012| vauthors = O'Malley MA, Simpson AG, Roger AJ | s2cid = 85406712}}{{cite journal | vauthors = Adl SM, Simpson AG, Farmer MA, Andersen RA, Anderson OR, Barta JR, Bowser SS, Brugerolle G, Fensome RA, Fredericq S, James TY, Karpov S, Kugrens P, Krug J, Lane CE, Lewis LA, Lodge J, Lynn DH, Mann DG, McCourt RM, Mendoza L, Moestrup O, Mozley-Standridge SE, Nerad TA, Shearer CA, Smirnov AV, Spiegel FW, Taylor MF | display-authors = 6 | title = The new higher level classification of eukaryotes with emphasis on the taxonomy of protists | journal = The Journal of Eukaryotic Microbiology | volume = 52 | issue = 5 | pages = 399–451 | year = 2005 | pmid = 16248873 | doi = 10.1111/j.1550-7408.2005.00053.x | s2cid = 8060916 | doi-access = free }} This more constrained definition excludes seaweeds and slime molds.{{Cite book | url = https://books.google.com/books?id=9IWaqAOGyt4C | title = Kingdoms and Domains: An Illustrated Guide to the Phyla of Life on Earth | vauthors = Margulis L, Chapman MJ | date = 2009-03-19 | publisher = Academic Press | isbn = 9780080920146 }}

{{See also|Microanimal|Ichthyoplankton}}

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As juveniles, animals develop from microscopic stages, which can include spores, eggs and larvae. At least one microscopic animal group, the parasitic cnidarian Myxozoa, is unicellular in its adult form, and includes marine species. Other adult marine microanimals are multicellular. Microscopic adult arthropods are more commonly found inland in freshwater, but there are marine species as well. Microscopic adult marine crustaceans include some copepods, cladocera and tardigrades (water bears). Some marine nematodes and rotifers are also too small to be recognized with the naked eye, as are many loricifera, including the recently discovered anaerobic species that spend their lives in an anoxic environment.{{cite journal | vauthors = Fang J | title = Animals thrive without oxygen at sea bottom | journal = Nature | volume = 464 | issue = 7290 | pages = 825 | date = April 2010 | pmid = 20376121 | doi = 10.1038/464825b | bibcode = 2010Natur.464..825F | doi-access = free }}{{cite web |url=http://www.sciencenews.org/view/generic/id/58154/description/Multicelled_animals_may_live_oxygen-free |title=Briny deep basin may be home to animals thriving without oxygen |work=Science News |date=2013-09-23}} Copepods contribute more to the secondary productivity and carbon sink of the world oceans than any other group of organisms.{{Cite journal |last1=Jónasdóttir |first1=Sigrún Huld |last2=Visser |first2=André W. |last3=Richardson |first3=Katherine |last4=Heath |first4=Michael R. |date=2015-09-29 |title=Seasonal copepod lipid pump promotes carbon sequestration in the deep North Atlantic |journal=Proceedings of the National Academy of Sciences |language=en |volume=112 |issue=39 |pages=12122–12126 |doi=10.1073/pnas.1512110112 |issn=0027-8424 |pmc=4593097 |pmid=26338976 |doi-access=free }}{{Cite journal |last1=Pinti |first1=Jérôme |last2=Jónasdóttir |first2=Sigrún H. |last3=Record |first3=Nicholas R. |last4=Visser |first4=André W. |date=2023-03-07 |title=The global contribution of seasonally migrating copepods to the biological carbon pump |journal=Limnology and Oceanography |language=en |volume=68 |issue=5 |pages=1147–1160 |doi=10.1002/lno.12335 |bibcode=2023LimOc..68.1147P |s2cid=257422956 |issn=0024-3590|doi-access=free }} While mites are not normally thought of as marine organisms, most species of the family Halacaridae live in the sea.{{Cite journal |last1=Pepato |first1=Almir R. |last2=Vidigal |first2=Teofânia H.D.A. |last3=Klimov |first3=Pavel B. |date=2018 |title=Molecular phylogeny of marine mites (Acariformes: Halacaridae), the oldest radiation of extant secondarily marine animals |journal=Molecular Phylogenetics and Evolution |language=en |volume=129 |pages=182–188 |doi=10.1016/j.ympev.2018.08.012 |pmid=30172010 |s2cid=52145427 |doi-access=free |bibcode=2018MolPE.129..182P }}

File:Copepod 2.jpg|Over 10,000 marine species are copepods, small, often microscopic crustaceans

File:Gastrotrich.jpg|Darkfield photo of a gastrotrich, a worm-like animal living between sediment particles

File:Echiniscus testudo Doyere 1840 Pl 12 Fig 1.png|Drawing of a tardigrade (water bear) on a grain of sand

File:Squatinella sp. (Rädertierchen - Rotifera) - 160x (13402418244).jpg|Rotifers, usually 0.1–0.5 mm long, may look like protists but have many cells and belongs to the Animalia.

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File:Lichen rock.jpeg on a rock in a marine splash zone. Lichens are mutualistic associations between a fungus and an alga or cyanobacterium.]]

{{See also|Marine fungi|Mycoplankton|Evolution of fungi}}

Over 1500 species of fungi are known from marine environments.{{cite journal |vauthors=Hyde KD, Jones EG, Leaño E, Pointing SB, Poonyth AD, Vrijmoed LL |title=Role of fungi in marine ecosystems |journal=Biodiversity and Conservation |year=1998 |volume=7 |issue=9 |pages=1147–1161 |doi=10.1023/A:1008823515157 |bibcode=1998BiCon...7.1147H |s2cid=22264931 }} These are parasitic on marine algae or animals, or are saprobes feeding on dead organic matter from algae, corals, protozoan cysts, sea grasses, wood and other substrata.{{cite book | vauthors = Kirk PM, Cannon PF, Minter DW, Stalpers J | title = Dictionary of the Fungi | edition = 10 | publisher = CABI | date = 2008 }} Spores of many species have special appendages which facilitate attachment to the substratum.{{cite journal |vauthors=Hyde KD, Greenwood R, Jones EG |title=Spore attachment in marine fungi |journal=Botanica Marina |year=1989 |volume=32 |issue=3 |pages=205–218 |doi=10.1515/botm.1989.32.3.205 |s2cid=84879817}} Marine fungi can also be found in sea foam and around hydrothermal areas of the ocean. A diverse range of unusual secondary metabolites is produced by marine fungi.{{cite journal |vauthors=San-Martin A, Orejarena S, Gallardo C, Silva M, Becerra J, Reinoso RO, Chamy MC, Vergara K, Rovirosa J |title=Steroids from the marine fungus Geotrichum sp |journal=Journal of the Chilean Chemical Society |year=2008 |volume=53 |issue=1 |pages=1377–1378 |doi=10.4067/S0717-97072008000100011 |doi-access=free }}

Mycoplankton are saprotropic members of the plankton communities of marine and freshwater ecosystems.{{cite book | veditors = Jones EB, Hyde KD, Pang KL | date = 2014 | url = https://books.google.com/books?id=mXfnBQAAQBAJ | title = Freshwater fungi: and fungal-like organisms | location = Berlin/Boston | publisher = De Gruyter | isbn = 9783110333480 }}{{cite book |year=2012 | veditors = Jones EB, Pang KL | title=Marine Fungi, and Fungal-like Organisms |url=http://www.degruyter.com/view/product/177990 |series=Marine and Freshwater Botany |location=Berlin, Boston |publisher=De Gruyter |publication-date=August 2012 |isbn=978-3-11-026406-7 |access-date=3 September 2015 |ref=jones2012 |doi=10.1515/9783110264067 }} They are composed of filamentous free-living fungi and yeasts associated with planktonic particles or phytoplankton.{{cite journal | vauthors = Wang X, Singh P, Gao Z, Zhang X, Johnson ZI, Wang G | title = Distribution and diversity of planktonic fungi in the West Pacific Warm Pool | journal = PLOS ONE | volume = 9 | issue = 7 | pages = e101523 | year = 2014 | pmid = 24992154 | pmc = 4081592 | doi = 10.1371/journal.pone.0101523.s001 | bibcode = 2014PLoSO...9j1523W | doi-access = free }} Similar to bacterioplankton, these aquatic fungi play a significant role in heterotrophic mineralization and nutrient cycling.{{cite book | vauthors = Wang G, Wang X, Liu X, Li Q | chapter = Diversity and biogeochemical function of planktonic fungi in the ocean| veditors = Raghukumar C |title=Biology of marine fungi | series = Progress in Molecular and Subcellular Biology|location=Berlin, Heidelberg |publisher=Springer-Verlag |year=2012 | volume = 53|pages=71–88 |isbn=978-3-642-23341-8 |chapter-url=https://www.springer.com/us/book/9783642233418 |access-date=3 September 2015 |doi=10.1007/978-3-642-23342-5 |s2cid=39378040 }} Mycoplankton can be up to 20 mm in diameter and over 50 mm in length.{{cite journal | vauthors = Damare S, Raghukumar C | title = Fungi and macroaggregation in deep-sea sediments | journal = Microbial Ecology | volume = 56 | issue = 1 | pages = 168–77 | date = July 2008 | pmid = 17994287 | doi = 10.1007/s00248-007-9334-y | bibcode = 2008MicEc..56..168D | s2cid = 21288251 }}

A typical milliliter of seawater contains about 10³ to 10⁴ fungal cells.{{cite journal | vauthors = Kubanek J, Jensen PR, Keifer PA, Sullards MC, Collins DO, Fenical W | title = Seaweed resistance to microbial attack: a targeted chemical defense against marine fungi | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 100 | issue = 12 | pages = 6916–21 | date = June 2003 | pmid = 12756301 | pmc = 165804 | doi = 10.1073/pnas.1131855100 | bibcode = 2003PNAS..100.6916K | doi-access = free }} This number is greater in coastal ecosystems and estuaries due to nutritional runoff from terrestrial communities. A higher diversity of mycoplankton is found around coasts and in surface waters down to 1000 meters, with a vertical profile that depends on how abundant phytoplankton is.{{cite journal | vauthors = Gao Z, Johnson ZI, Wang G | title = Molecular characterization of the spatial diversity and novel lineages of mycoplankton in Hawaiian coastal waters | journal = The ISME Journal | volume = 4 | issue = 1 | pages = 111–20 | date = January 2010 | pmid = 19641535 | doi = 10.1038/ismej.2009.87 | doi-access = free | bibcode = 2010ISMEJ...4..111G }}{{cite journal | vauthors = Panzer K, Yilmaz P, Weiß M, Reich L, Richter M, Wiese J, Schmaljohann R, Labes A, Imhoff JF, Glöckner FO, Reich M | display-authors = 6 | title = Identification of Habitat-Specific Biomes of Aquatic Fungal Communities Using a Comprehensive Nearly Full-Length 18S rRNA Dataset Enriched with Contextual Data | journal = PLOS ONE | volume = 10 | issue = 7 | pages = e0134377 | date = 2015-07-30 | pmid = 26226014 | pmc = 4520555 | doi = 10.1371/journal.pone.0134377 | bibcode = 2015PLoSO..1034377P | doi-access = free }} This profile changes between seasons due to changes in nutrient availability.{{cite journal | vauthors = Gutierrez MH, Pantoja S, Quinones RA, Gonzalez RR | trans-title = First record of filamentous fungi in the coastal upwelling ecosystem off central Chile | title = Primer registro de hongos filamentosos en el ecosistema de surgencia costero frente a Chile central | language = Spanish | journal = Gayana |year=2010 | volume = 74 | issue = 1 | pages = 66–73 }} Marine fungi survive in a constant oxygen deficient environment, and therefore depend on oxygen diffusion by turbulence and oxygen generated by photosynthetic organisms.{{Cite book |title = Plankton Dynamics of Indian Waters | vauthors = Sridhar KR |publisher = Pratiksha Publications |year = 2009 |location = Jaipur, India |pages = 133–148 |chapter = 10. Aquatic fungi – Are they planktonic?}}

Marine fungi can be classified as:

• Lower fungi - adapted to marine habitats (zoosporic fungi, including mastigomycetes: oomycetes and chytridiomycetes)
• Higher fungi - filamentous, modified to planktonic lifestyle (hyphomycetes, ascomycetes, basidiomycetes). Most mycoplankton species are higher fungi.

Lichens are mutualistic associations between a fungus, usually an ascomycete, and an alga or a cyanobacterium. Several lichens are found in marine environments.{{cite web | url = http://ocean.otr.usm.edu/~w529014/index_files/Page2025.htm | title = Species of Higher Marine Fungi | archive-url = https://web.archive.org/web/20130422084649/http://ocean.otr.usm.edu/~w529014/index_files/Page2025.htm | archive-date=22 April 2013 | publisher = University of Mississippi | access-date = 5 February 2012 }} Many more occur in the splash zone, where they occupy different vertical zones depending on how tolerant they are to submersion.{{cite journal | vauthors = Hawksworth DL | title = Freshwater and marine lichen-forming fungi. | journal = Fungal Diversity | date = 2000 | volume = 5 | pages = 1–7 | url = http://www.fungaldiversity.org/fdp/sfdp/FD_5_1-7.pdf }} Some lichens live a long time; one species has been dated at 8,600 years.{{cite web |url=https://www.nps.gov/glac/learn/nature/lichens.htm |title=Lichens |publisher=National Park Service, US Department of the Interior, Government of the United States |date=22 May 2016 |access-date=4 April 2018 }} However their lifespan is difficult to measure because what defines the same lichen is not precise.{{cite web |url=http://www.earthlife.net/lichens/growth.html |title=The Earth Life Web, Growth and Development in Lichens |date=14 February 2020 |publisher=earthlife.net }} Lichens grow by vegetatively breaking off a piece, which may or may not be defined as the same lichen, and two lichens of different ages can merge, raising the issue of whether it is the same lichen.

The sea snail Littoraria irrorata damages plants of Spartina in the sea marshes where it lives, which enables spores of intertidal ascomycetous fungi to colonize the plant. The snail then eats the fungal growth in preference to the grass itself.{{cite journal | vauthors = Silliman BR, Newell SY | title = Fungal farming in a snail | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 100 | issue = 26 | pages = 15643–8 | date = December 2003 | pmid = 14657360 | pmc = 307621 | doi = 10.1073/pnas.2535227100 | bibcode = 2003PNAS..10015643S | doi-access = free }}

According to fossil records, fungi date back to the late Proterozoic era 900–570 million years ago. Fossil marine lichens 600 million years old have been discovered in China.{{cite journal | vauthors = Yuan X, Xiao S, Taylor TN | title = Lichen-like symbiosis 600 million years ago | journal = Science | volume = 308 | issue = 5724 | pages = 1017–20 | date = May 2005 | pmid = 15890881 | doi = 10.1126/science.1111347 | s2cid = 27083645 | bibcode = 2005Sci...308.1017Y }} It has been hypothesized that mycoplankton evolved from terrestrial fungi, likely in the Paleozoic era (390 million years ago).{{Cite book|title = Marine Fungi: and Fungal-like Organisms|url = https://books.google.com/books?id=RcF97cHppPsC|publisher = Walter de Gruyter|date = 2012-08-31|isbn = 9783110264067| vauthors = Jones EB, Pang KL }}

File:DickinsoniaCostata.jpg may be the earliest animal. They appear in the fossil record 571 million to 541 million years ago.]]

{{Further|Marine invertebrates|Origin of eukaryotes|Evolutionary origin of animals|Avalon explosion|Cambrian explosion}}

The earliest animals were marine invertebrates, that is, vertebrates came later. Animals are multicellular eukaryotes, and are distinguished from plants, algae, and fungi by lacking cell walls.{{cite web |url=http://micro.magnet.fsu.edu/cells/animalcell.html |title=Animal Cell Structure | vauthors = Davidson MW |date=26 May 2005 |website=Molecular Expressions |publisher=Florida State University |location=Tallahassee, Fla. |access-date=2008-09-03}} Marine invertebrates are animals that inhabit a marine environment apart from the vertebrate members of the chordate phylum; invertebrates lack a vertebral column. Some have evolved a shell or a hard exoskeleton.

The earliest animal fossils may belong to the genus Dickinsonia,{{cite web | vauthors = Vogel G |title=This fossil is one of the world's earliest animals, according to fat molecules preserved for a half-billion years |url=https://www.science.org/content/article/fossil-one-world-s-earliest-animals-according-fat-molecules-preserved-half-billion |website=Science |publisher=AAAS |access-date=21 September 2018|date=2018-09-20 }} 571 million to 541 million years ago.{{cite journal | vauthors = Bobrovskiy I, Hope JM, Ivantsov A, Nettersheim BJ, Hallmann C, Brocks JJ | title = Ancient steroids establish the Ediacaran fossil Dickinsonia as one of the earliest animals | journal = Science | volume = 361 | issue = 6408 | pages = 1246–1249 | date = September 2018 | pmid = 30237355 | doi = 10.1126/science.aat7228 | doi-access = free | bibcode = 2018Sci...361.1246B | hdl = 1885/230014 | hdl-access = free }} Individual Dickinsonia typically resemble a bilaterally symmetrical ribbed oval. They kept growing until they were covered with sediment or otherwise killed,{{cite journal | vauthors = Retallack GJ | year = 2007 | title = Growth, decay and burial compaction of Dickinsonia, an iconic Ediacaran fossil | journal = Alcheringa: An Australasian Journal of Palaeontology | volume = 31 | issue = 3 | pages = 215–240 | url = http://www.informaworld.com/index/781217204.pdf | doi = 10.1080/03115510701484705| bibcode = 2007Alch...31..215R | s2cid = 17181699 }} and spent most of their lives with their bodies firmly anchored to the sediment.{{cite journal | vauthors = Sperling EA, Vinther J | title = A placozoan affinity for Dickinsonia and the evolution of late Proterozoic metazoan feeding modes | journal = Evolution & Development | volume = 12 | issue = 2 | pages = 201–9 | date = 2010 | pmid = 20433459 | doi = 10.1111/j.1525-142X.2010.00404.x | s2cid = 38559058 }} Their taxonomic affinities are presently unknown, but their mode of growth is consistent with a bilaterian affinity.{{cite journal | vauthors = Gold DA, Runnegar B, Gehling JG, Jacobs DK | title = Ancestral state reconstruction of ontogeny supports a bilaterian affinity for Dickinsonia | journal = Evolution & Development | volume = 17 | issue = 6 | pages = 315–24 | year = 2015 | pmid = 26492825 | doi = 10.1111/ede.12168 | bibcode = 2015EvDev..17..315G | s2cid = 26099557 }}

Apart from Dickinsonia, the earliest widely accepted animal fossils are the rather modern-looking cnidarians (the group that includes coral, jellyfish, sea anemones and Hydra), possibly from around {{ma|580|Ma}}{{cite journal | vauthors = Chen JY, Oliveri P, Gao F, Dornbos SQ, Li CW, Bottjer DJ, Davidson EH | title = Precambrian animal life: probable developmental and adult cnidarian forms from Southwest China | journal = Developmental Biology | volume = 248 | issue = 1 | pages = 182–96 | date = August 2002 | pmid = 12142030 | doi = 10.1006/dbio.2002.0714 | url = https://pantherfile.uwm.edu/sdornbos/www/PDF%27s/Chen%20et%20al.%202002.pdf | archive-url = https://web.archive.org/web/20130526203745/https://pantherfile.uwm.edu/sdornbos/www/PDF%27s/Chen%20et%20al.%202002.pdf | url-status = dead | df = dmy-all | author-link7 = Eric H. Davidson | archive-date = 26 May 2013 | first6 = David J. | first7 = Eric H. | access-date = 2015-02-04 }} The Ediacara biota, which flourished for the last 40 million years before the start of the Cambrian,{{cite journal | vauthors = Grazhdankin D |date=June 2004 |title=Patterns of distribution in the Ediacaran biotas: facies versus biogeography and evolution |journal=Paleobiology |volume=30 |issue=2 |pages=203–221 |doi=10.1666/0094-8373(2004)030<0203:PODITE>2.0.CO;2 |bibcode=2004Pbio...30..203G |s2cid=129376371 |issn=0094-8373 }} were the first animals more than a very few centimeters long. Like Dickinsonia, many were flat with a "quilted" appearance, and seemed so strange that there was a proposal to classify them as a separate kingdom, Vendozoa.{{cite journal | vauthors = Seilacher A |author-link=Adolf Seilacher |date=August 1992 |title=Vendobionta and Psammocorallia: lost constructions of Precambrian evolution |url=http://jgs.lyellcollection.org/content/149/4/607.abstract |journal=Journal of the Geological Society |volume=149 |issue=4 |pages=607–613 |doi=10.1144/gsjgs.149.4.0607 |issn=0016-7649 |access-date=2015-02-04 |bibcode=1992JGSoc.149..607S |s2cid=128681462 }} Others, however, have been interpreted as early molluscs (Kimberella{{cite journal | vauthors = Martin MW, Grazhdankin DV, Bowring SA, Evans DA, Fedonkin MA, Kirschvink JL | title = Age of Neoproterozoic bilatarian body and trace fossils, White Sea, Russia: implications for metazoan evolution | journal = Science | volume = 288 | issue = 5467 | pages = 841–5 | date = May 2000 | pmid = 10797002 | doi = 10.1126/science.288.5467.841 | s2cid = 1019572 | bibcode = 2000Sci...288..841M }}{{cite journal | vauthors = Fedonkin MA, Waggoner BM |date=28 August 1997 |title=The late Precambrian fossil Kimberella is a mollusc-like bilaterian organism |journal=Nature |volume=388 |issue=6645 |pages=868–871 |bibcode=1997Natur.388..868F |doi=10.1038/42242 |s2cid=4395089 |issn=0028-0836 |doi-access=free }}), echinoderms (Arkarua{{cite journal | vauthors = Mooi R, David B | date=December 1998 |title=Evolution Within a Bizarre Phylum: Homologies of the First Echinoderms |journal=American Zoologist |volume=38 |issue=6 |pages=965–974 |doi=10.1093/icb/38.6.965 |issn=1540-7063 |doi-access=free }}), and arthropods (Spriggina,{{cite conference |url= https://gsa.confex.com/gsa/2003AM/finalprogram/abstract_62056.htm |title=Spriggina is a trilobitoid ecdysozoan | vauthors = McMenamin MA |author-link=Mark McMenamin |date=September 2003 |conference=Geoscience Horizons Seattle 2003 |conference-url=https://www.geosociety.org/meetings/2003/ |volume=35 |issue=6 |series=Abstracts with Programs |publisher=Geological Society of America |location=Boulder, Colo. |page=105 |oclc=249088612 |access-date=2007-11-24 |archive-url=https://web.archive.org/web/20160412064305/https://gsa.confex.com/gsa/2003AM/finalprogram/abstract_62056.htm |archive-date=12 April 2016 |url-status=dead }} Paper No. 40-2 presented at the Geological Society of America's 2003 Seattle Annual Meeting (2–5 November 2003) on 2 November 2003, at the Washington State Convention Center. Parvancorina{{cite journal | vauthors = Lin JP, Gon III SM, Gehling JG, Babcock LE, Zhao YL, Zhang XL, Hu SX, Yuan JL, Yu MY, Peng J |display-authors=6 |year=2006 |title=A Parvancorina-like arthropod from the Cambrian of South China |journal=Historical Biology: An International Journal of Paleobiology |volume=18 |issue=1 |pages=33–45 |doi=10.1080/08912960500508689 |bibcode=2006HBio...18...33L |s2cid=85821717 |issn=1029-2381}}). There is still debate about the classification of these specimens, mainly because the diagnostic features which allow taxonomists to classify more recent organisms, such as similarities to living organisms, are generally absent in the Ediacarans. However, there seems little doubt that Kimberella was at least a triploblastic bilaterian animal, in other words, an animal significantly more complex than the cnidarians.{{cite journal | vauthors = Butterfield NJ | title = Hooking some stem-group "worms": fossil lophotrochozoans in the Burgess Shale | journal = BioEssays | volume = 28 | issue = 12 | pages = 1161–6 | date = December 2006 | pmid = 17120226 | doi = 10.1002/bies.20507 | s2cid = 29130876 }}

Small shelly fauna are a very mixed collection of fossils found between the Late Ediacaran and Middle Cambrian periods. The earliest, Cloudina, shows signs of successful defense against predation and may indicate the start of an evolutionary arms race. Some tiny Early Cambrian shells almost certainly belonged to molluscs, while the owners of some "armor plates," Halkieria and Microdictyon, were eventually identified when more complete specimens were found in Cambrian lagerstätten that preserved soft-bodied animals.{{cite journal | vauthors = Bengtson S | title = Early skeletal fossils | journal = Paleontological Society Papers | date = November 2004 | volume = 10 | pages = 67–78 | doi = 10.1017/S1089332600002345 }}

File:Kimberella NT.jpg, an early mollusc important for understanding the Cambrian explosion. Invertebrates are grouped into different phyla (body plans).]]

Invertebrates are grouped into different phyla. Informally phyla can be thought of as a way of grouping organisms according to their body plan.{{cite book | vauthors = Valentine JW | year = 2004 | title = On the Origin of Phyla | publisher = University Of Chicago Press | location = Chicago | isbn = 978-0-226-84548-7 | page = 7 | quote = Classifications of organisms in hierarchical systems were in use by the seventeenth and eighteenth centuries. Usually organisms were grouped according to their morphological similarities as perceived by those early workers, and those groups were then grouped according to their similarities, and so on, to form a hierarchy. }}{{Cite book|url=https://books.google.com/books?id=DMBkmHm5fe4C |title=On the Origin of Phyla |isbn=9780226845487 | vauthors = Valentine JW |date=2004-06-18|publisher=University of Chicago Press }}{{rp|33}} A body plan refers to a blueprint which describes the shape or morphology of an organism, such as its symmetry, segmentation and the disposition of its appendages. The idea of body plans originated with vertebrates, which were grouped into one phylum. But the vertebrate body plan is only one of many, and invertebrates consist of many phyla or body plans. The history of the discovery of body plans can be seen as a movement from a worldview centered on vertebrates, to seeing the vertebrates as one body plan among many. Among the pioneering zoologists, Linnaeus identified two body plans outside the vertebrates; Cuvier identified three; and Haeckel had four, as well as the Protista with eight more, for a total of twelve. For comparison, the number of phyla recognized by modern zoologists has risen to 35.

File:Marine animal biodiversity.png, 18 October 2019.{{Cite web|url=http://www.marinespecies.org/|title=WoRMS - World Register of Marine Species|website=www.marinespecies.org}}{{cite journal | vauthors = Novak BJ, Fraser D, Maloney TH | title = Transforming Ocean Conservation: Applying the Genetic Rescue Toolkit | journal = Genes | volume = 11 | issue = 2 | page = 209 | date = February 2020 | pmid = 32085502 | pmc = 7074136 | doi = 10.3390/genes11020209 | doi-access = free }}]]Historically body plans were thought of as having evolved rapidly during the Cambrian explosion,{{cite journal| vauthors = Erwin D, Valentine J, Jablonski D |title=Recent fossil finds and new insights into animal development are providing fresh perspectives on the riddle of the explosion of animals during the Early Cambrian |journal=American Scientist |date=1997 |issue=March–April |url=http://www.americanscientist.org/issues/num2/the-origin-of-animal-body-plans/3}} but a more nuanced understanding of animal evolution suggests a gradual development of body plans throughout the early Palaeozoic and beyond. More generally a phylum can be defined in two ways: as described above, as a group of organisms with a certain degree of morphological or developmental similarity (the phenetic definition), or a group of organisms with a certain degree of evolutionary relatedness (the phylogenetic definition).{{cite journal | vauthors = Budd GE, Jensen S | title = A critical reappraisal of the fossil record of the bilaterian phyla | journal = Biological Reviews of the Cambridge Philosophical Society | volume = 75 | issue = 2 | pages = 253–95 | date = May 2000 | pmid = 10881389 | doi = 10.1111/j.1469-185X.1999.tb00046.x | s2cid = 39772232 }}

In the 1970s there was already a debate about whether the emergence of the modern phyla was "explosive" or gradual but hidden by the shortage of Precambrian animal fossils. A re-analysis of fossils from the Burgess Shale lagerstätte increased interest in the issue when it revealed animals, such as Opabinia, which did not fit into any known phylum. At the time these were interpreted as evidence that the modern phyla had evolved very rapidly in the Cambrian explosion and that the Burgess Shale's "weird wonders" showed that the Early Cambrian was a uniquely experimental period of animal evolution.{{cite book | vauthors = Gould SJ |title=Wonderful life: the Burgess Shale and the nature of history |date=1989 |location=New York|publisher=W.W. Norton |isbn=978-0-393-02705-1 |edition=First}} Later discoveries of similar animals and the development of new theoretical approaches led to the conclusion that many of the "weird wonders" were evolutionary "aunts" or "cousins" of modern groups{{cite journal | vauthors = Budd GE | title = The Cambrian fossil record and the origin of the phyla | journal = Integrative and Comparative Biology | volume = 43 | issue = 1 | pages = 157–65 | date = February 2003 | pmid = 21680420 | doi = 10.1093/icb/43.1.157 | doi-access = free | author-link = Graham Budd }}—for example that Opabinia was a member of the lobopods, a group which includes the ancestors of the arthropods, and that it may have been closely related to the modern tardigrades.{{cite journal | vauthors = Budd GE | date=March 1996 |title=The morphology of Opabinia regalis and the reconstruction of the arthropod stem-group |journal=Lethaia |volume=29 |issue=1 |pages=1–14 |doi=10.1111/j.1502-3931.1996.tb01831.x | bibcode=1996Letha..29....1B |issn=0024-1164 }} Nevertheless, there is still much debate about whether the Cambrian explosion was really explosive and, if so, how and why it happened and why it appears unique in the history of animals.{{cite journal | vauthors = Marshall CR | date=May 2006 |title=Explaining the Cambrian 'Explosion' of Animals |journal=Annual Review of Earth and Planetary Sciences |volume=34 |pages=355–384 |bibcode=2006AREPS..34..355M |doi=10.1146/annurev.earth.33.031504.103001 |s2cid=85623607 |issn=1545-4495 }}

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{{Further|Animal#Phylogeny}}

The deepest-branching animals — the earliest animals that appeared during evolution — are marine non-vertebrate organisms. The earliest animal phyla are the Porifera, Ctenophora, Placozoa and Cnidaria. No member of these clades exhibit body plans with bilateral symmetry.

{{Clade |style= float:left;

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|1=Choanoflagellata 50 px unicellular protists thought to be the closest living relatives of animals

|sublabel1=950 mya

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|1=Porifera 30 px sponges – asymmetric

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|1=Ctenophora 45 px comb jellies – biradial symmetry

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|1=Placozoa 40 px simplest animals – asymmetric

|2={{Clade

|1=Cnidaria 30 px have tentacles with stingers – radial symmetry

|2={{Clade

|1=bilaterians 45 px all remaining animals – bilateral symmetry →

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| footer = There has been much controversy over which invertebrate phyla, sponges or comb jellies, is the most basal.{{cite journal | vauthors = King N, Rokas A | title = Embracing Uncertainty in Reconstructing Early Animal Evolution | journal = Current Biology | volume = 27 | issue = 19 | pages = R1081–R1088 | date = October 2017 | pmid = 29017048 | pmc = 5679448 | doi = 10.1016/j.cub.2017.08.054 | bibcode = 2017CBio...27R1081K }} Currently, sponges are more widely considered to be the most basal.{{cite journal | vauthors = Feuda R, Dohrmann M, Pett W, Philippe H, Rota-Stabelli O, Lartillot N, Wörheide G, Pisani D | display-authors = 6 | title = Improved Modeling of Compositional Heterogeneity Supports Sponges as Sister to All Other Animals | journal = Current Biology | volume = 27 | issue = 24 | pages = 3864–3870.e4 | date = December 2017 | pmid = 29199080 | doi = 10.1016/j.cub.2017.11.008 | doi-access = free | bibcode = 2017CBio...27E3864F | hdl = 11572/302898 | hdl-access = free }}{{cite journal | vauthors = Nielsen C | title = Early animal evolution: a morphologist's view | journal = Royal Society Open Science | volume = 6 | issue = 7 | pages = 190638 | date = July 2019 | pmid = 31417759 | pmc = 6689584 | doi = 10.1098/rsos.190638 | bibcode = 2019RSOS....690638N }}

| image1 = Aplysina archeri (Stove-pipe Sponge-pink variation).jpg|

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File:Callyspongia sp. (Tube sponge).jpg

Sponges are animals of the phylum Porifera (from Modern Latin for bearing pores{{cite web | url = http://www.etymonline.com/index.php?term=Porifera | title = Porifera (n.) | work = Online Etymology Dictionary | access-date = 18 August 2016 }}). They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through them, consisting of jelly-like mesohyl sandwiched between two thin layers of cells. They have non-specialized cells that can transform into other types and that often migrate between the main cell layers and the mesohyl in the process. Sponges do not have nervous, digestive or circulatory systems. Instead, most rely on maintaining a constant water flow through their bodies to obtain food and oxygen and to remove wastes.

Sponges are similar to other animals in that they are multicellular, heterotrophic, lack cell walls and produce sperm cells. Unlike other animals, they lack true tissues and organs, and have no body symmetry. The shapes of their bodies are adapted for maximal efficiency of water flow through the central cavity, where it deposits nutrients, and leaves through a hole called the osculum. Many sponges have internal skeletons of spongin and/or spicules of calcium carbonate or silicon dioxide. All sponges are sessile aquatic animals. Although there are freshwater species, the great majority are marine (salt water) species, ranging from tidal zones to depths exceeding {{convert|8800|m|mi|abbr=on}}. Some sponges live to great ages; there is evidence of the deep-sea glass sponge Monorhaphis chuni living about 11,000 years.{{cite journal | vauthors = Petralia RS, Mattson MP, Yao PJ | title = Aging and longevity in the simplest animals and the quest for immortality | journal = Ageing Research Reviews | volume = 16 | pages = 66–82 | date = July 2014 | pmid = 24910306 | pmc = 4133289 | doi = 10.1016/j.arr.2014.05.003 }}{{cite journal | vauthors = Jochum KP, Wang X, Vennemann TW, Sinha B, Müller WE | year = 2012 | title = Siliceous deep-sea sponge Monorhaphis chuni: A potential paleoclimate archive in ancient animals | journal = Chemical Geology | volume = 300 | pages = 143–151 | doi = 10.1016/j.chemgeo.2012.01.009 | bibcode = 2012ChGeo.300..143J }}

While most of the approximately 5,000–10,000 known species feed on bacteria and other food particles in the water, some host photosynthesizing micro-organisms as endosymbionts and these alliances often produce more food and oxygen than they consume. A few species of sponge that live in food-poor environments have become carnivores that prey mainly on small crustaceans.{{cite journal | vauthors = Vacelet J, Duport E | title = Prey capture and digestion in the carnivorous sponge Asbestopluma hypogea (Porifera: Demospongiae). | journal = Zoomorphology | date = November 2004 | volume = 123 | issue = 4 | pages = 179–90 | doi = 10.1007/s00435-004-0100-0 | s2cid = 24484610 }}

File:Sponges in Caribbean Sea, Cayman Islands.jpg|Sponge biodiversity. There are four sponge species in this photo.

File:Euplectella aspergillum (cropped).jpg|Venus' flower basket at a depth of 2572 meters

File:Barrel sponge (Xestospongia testudinaria).jpg|Barrel sponge

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Linnaeus mistakenly identified sponges as plants in the order Algae.{{cite web |url=http://www.marinespecies.org/aphia.php?p=sourceget&id=44036 |title=Spongia Linnaeus, 1759 |publisher=World Register of Marine Species |access-date=18 July 2012 }} For a long time thereafter sponges were assigned to a separate subkingdom, Parazoa (meaning beside the animals).{{cite journal | vauthors = Rowland SM, Stephens T |year=2001 |title=Archaeocyatha: A history of phylogenetic interpretation |journal=Journal of Paleontology |volume=75 |pages=1065–1078 |issue=6 |jstor=1307076 |doi=10.1666/0022-3360(2001)075<1065:AAHOPI>2.0.CO;2 |s2cid=86211946 }} They are now classified as a paraphyletic phylum from which the higher animals have evolved.{{cite journal |vauthors=Sperling EA, Pisani D, Peterson KJ |title=Poriferan paraphyly and its implications for Precambrian palaeobiology |journal=Geological Society, London, Special Publications |date=1 January 2007 |volume=286 |issue=1 |pages=355–368 |doi=10.1144/SP286.25 |url=http://www.dartmouth.edu/~peterson/Sperling,%20Pisani%20and%20Peterson.pdf |access-date=22 August 2012 |url-status=dead |archive-url=https://web.archive.org/web/20090509061759/http://www.dartmouth.edu/~peterson/Sperling,%20Pisani%20and%20Peterson.pdf |archive-date=9 May 2009 |bibcode=2007GSLSP.286..355S |s2cid=34175521 }}

Ctenophores (from Greek for carrying a comb), commonly known as comb jellies, are a phylum that live worldwide in marine waters. They are the largest non-colonial animals to swim with the help of cilia (hairs or combs).{{cite book | vauthors = Ruppert EE, Fox RS, Barnes RD | title=Invertebrate Zoology | publisher=Brooks / Cole | edition=7 | isbn=978-0-03-025982-1 | year=2004 | pages=[https://archive.org/details/isbn_9780030259821/page/182 182–195] | url=https://archive.org/details/isbn_9780030259821/page/182 }} Coastal species need to be tough enough to withstand waves and swirling sediment, but some oceanic species are so fragile and transparent that it is very difficult to capture them intact for study.{{cite web| title=Ctenophores – some notes from an expert | vauthors = Mills CE | url= http://faculty.washington.edu/cemills/Ctenophores.html | access-date=2009-02-05}} In the past ctenophores were thought to have only a modest presence in the ocean, but it is now known they are often significant and even dominant parts of the planktonic biomass.{{rp|269}}

The phylum has about 150 known species with a wide range of body forms. Sizes range from a few millimeters to {{convert|1.5|m|abbr=on}}. Cydippids are egg-shaped with their cilia arranged in eight radial comb rows, and deploy retractable tentacles for capturing prey. The benthic platyctenids are generally combless and flat. The coastal beroids have gaping mouths and lack tentacles. Most adult ctenophores prey on microscopic larvae and rotifers and small crustaceans but beroids prey on other ctenophores.

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File:LightRefractsOf comb-rows of ctenophore Mertensia ovum.jpg|Light diffracting along the comb rows of a cydippid, left tentacle deployed, right retracted

File:Ctenophore.jpg|Deep-sea ctenophore trailing tentacles studded with tentilla (sub-tentacles)

File:Aulacoctena cydippid ctenophore.jpg|Egg-shaped cydippid ctenophore

File:Coeloplana astericola (Benthic ctenophores) on Echniaster luzonicus (Seastar).jpg|Group of small benthic creeping comb jellies streaming tentacles and living symbiotically on a starfish.

File:Lobate ctenophore.jpg|Lobata sp. with paired thick lobes

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File:Ctenophore2.jpg ctenophore, mouth gaping, preys on other ctenophores.]]

Early writers combined ctenophores with cnidarians. Ctenophores resemble cnidarians in relying on water flow through the body cavity for both digestion and respiration, as well as in having a decentralized nerve net rather than a brain. Also like cnidarians, the bodies of ctenophores consist of a mass of jelly, with one layer of cells on the outside and another lining the internal cavity. In ctenophores, however, these layers are two cells deep, while those in cnidarians are only a single cell deep. While cnidarians exhibit radial symmetry, ctenophores have two anal canals which exhibit biradial symmetry (half-turn rotational symmetry).{{cite journal | vauthors = Martindale MQ, Finnerty JR, Henry JQ | title = The Radiata and the evolutionary origins of the bilaterian body plan | journal = Molecular Phylogenetics and Evolution | volume = 24 | issue = 3 | pages = 358–65 | date = September 2002 | pmid = 12220977 | doi = 10.1016/s1055-7903(02)00208-7 | bibcode = 2002MolPE..24..358M }} The position of the ctenophores in the evolutionary family tree of animals has long been debated, and the majority view at present, based on molecular phylogenetics, is that cnidarians and bilaterians are more closely related to each other than either is to ctenophores.{{cite book | vauthors = Brusca RC, Brusca GJ | date = 2003 | title = Invertebrates | edition = Second | publisher = Sinauer Associates | isbn = 978-0-87893-097-5}}{{rp|222}}

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Placozoa (from Greek for flat animals) have the simplest structure of all animals. They are a basal form of free-living (non-parasitic) multicellular organism{{MeshName|Placozoa}} that do not yet have a common name.{{cite book|language=de| vauthors = Wehner R, Gehring W | title=Zoologie|edition=24th|publisher=Thieme|location= Stuttgart|date=June 2007|page=696}} They live in marine environments and form a phylum containing so far only three described species, of which the first, the classical Trichoplax adhaerens, was discovered in 1883.{{cite book | vauthors = Schulze FE | chapter = Trichoplax adhaerens n. g., n. s. | title = Zoologischer Anzeiger | publisher = Elsevier | location = Amsterdam and Jena | volume = 6 | date = 1883 | page = 92 }} Two more species have been discovered since 2017,{{Cite journal |vauthors=Eitel M, Francis WR, Osigus HJ, Krebs S, Vargas S, Blum H, Williams GA, Schierwater B, Wörheide G |display-authors=6 |date=2017-10-13 |title=A taxogenomics approach uncovers a new genus in the phylum Placozoa |url=https://www.biorxiv.org/content/early/2017/10/13/202119 |journal=bioRxiv |language=en |pages=202119 |doi=10.1101/202119 |s2cid=89829846 }}{{cite journal | vauthors = Osigus HJ, Rolfes S, Herzog R, Kamm K, Schierwater B | title = Polyplacotoma mediterranea is a new ramified placozoan species | journal = Current Biology | volume = 29 | issue = 5 | pages = R148–R149 | date = March 2019 | pmid = 30836080 | doi = 10.1016/j.cub.2019.01.068 | doi-access = free | bibcode = 2019CBio...29.R148O }} and genetic methods indicate this phylum has a further 100 to 200 undescribed species.{{cite web | url = http://www.marinespecies.org/photogallery.php?album=5163&pic=24069 | title = Trichoplax adhaerens | work = WoRMS | date = 2009 }}

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File:Exodigestion in Trichoplax adhaerens.jpg]]

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Trichoplax is a small, flattened, animal about one mm across and usually about 25 μm thick. Like the amoebae they superficially resemble, they continually change their external shape. In addition, spherical phases occasionally form which may facilitate movement. Trichoplax lacks tissues and organs. There is no manifest body symmetry, so it is not possible to distinguish anterior from posterior or left from right. It is made up of a few thousand cells of six types in three distinct layers.{{cite journal | vauthors = Smith CL, Varoqueaux F, Kittelmann M, Azzam RN, Cooper B, Winters CA, Eitel M, Fasshauer D, Reese TS | display-authors = 6 | title = Novel cell types, neurosecretory cells, and body plan of the early-diverging metazoan Trichoplax adhaerens | journal = Current Biology | volume = 24 | issue = 14 | pages = 1565–1572 | date = July 2014 | pmid = 24954051 | pmc = 4128346 | doi = 10.1016/j.cub.2014.05.046 | bibcode = 2014CBio...24.1565S }} The outer layer of simple epithelial cells bear cilia which the animal uses to help it creep along the seafloor.{{cite book | vauthors = Barnes RD |year=1982 |title= Invertebrate Zoology |publisher= Holt-Saunders International |location= Philadelphia|pages= 84–85|isbn= 978-0-03-056747-6}} Trichoplax feed by engulfing and absorbing food particles – mainly microbes and organic detritus – with their underside.

File:Nematostella vectensis (I1419) 999 (30695685804).jpg, are the simplest animals to organise cells into tissue. Yet they have the same genes that form the vertebrate (including human) head.]]

Cnidarians (from Greek for nettle) are distinguished by the presence of stinging cells, specialized cells that they use mainly for capturing prey. Cnidarians include corals, sea anemones, jellyfish and hydrozoans. They form a phylum containing over 10,000{{cite journal| vauthors = Zhang ZQ | title=Animal biodiversity: An introduction to higher-level classification and taxonomic richness | journal=Zootaxa| volume=3148| year=2011| pages=7–12| url=http://mapress.com/zootaxa/2011/f/zt03148p012.pdf| doi=10.11646/zootaxa.3148.1.3 }} species of animals found exclusively in aquatic (mainly marine) environments. Their bodies consist of mesoglea, a non-living jelly-like substance, sandwiched between two layers of epithelium that are mostly one cell thick. They have two basic body forms: swimming medusae and sessile polyps, both of which are radially symmetrical with mouths surrounded by tentacles that bear cnidocytes. Both forms have a single orifice and body cavity that are used for digestion and respiration.

Fossil cnidarians have been found in rocks formed about {{ma|580}}. Fossils of cnidarians that do not build mineralized structures are rare. Scientists currently think cnidarians, ctenophores and bilaterians are more closely related to calcareous sponges than these are to other sponges, and that anthozoans are the evolutionary "aunts" or "sisters" of other cnidarians, and the most closely related to bilaterians.

Cnidarians are the simplest animals in which the cells are organized into tissues.{{cite web |url=http://genome.jgi-psf.org/Nemve1/Nemve1.home.html |title=Nematostella vectensis v1.0 |work=Genome Portal |publisher=University of California |access-date=2014-01-19}} The starlet sea anemone is used as a model organism in research.{{cite web |url=http://www.nematostella.org/ |title=Nematostella |publisher=Nematostella.org |access-date=2014-01-18 |archive-url=https://web.archive.org/web/20060508222254/http://www.nematostella.org/ |archive-date=8 May 2006 |url-status=dead }} It is easy to care for in the laboratory and a protocol has been developed which can yield large numbers of embryos on a daily basis.{{cite journal | vauthors = Genikhovich G, Technau U | title = The starlet sea anemone Nematostella vectensis: an anthozoan model organism for studies in comparative genomics and functional evolutionary developmental biology | journal = Cold Spring Harbor Protocols | volume = 2009 | issue = 9 | pages = pdb.emo129 | date = September 2009 | pmid = 20147257 | doi = 10.1101/pdb.emo129 }} There is a remarkable degree of similarity in the gene sequence conservation and complexity between the sea anemone and vertebrates. In particular, genes concerned in the formation of the head in vertebrates are also present in the anemone.{{cite news |title=Where Does Our Head Come From? Brainless Sea Anemone Sheds New Light on the Evolutionary Origin of the Head |url=https://www.sciencedaily.com/releases/2013/02/130220084436.htm#.USs4Y9Dvwpg.twitter |newspaper=Science Daily |date=2013-02-12 |access-date=2014-01-18}}{{cite journal | vauthors = Sinigaglia C, Busengdal H, Leclère L, Technau U, Rentzsch F | title = The bilaterian head patterning gene six3/6 controls aboral domain development in a cnidarian | journal = PLOS Biology | volume = 11 | issue = 2 | pages = e1001488 | year = 2013 | pmid = 23483856 | pmc = 3586664 | doi = 10.1371/journal.pbio.1001488 | doi-access = free }}

File:Sea anemone in tidepools.jpg|Sea anemones are common in tidepools.

File:Coral detail.jpg|Close up of polyps on the surface of a coral, waving their tentacles.

File:Maldives small island.jpg|If an island sinks below the sea, coral growth can keep up with rising water and form an atoll.

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File:Portuguese Man-O-War (Physalia physalis).jpg|The Portuguese man o' war is a colonial siphonophore

File:Porpita porpita.jpg|Porpita porpita consists of a colony of hydroids{{cite web | url = http://www.beachhunter.net/thingstoknow/jellyfish/blue-button-jellyfish.htm | title = Blue Buttons in Florida | work = BeachHunter.net }}

File:Largelionsmanejellyfish.jpg|Lion's mane jellyfish, largest known jellyfish{{cite book | vauthors = Karleskint G, Turner R, Small J | date = 2012 | url = https://books.google.com/books?id=uBXTCQAAQBAJ&q=%22lion%27s+mane+jellyfish%22+%22Introduction+to+Marine+Biology%22&pg=PA445 | title = Introduction to Marine Biology | publisher = Cengage Learning | edition = 4th | page = 445 | isbn = 978-1-133-36446-7 }}

File:Turritopsis dohrnii (cropped).jpg|Turritopsis dohrnii achieves biological immortality by transferring its cells back to childhood.{{cite journal| vauthors = Bavestrello G, Sommer C, Sarà M |year=1992|title=Bi-directional conversion in Turritopsis nutricula (Hydrozoa)|journal=Scientia Marina|volume=56|issue=2–3|pages=137–140}}{{cite journal| vauthors = Piraino S, Boero F, Aeschbach B, Schmid V |year=1996|title=Reversing the life cycle: medusae transforming into polyps and cell transdifferentiation in Turritopsis nutricula (Cnidaria, Hydrozoa)|journal=Biological Bulletin|volume=190|issue=3|pages=302–312|doi=10.2307/1543022|pmid=29227703|jstor=1543022|s2cid=3956265}}

File:Chironex fleckeri (sea wasp).jpg|The sea wasp is the most lethal jellyfish in the world.{{Cite journal |vauthors=Fenner PJ, Williamson JA |title=Worldwide deaths and severe envenomation from jellyfish stings |journal=The Medical Journal of Australia |volume=165 |issue=11–12 |pages=658–61 |year=1996 |pmid=8985452 |doi=10.5694/j.1326-5377.1996.tb138679.x |s2cid=45032896}}

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File:Bilaterian body plan.svg.]]

Some of the earliest bilaterians were wormlike, and the original bilaterian may have been a bottom dwelling worm with a single body opening. A bilaterian body can be conceptualized as a cylinder with a gut running between two openings, the mouth and the anus. Around the gut it has an internal body cavity, a coelom or pseudocoelom.{{efn|The earliest Bilateria may have had only a single opening, and no coelom.{{cite journal | vauthors = Cannon JT, Vellutini BC, Smith J, Ronquist F, Jondelius U, Hejnol A | title = Xenacoelomorpha is the sister group to Nephrozoa | journal = Nature | volume = 530 | issue = 7588 | pages = 89–93 | date = February 2016 | pmid = 26842059 | doi = 10.1038/nature16520 | s2cid = 205247296 | bibcode = 2016Natur.530...89C | url = http://urn.kb.se/resolve?urn=urn:nbn:se:nrm:diva-1844 }}}} Animals with this bilaterally symmetric body plan have a head (anterior) end and a tail (posterior) end as well as a back (dorsal) and a belly (ventral); therefore they also have a left side and a right side.

Having a front end means that this part of the body encounters stimuli, such as food, favoring cephalisation, the development of a head with sense organs and a mouth.{{cite journal | vauthors = Finnerty JR | title = Did internal transport, rather than directed locomotion, favor the evolution of bilateral symmetry in animals? | journal = BioEssays | volume = 27 | issue = 11 | pages = 1174–80 | date = November 2005 | pmid = 16237677 | doi = 10.1002/bies.20299 | url = http://faculty.weber.edu/rmeyers/PDFs/Finnerty%20-%20symmetry%20evol.pdf | access-date = 27 August 2019 | url-status = dead | archive-url = https://web.archive.org/web/20140810012815/http://faculty.weber.edu/rmeyers/PDFs/Finnerty%20-%20symmetry%20evol.pdf | archive-date = 10 August 2014 }} The body stretches back from the head, and many bilaterians have a combination of circular muscles that constrict the body, making it longer, and an opposing set of longitudinal muscles, that shorten the body; these enable soft-bodied animals with a hydrostatic skeleton to move by peristalsis.{{cite journal | vauthors = Quillin KJ | title = Ontogenetic scaling of hydrostatic skeletons: geometric, static stress and dynamic stress scaling of the earthworm lumbricus terrestris | journal = The Journal of Experimental Biology | volume = 201 | issue = 12 | pages = 1871–83 | date = May 1998 | pmid = 9600869 | doi = 10.1242/jeb.201.12.1871 | doi-access = free | bibcode = 1998JExpB.201.1871Q }} They also have a gut that extends through the basically cylindrical body from mouth to anus. Many bilaterian phyla have primary larvae which swim with cilia and have an apical organ containing sensory cells. However, there are exceptions to each of these characteristics; for example, adult echinoderms are radially symmetric (unlike their larvae), and certain parasitic worms have extremely simplified body structures.{{cite book| vauthors = Minelli A |title=Perspectives in Animal Phylogeny and Evolution |url=https://books.google.com/books?id=jIASDAAAQBAJ&pg=PA53 |year=2009 |publisher=Oxford University Press |isbn=978-0-19-856620-5 |page=53}}{{Cite book |chapter-url=http://www.sinauer.com/media/wysiwyg/samples/Brusca3e_Chapter_9.pdf |chapter=Introduction to the Bilateria and the Phylum Xenacoelomorpha {{!}} Triploblasty and Bilateral Symmetry Provide New Avenues for Animal Radiation |title=Invertebrates | vauthors = Brusca RC |date=2016 |publisher=Sinauer Associates |pages=345–372 |isbn=978-1605353753}}

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File:Ikaria wariootia (cropped).jpg, an early bilaterian{{cite web | vauthors = Specktor B | url = https://www.livescience.com/bilaterian-worm-animal-ancestor.html | title = This primeval worm may be the ancestor of all animals] | work = Live Science' | date = 26 March 2020 }}]]

{{clade

|label1= ← bilaterians

|1={{clade |style = float:right;

|label1=Xenacoelomorpha

|1={{clade

|1=60 px   basal bilaterians (lack a true gut)

}}

|label3=Nephrozoa

|3={{clade

|label1=protostomes

|sublabel1=610 mya

|1={{clade

|1=55 px develops mouth first →

}}

|label2= deuterostomes

|sublabel2=650 mya

|2={{clade

|1= 65 px develops anus first →

}}

}}

}}

}}

{{See also|Embryological origins of the mouth and anus}}

Protostomes (from Greek for first mouth) are a superphylum of animals. It is a sister clade of the deuterostomes (from Greek for second mouth), with which it forms the Nephrozoa clade. Protostomes are distinguished from deuterostomes by the way their embryos develop. In protostomes the first opening that develops becomes the mouth, while in deuterostomes it becomes the anus.{{cite news | vauthors = Wade N |title=This Prehistoric Human Ancestor Was All Mouth |url=https://www.nytimes.com/2017/01/30/science/this-prehistoric-human-ancestor-was-all-mouth.html |date=30 January 2017 |work=The New York Times |access-date=31 January 2017}}{{cite journal | vauthors = Han J, Morris SC, Ou Q, Shu D, Huang H | title = Meiofaunal deuterostomes from the basal Cambrian of Shaanxi (China) | journal = Nature | volume = 542 | issue = 7640 | pages = 228–231 | date = February 2017 | pmid = 28135722 | doi = 10.1038/nature21072 | s2cid = 353780 | bibcode = 2017Natur.542..228H }}

{{Clade

|label1 = ← Protostomes

|sublabel1= (extant)

|1={{clade

|label1=Ecdysozoa |sublabel1=>529 mya

|1={{clade

|1=Scalidophora 40 px penis worms and mud dragons

|2={{clade

|1=arthropods 60 px mainly crustaceans

|2=nematodes 50 px roundworms

}}

}}

|label2=Spiralia

|2={{clade

|label1=Gnathifera

|1={{clade

|1=rotifers 40 px

|2=arrow worms 70 px

}}

|label2=Platytrochozoa |sublabel2=580 mya

|2={{clade

|1=flatworms 80 px

|label2=Lophotrochozoa |sublabel2=550 mya

|2={{Clade

|1=molluscs 60 px gastropods, bivalves and cephalopods

|2=ringed worms 60 px

}}

}}

}}

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{{Further|Marine worm|Sea worm}}

File:Chaetoblack.png, found worldwide as a predatory component of plankton.]]

Worms (Old English for serpents) form a number of phyla. Different groups of marine worms are related only distantly, so they are found in several different phyla such as the Annelida (segmented worms), Chaetognatha (arrow worms), Phoronida (horseshoe worms), and Hemichordata. All worms, apart from the Hemichordata, are protostomes. The Hemichordata are

deuterostomes and are discussed in their own section below.

The typical body plan of a worm involves long cylindrical tube-like bodies and no limbs. Marine worms vary in size from microscopic to over {{convert|1|m|ft}} in length for some marine polychaete worms (bristle worms){{cite web |url=https://www.bbc.co.uk/cornwall/content/articles/2009/04/07/nature_worm_feature.shtml |title=Cornwall – Nature – Superstar Worm |work=BBC }} and up to {{convert|58|m|ft|}} for the marine nemertean worm (bootlace worm).Mark Carwardine (1995) The Guinness Book of Animal Records. Guinness Publishing. p. 232. Some marine worms occupy a small variety of parasitic niches, living inside the bodies of other animals, while others live more freely in the marine environment or by burrowing underground. Many of these worms have specialized tentacles used for exchanging oxygen and carbon dioxide and also may be used for reproduction. Some marine worms are tube worms, such as the giant tube worm which lives in waters near underwater volcanoes and can withstand temperatures up to 90 degrees Celsius. Platyhelminthes (flatworms) form another worm phylum which includes a class of parasitic tapeworms. The marine tapeworm Polygonoporus giganticus, found in the gut of sperm whales, can grow to over 30 m (100 ft).{{cite news |date=1957-04-08 |url=http://www.time.com/time/magazine/article/0,9171,809356-1,00.html |archive-url=https://web.archive.org/web/20080627143218/http://www.time.com/time/magazine/article/0,9171,809356-1,00.html |url-status=dead |archive-date=27 June 2008 |title=The Persistent Parasites |magazine=Time}}{{cite tech report |year=1985 |veditors=Hargis W |title=Parasitology and pathology of marine organisms of the world ocean |url=https://spo.nmfs.noaa.gov/content/tr-25-parasitology-and-pathology-marine-organisms-world-ocean |publisher=National Oceanic and Atmospheric Administration}}

Nematodes (roundworms) constitute a further worm phylum with tubular digestive systems and an opening at both ends.{{cite web |url=http://plpnemweb.ucdavis.edu/nemaplex/General/animpara.htm |url-status=dead |archive-url=https://web.archive.org/web/20060914101343/http://plpnemweb.ucdavis.edu/nemaplex/General/animpara.htm |archive-date=14 September 2006 |title=Classification of Animal Parasites}}{{cite journal | vauthors = Garcia LS | title = Classification of human parasites, vectors, and similar organisms | journal = Clinical Infectious Diseases | volume = 29 | issue = 4 | pages = 734–6 | date = October 1999 | pmid = 10589879 | doi = 10.1086/520425 | doi-access = free }} Over 25,000 nematode species have been described,{{cite journal | vauthors = Hodda M | year = 2011 | title = Phylum Nematoda Cobb, 1932. In: Zhang, Z.-Q. Animal biodiversity: An outline of higher-level classification and survey of taxonomic richness | journal = Zootaxa | volume = 3148 | pages = 63–95 | doi = 10.11646/zootaxa.3148.1.11 }}{{cite journal | vauthors = Zhang Z | year = 2013 | title = Animal biodiversity: An update of classification and diversity in 2013. In: Zhang, Z.-Q. (Ed.) Animal Biodiversity: An Outline of Higher-level Classification and Survey of Taxonomic Richness (Addenda 2013) | journal = Zootaxa | volume = 3703 | issue = 1| pages = 5–11 | doi = 10.11646/zootaxa.3703.1.3 | s2cid = 85252974 | doi-access = free }} of which more than half are parasitic. It has been estimated that another million are beyond our current knowledge.

{{cite journal | vauthors = Lambshead PJ | title = Recent developments in marine benthic biodiversity research | journal = Oceanis | year = 1993 | volume = 19 | issue = 6 | pages = 5–24 }} They are ubiquitous in marine, freshwater and terrestrial environments, where they often outnumber other animals in both individual and species counts. They are found in every part of the Earth's lithosphere, from the top of mountains to the bottom of oceanic trenches.{{cite journal | vauthors = Borgonie G, García-Moyano A, Litthauer D, Bert W, Bester A, van Heerden E, Möller C, Erasmus M, Onstott TC | display-authors = 6 | title = Nematoda from the terrestrial deep subsurface of South Africa | journal = Nature | volume = 474 | issue = 7349 | pages = 79–82 | date = June 2011 | pmid = 21637257 | doi = 10.1038/nature09974 | hdl-access = free | s2cid = 4399763 | bibcode = 2011Natur.474...79B | hdl = 1854/LU-1269676 | url = https://biblio.ugent.be/publication/1269676 }} By count they represent 90% of all animals on the ocean floor.{{cite journal | vauthors = Danovaro R, Gambi C, Dell'Anno A, Corinaldesi C, Fraschetti S, Vanreusel A, Vincx M, Gooday AJ | display-authors = 6 | title = Exponential decline of deep-sea ecosystem functioning linked to benthic biodiversity loss | journal = Current Biology | volume = 18 | issue = 1 | pages = 1–8 | date = January 2008 | pmid = 18164201 | doi = 10.1016/j.cub.2007.11.056 | s2cid = 15272791 | doi-access = free | bibcode = 2008CBio...18....1D }}

• {{cite press release |date=27 December 2007 |title=Deep-sea species' loss could lead to oceans' collapse, study suggests |website=EurekAlert! |url=http://www.eurekalert.org/pub_releases/2007-12/cp-dsl122007.php}} Their numerical dominance, often exceeding a million individuals per square meter and accounting for about 80% of all individual animals on Earth, their diversity of life cycles, and their presence at various trophic levels point at an important role in many ecosystems.{{cite book | vauthors = Platt HM | chapter = foreword |veditors=Lorenzen S, Lorenzen SA | title = The phylogenetic systematics of freeliving nematodes | publisher = The Ray Society | location = London | year = 1994 | isbn = 978-0-903874-22-9 }}

File:Riftia tube worm colony Galapagos 2011.jpg|Giant tube worms cluster around hydrothermal vents.

File:CSIRO ScienceImage 2818 Group of Nematodes.jpg|Nematodes are ubiquitous pseudocoelomates which can parasite marine plants and animals.

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| caption1 = Bigfin reef squid displaying vivid iridescence at night. Cephalopods are the most neurologically advanced invertebrates.{{Cite book| vauthors = Barnes RS, Calow P, Olive PJ | year=2001| title=The Invertebrates, A Synthesis | edition=3rd| publisher=Blackwell Science | location =UK}}

| image2 = Glaucus atlanticus 1 cropped.jpg

| caption2 = Blue dragon, a pelagic sea slug

| image3 = Bolinus brandaris 2.jpg

| caption3 = Bolinus brandaris, a sea snail from which the Phoenicians extracted royal Tyrian purple dye colour code: #66023C _____{{cite web | url = http://www.green-lion.net/colour_purple.html | archive-url = https://web.archive.org/web/20140228184525/http://www.green-lion.net/colour_purple.html | url-status = dead | archive-date = 28 February 2014 | title = Tyrian Purple | work = Green Lion | date = 28 February 2014 }}

| image4 = Archimollusc-en.svg

| caption4 = Hypothetical ancestral mollusc

}}

{{See also|Evolution of molluscs|Evolution of cephalopods}}

Molluscs (Latin for soft) form a phylum with about 85,000 extant recognized species.{{cite book | vauthors = Chapman AD | date = 2009 | url = http://www.environment.gov.au/biodiversity/abrs/publications/other/species-numbers/2009/04-02-groups-invertebrates.html#mollusca | title = Numbers of Living Species in Australia and the World | edition = 2nd | publisher = Australian Biological Resources Study | location = Canberra | isbn = 978-0-642-56860-1}} They are the largest marine phylum in terms of species count, containing about 23% of all the named marine organisms.{{cite web|url=http://www.austmus.gov.au/display.cfm?id=2897|title=Recognising research on molluscs|access-date=2009-03-09|publisher=Australian Museum|year=2008| vauthors = Hancock R |url-status=dead|archive-url=https://web.archive.org/web/20090530042720/http://www.austmus.gov.au/display.cfm?id=2897|archive-date=30 May 2009|df=dmy-all}} Molluscs have more varied forms than other invertebrate phyla. They are highly diverse, not just in size and in anatomical structure, but also in behavior and in habitat.

The mollusc phylum is divided into 9 or 10 taxonomic classes. These classes include gastropods, bivalves and cephalopods, as well as other lesser-known but distinctive classes. Gastropods with protective shells are referred to as snails, whereas gastropods without protective shells are referred to as slugs. Gastropods are by far the most numerous molluscs in terms of species.{{Cite book | veditors = Ponder WF, Lindberg DR | year = 2008 | title = Phylogeny and Evolution of the Mollusca | publisher = Berkeley: University of California Press | page = 481 | isbn = 978-0-520-25092-5 }} Bivalves include clams, oysters, cockles, mussels, scallops, and numerous other families. There are about 8,000 marine bivalves species (including brackish water and estuarine species). A deep sea ocean quahog clam has been reported as having lived 507 years{{cite journal | vauthors = Munro D, Blier PU | title = The extreme longevity of Arctica islandica is associated with increased peroxidation resistance in mitochondrial membranes | journal = Aging Cell | volume = 11 | issue = 5 | pages = 845–55 | date = October 2012 | pmid = 22708840 | doi = 10.1111/j.1474-9726.2012.00847.x | s2cid = 205634828 | doi-access = free }} making it the longest recorded life of all animals apart from colonial animals, or near-colonial animals like sponges.

File:Nembrotha aurea B.jpg|Marine gastropods are sea snails or sea slugs. This nudibranch is a sea slug.

File:Catalonia VilassarDeDalt CaudelCargol Conquilla.JPG|The sea snail Syrinx aruanus has a shell up to 91 cm long, the largest of any living gastropod.

File:Placopecten magellanicus.jpg|Molluscs usually have eyes. Bordering the edge of the mantle of a scallop, a bivalve mollusc, can be over 100 simple eyes.

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Cephalopods include octopus, squid and cuttlefish. About 800 living species of marine cephalopods have been identified,{{cite web |url=http://cephbase.eol.org/ |title=Welcome to CephBase |work=CephBase |access-date=29 January 2016 |archive-date=12 January 2016 |archive-url=https://web.archive.org/web/20160112225313/http://cephbase.eol.org/ |url-status=dead }} and an estimated 11,000 extinct taxa have been described.{{The Mollusca|volume=12|name-list-style=vanc}} They are found in all oceans, but there are no fully freshwater cephalopods.{{Cite web | url=http://www.abc.net.au/science/articles/2013/01/16/3670198.htm | title=Are there any freshwater cephalopods?| website=Australian Broadcasting Corporation| date=2013-01-16}}

File:Nautilus belauensis from Palau.jpg|The nautilus is a living fossil little changed since it evolved 500 million years ago as one of the first cephalopods.{{cite magazine|url=https://www.newscientist.com/article/dn14033-simpleminded-nautilus-reveals-flash-of-memory.html|title=Simple-Minded Nautilus Shows Flash of Memory| vauthors = Callaway E |date=2 June 2008|magazine=New Scientist|access-date=7 March 2012}}{{cite journal|title=Living Fossil Memories| vauthors = Phillips K |date=15 June 2008|page=iii|volume=211|doi=10.1242/jeb.020370|journal=Journal of Experimental Biology|issue=12| bibcode = 2008JExpB.211Y...3P |s2cid=84279320}}{{cite journal| vauthors = Crook R, Basil J |year=2008|title=A biphasic memory curve in the chambered nautilus, Nautilus pompilius L. (Cephalopoda: Nautiloidea)|journal=Journal of Experimental Biology|volume=211|pages=1992–1998|doi=10.1242/jeb.018531|issue=12|pmid=18515730|bibcode=2008JExpB.211.1992C |s2cid=6305526}}

File:Dactylioceras NT.jpg|Reconstruction of an ammonite, a highly successful early cephalopod that appeared 400 mya.

File:HPIM1795.JPG|Cephalopods, like this cuttlefish, use their mantle cavity for jet propulsion.

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Molluscs have such diverse shapes that many textbooks base their descriptions of molluscan anatomy on a generalized or hypothetical ancestral mollusc. This generalized mollusc is unsegmented and bilaterally symmetrical with an underside consisting of a single muscular foot. Beyond that it has three further key features. Firstly, it has a muscular cloak called a mantle covering its viscera and containing a significant cavity used for breathing and excretion. A shell secreted by the mantle covers the upper surface. Secondly (apart from bivalves) it has a rasping tongue called a radula used for feeding. Thirdly, it has a nervous system including a complex digestive system using microscopic, muscle-powered hairs called cilia to exude mucus. The generalized mollusc has two paired nerve cords (three in bivalves). The brain, in species that have one, encircles the esophagus. Most molluscs have eyes and all have sensors detecting chemicals, vibrations, and touch.{{cite book |title=Invertebrate Zoology |edition=7th | vauthors = Ruppert RE, Fox RS, Barnes RD |year=2004 |publisher=Cengage Learning |isbn=978-81-315-0104-7 }}{{Cite book| vauthors = Hayward PJ |title=Handbook of the Marine Fauna of North-West Europe|year=1996|publisher=Oxford University Press|isbn=978-0-19-854055-7}}

Good evidence exists for the appearance of marine gastropods, cephalopods and bivalves in the Cambrian period {{ma|Cambrian|{{period end|Cambrian}}}}.

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{{Annotated image |float=right| image=Lobster line drawing.jpg |caption=Segments and tagmata of an arthropod{{rp|518–52}} The thorax bears the main locomotory appendages. The head and thorax are fused in some arthropods, such as crabs and lobsters.|width=220 |image-width=120 |height=195

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{{Annotation|164|45|Head}}

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File:Pneumodesmus newmani - MUSE.JPG,{{cite journal | vauthors = Wilson HM, Anderson LI |date=January 2004 |title=Morphology and taxonomy of Paleozoic millipedes (Diplopoda: Chilognatha: Archipolypoda) from Scotland |url= http://jpaleontol.geoscienceworld.org/content/78/1/169.abstract |journal=Journal of Paleontology |volume=78 |issue=1 |pages=169–184 |doi=10.1666/0022-3360(2004)078<0169:MATOPM>2.0.CO;2 |bibcode=2004JPal...78..169W |s2cid=131201588 }} lived in the Early Devonian.{{cite journal | vauthors = Suarez SE, Brookfield ME, Catlos EJ, Stöckli DF | title = A U-Pb zircon age constraint on the oldest-recorded air-breathing land animal | journal = PLOS ONE | volume = 12 | issue = 6 | pages = e0179262 | year = 2017 | pmid = 28658320 | pmc = 5489152 | doi = 10.1371/journal.pone.0179262 | bibcode = 2017PLoSO..1279262S | doi-access = free }}]]

Arthropods (Greek for jointed feet) have an exoskeleton (external skeleton), a segmented body, and jointed appendages (paired appendages). They form a phylum which includes insects, arachnids, myriapods, and crustaceans. Arthropods are characterized by their jointed limbs and cuticle made of chitin, often mineralized with calcium carbonate. The arthropod body plan consists of segments, each with a pair of appendages. The rigid cuticle inhibits growth, so arthropods replace it periodically by moulting. Their versatility has enabled them to become the most species-rich members of all ecological guilds in most environments.

The evolutionary ancestry of arthropods dates back to the Cambrian period and is generally regarded as monophyletic. However, basal relationships of arthropods with extinct phyla such as lobopodians have recently been debated.{{cite journal | vauthors = Campbell LI, Rota-Stabelli O, Edgecombe GD, Marchioro T, Longhorn SJ, Telford MJ, Philippe H, Rebecchi L, Peterson KJ, Pisani D | display-authors = 6 | title = MicroRNAs and phylogenomics resolve the relationships of Tardigrada and suggest that velvet worms are the sister group of Arthropoda | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 108 | issue = 38 | pages = 15920–4 | date = September 2011 | pmid = 21896763 | pmc = 3179045 | doi = 10.1073/pnas.1105499108 | bibcode = 2011PNAS..10815920C | doi-access = free }}{{cite journal | vauthors = Smith FW, Goldstein B | title = Segmentation in Tardigrada and diversification of segmental patterns in Panarthropoda | journal = Arthropod Structure & Development | volume = 46 | issue = 3 | pages = 328–340 | date = May 2017 | pmid = 27725256 | doi = 10.1016/j.asd.2016.10.005 | bibcode = 2017ArtSD..46..328S }}

{{clade

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File:Cheirurus ingricus.png| Fossil trilobite. Trilobites first appeared about 521 Ma. They were highly successful and were found everywhere in the ocean for 270 Ma.{{cite web |url=http://firstlifeseries.com/learn-more/ |title= David Attenborough's First Life |access-date=2011-03-10 |url-status=dead |archive-url=https://web.archive.org/web/20110126162514/http://firstlifeseries.com/learn-more/ |archive-date=26 January 2011 |df=dmy-all }}

File:20191203 Anomalocaris canadensis.png| The Anomalocaris ("abnormal shrimp") was one of the first apex predators and first appeared about 515 Ma.

File:Jaekelopterus rhenaniae reconstruction.jpg|The largest known arthropod, the sea scorpion Jaekelopterus rhenaniae, has been found in estuarine strata from about 390 Ma. It was up to {{convert|2.5|m|ft|abbr=on}} long.{{cite journal | vauthors = Braddy SJ, Poschmann M, Tetlie OE | title = Giant claw reveals the largest ever arthropod | journal = Biology Letters | volume = 4 | issue = 1 | pages = 106–9 | date = February 2008 | pmid = 18029297 | pmc = 2412931 | doi = 10.1098/rsbl.2007.0491 }}{{cite journal| vauthors = Daniel C |title=Giant sea scorpion discovered|journal=Nature|date=21 November 2007|access-date=10 June 2013|url=http://www.nature.com/news/2007/071120/full/news.2007.272.html|doi=10.1038/news.2007.272}}

File:Limulus (cropped).jpg| Xiphosurans, the group including modern Horseshoe crabs appeared around 480 Ma.{{Cite journal |last1=Bicknell |first1=Russell D. C. |last2=Pates |first2=Stephen |date=2020 |title=Pictorial Atlas of Fossil and Extant Horseshoe Crabs, With Focus on Xiphosurida |journal=Frontiers in Earth Science |volume=8 |page=98 |doi=10.3389/feart.2020.00098 |bibcode=2020FrEaS...8...98B |issn=2296-6463 |doi-access=free }}

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Extant marine arthropods range in size from the microscopic crustacean Stygotantulus to the Japanese spider crab. Arthropods' primary internal cavity is a hemocoel, which accommodates their internal organs, and through which their haemolymph - analogue of blood - circulates; they have open circulatory systems. Like their exteriors, the internal organs of arthropods are generally built of repeated segments. Their nervous system is "ladder-like", with paired ventral nerve cords running through all segments and forming paired ganglia in each segment. Their heads are formed by fusion of varying numbers of segments, and their brains are formed by fusion of the ganglia of these segments and encircle the esophagus. The respiratory and excretory systems of arthropods vary, depending as much on their environment as on the subphylum to which they belong.

File:Hyperia.jpg|Many crustaceans are very small, like this tiny amphipod, and make up a significant part of the ocean's zooplankton.

File:Odontodactylus scyllarus 2.png|Mantis shrimp have the most advanced eyes in the animal kingdom,{{cite news |url=http://archive.dailycal.org/article/19671/mantis_shrimp_boasts_most_advanced_eyes |title=Mantis shrimp boasts most advanced eyes |vauthors=Kilday P |newspaper=The Daily Californian |date=28 September 2005 |access-date=23 September 2016 |archive-date=29 September 2012 |archive-url=https://web.archive.org/web/20120929000217/http://archive.dailycal.org/article/19671/mantis_shrimp_boasts_most_advanced_eyes |url-status=dead}} and smash prey by swinging their club-like raptorial claws.{{cite journal| vauthors = Patek SN, Caldwell RL |year=2005|journal=Journal of Experimental Biology|volume=208|pages=3655–3664|title=Extreme impact and cavitation forces of a biological hammer: strike forces of the peacock mantis shrimp|doi=10.1242/jeb.01831|pmid=16169943|issue=19|s2cid=312009|url=http://www.escholarship.org/uc/item/9938507n }}

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Arthropod vision relies on various combinations of compound eyes and pigment-pit ocelli: in most species the ocelli can only detect the direction from which light is coming, and the compound eyes are the main source of information. Arthropods also have a wide range of chemical and mechanical sensors, mostly based on modifications of the many setae (bristles) that project through their cuticles. Arthropod methods of reproduction are diverse: terrestrial species use some form of internal fertilization while marine species lay eggs using either internal or external fertilization. Arthropod hatchlings vary from miniature adults to grubs that lack jointed limbs and eventually undergo a total metamorphosis to produce the adult form.

{{See also|Evolution of brachiopods}}

In deuterostomes the first opening that develops in the growing embryo becomes the anus, while in protostomes it becomes the mouth. Deuterostomes form a superphylum of animals and are the sister clade of the protostomes. It is once considered that the earliest known deuterostomes are Saccorhytus fossils from about 540 million years ago.{{Cite journal |last1=Han |first1=Jian |last2=Morris |first2=Simon Conway |last3=Ou |first3=Qiang |last4=Shu |first4=Degan |last5=Huang |first5=Hai |date=2017 |title=Meiofaunal deuterostomes from the basal Cambrian of Shaanxi (China) |url=https://www.nature.com/articles/nature21072 |journal=Nature |language=en |volume=542 |issue=7640 |pages=228–231 |doi=10.1038/nature21072 |pmid=28135722 |bibcode=2017Natur.542..228H |s2cid=353780 |issn=1476-4687}} However, another study considered that Saccorhytus is more likely to be an ecdysozoan.{{Cite journal |last1=Liu |first1=Yunhuan |last2=Carlisle |first2=Emily |last3=Zhang |first3=Huaqiao |last4=Yang |first4=Ben |last5=Steiner |first5=Michael |last6=Shao |first6=Tiequan |last7=Duan |first7=Baichuan |last8=Marone |first8=Federica |last9=Xiao |first9=Shuhai |last10=Donoghue |first10=Philip C. J. |date=2022-08-17 |title=Saccorhytus is an early ecdysozoan and not the earliest deuterostome |url=https://www.nature.com/articles/s41586-022-05107-z |journal=Nature |volume=609 |issue=7927 |language=en |pages=541–546 |doi=10.1038/s41586-022-05107-z |pmid=35978194 |bibcode=2022Natur.609..541L |s2cid=251646316 |issn=1476-4687|hdl=1983/454e7bec-4cd4-4121-933e-abeab69e96c1 |hdl-access=free }}

{{clade |label1 = deuterostomes

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File:Haeckel Asteridea Larvae.jpgs have fivefold symmetry but as larvae have bilateral symmetry. This is why they are in the Bilateria.]]

Echinoderms (Greek for spiny skin) is a phylum which contains only marine invertebrates. The phylum contains about 7000 living species,{{cite web|url=http://animaldiversity.ummz.umich.edu/site/accounts/information/Echinodermata.html|title=Animal Diversity Web - Echinodermata|publisher=University of Michigan Museum of Zoology|access-date=26 August 2012}} making it the second-largest grouping of deuterostomes, after the chordates.

Adult echinoderms are recognizable by their radial symmetry (usually five-point) and include starfish, sea urchins, sand dollars, and sea cucumbers, as well as the sea lilies.{{Cite web |date=2023-06-08 |title=Echinoderm {{!}} Definition, Characteristics, Species, & Facts {{!}} Britannica |url=https://www.britannica.com/animal/echinoderm |access-date=2023-06-24 |website=www.britannica.com |language=en}} Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone. They are unique among animals in having bilateral symmetry at the larval stage, but five-fold symmetry (pentamerism, a special type of radial symmetry) as adults.{{cite web | url=http://lanwebs.lander.edu/faculty/rsfox/invertebrates/asterias.html | title=Asterias forbesi | vauthors = Fox R | work=Invertebrate Anatomy OnLine | publisher=Lander University | access-date=14 June 2014}}

Echinoderms are important both biologically and geologically. Biologically, there are few other groupings so abundant in the biotic desert of the deep sea, as well as shallower oceans. Most echinoderms are able to regenerate tissue, organs, limbs, and reproduce asexually; in some cases, they can undergo complete regeneration from a single limb. Geologically, the value of echinoderms is in their ossified skeletons, which are major contributors to many limestone formations, and can provide valuable clues as to the geological environment. They were the most used species in regenerative research in the 19th and 20th centuries.

File:Riccio Melone a Capo Caccia adventurediving.it.jpg|Echinoderm literally means "spiny skin", as this water melon sea urchin illustrates.

File:Ochre sea star.jpg|The ochre sea star was the first keystone predator to be studied. They limit mussels which can overwhelm intertidal communities.Holsinger, K. (2005). Keystone species. Retrieved 10 May 2010, from {{cite web |url=http://darwin.eeb.uconn.edu/eeb310/lecture-notes/interactions/node2.html | vauthors = Holsinger K | date = 11 October 2005 | work = University of Connecticut |title=Keystone species |access-date=2010-05-12 |url-status=dead |archive-url=https://web.archive.org/web/20100630134633/http://darwin.eeb.uconn.edu/eeb310/lecture-notes/interactions/node2.html |archive-date=30 June 2010 |df=dmy-all }}

File:Colorful crinoids at shallow waters of Gili Lawa Laut.JPG|Colorful sea lilies in shallow waters

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File:Espardenya (animal).jpg|Sea cucumbers filter feed on plankton and suspended solids.

File:Scotoplanes globosa and crab (cropped).jpg|The sea pig, a deep water sea cucumber, is the only echinoderm that uses legged locomotion.

File:Enypniastes sp Indonesia.jpg| A benthopelagic and bioluminescent swimming sea cucumber, 3200 meters deep

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It is held by some scientists that the radiation of echinoderms was responsible for the Mesozoic Marine Revolution. Aside from the hard-to-classify Arkarua (a Precambrian animal with echinoderm-like pentamerous radial symmetry), the first definitive members of the phylum appeared near the start of the Cambrian.

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| alt2 =

| caption2 = Gill slits in an acorn worm (left) and tunicate (right)

| footer = Gill slits have been described as "the foremost morphological innovation of early deuterostomes".{{cite journal | vauthors = Simakov O, Kawashima T, Marlétaz F, Jenkins J, Koyanagi R, Mitros T, Hisata K, Bredeson J, Shoguchi E, Gyoja F, Yue JX, Chen YC, Freeman RM, Sasaki A, Hikosaka-Katayama T, Sato A, Fujie M, Baughman KW, Levine J, Gonzalez P, Cameron C, Fritzenwanker JH, Pani AM, Goto H, Kanda M, Arakaki N, Yamasaki S, Qu J, Cree A, Ding Y, Dinh HH, Dugan S, Holder M, Jhangiani SN, Kovar CL, Lee SL, Lewis LR, Morton D, Nazareth LV, Okwuonu G, Santibanez J, Chen R, Richards S, Muzny DM, Gillis A, Peshkin L, Wu M, Humphreys T, Su YH, Putnam NH, Schmutz J, Fujiyama A, Yu JK, Tagawa K, Worley KC, Gibbs RA, Kirschner MW, Lowe CJ, Satoh N, Rokhsar DS, Gerhart J | display-authors = 6 | title = Hemichordate genomes and deuterostome origins | journal = Nature | volume = 527 | issue = 7579 | pages = 459–65 | date = November 2015 | pmid = 26580012 | pmc = 4729200 | doi = 10.1038/nature16150 | bibcode = 2015Natur.527..459S }}{{cite web | url = https://www.futurity.org/acorn-worm-pharynx-1054372-2/ | title = How humans got a pharynx from this 'ugly beast' | work = Futurity | date = 23 November 2015 }} In aquatic organisms, gill slits allow water that enters the mouth during feeding to exit. Some invertebrate chordates also use the slits to filter food from the water.

}}

Hemichordates form a sister phylum to the echinoderms. They are solitary worm-shaped organisms rarely seen by humans because of their lifestyle. They include two main groups, the acorn worms and the Pterobranchia. Pterobranchia form a class containing about 30 species of small worm-shaped animals that live in secreted tubes on the ocean floor. Acorn worms form a class containing about 111 species that generally live in U-shaped burrows on the seabed, from the shoreline to a depth of 3000 meters. The worms lie there with the proboscis sticking out of one opening in the burrow, subsisting as deposit feeders or suspension feeders. It is supposed the ancestors of acorn worms used to live in tubes like their relatives, the Pterobranchia, but eventually started to live a safer and more sheltered existence in sediment burrows.{{Cite web|url=https://phys.org/news/2016-07-secret-oesia-life-prehistoric-worm.html|title=The secret to an Oesia life: Prehistoric worm built tube-like 'houses' on sea floor|website=phys.org}} Some of these worms may grow to be very long; one particular species may reach a length of 2.5 meters (8 ft 2 in), although most acorn worms are much smaller.

Acorn worms are more highly specialized and advanced than other worm-like organisms. They have a circulatory system with a heart that also functions as a kidney. Acorn worms have gill-like structures they use for breathing, similar to the gills of fish. Therefore, acorn worms are sometimes said to be a link between classical invertebrates and vertebrates. Acorn worms continually form new gill slits as they grow in size, and some older individuals have more than a hundred on each side. Each slit consists of a branchial chamber opening to the pharynx through a U-shaped cleft. Cilia push water through the slits, maintaining a constant flow, just as in fish.{{cite book | vauthors = Barnes RD |year=1982 |title= Invertebrate Zoology |publisher= Holt-Saunders International |location= Philadelphia, PA|pages= 1018–1026|isbn= 978-0-03-056747-6}} Some acorn worms also have a postanal tail which may be homologous to the post-anal tail of vertebrates.

The three-section body plan of the acorn worm is no longer present in the vertebrates, except in the anatomy of the frontal neural tube, later developed into a brain divided into three parts. This means some of the original anatomy of the early chordate ancestors is still present in vertebrates even if it is not always visible. One theory is the three-part body originated from an early common ancestor of the deuterostomes, and maybe even from a common bilateral ancestor of both deuterostomes and protostomes. Studies have shown the gene expression in the embryo share three of the same signaling centers that shape the brains of all vertebrates, but instead of taking part in the formation of their neural system,{{cite web | url = http://discovery.lifemapsc.com/library/images/secondary-organizers-of-the-early-brain-and-the-location-of-the-meso-diencephalic-dopaminergic-precursor-cells | title = Secondary organizers of the early brain and the location of the meso-diencephalic dopaminergic precursor cells | work = Life Map | archive-url = https://web.archive.org/web/20140310172544/http://discovery.lifemapsc.com/library/images/secondary-organizers-of-the-early-brain-and-the-location-of-the-meso-diencephalic-dopaminergic-precursor-cells | archive-date=10 March 2014 | access-date = 10 March 2014 }} they are controlling the development of the different body regions.{{cite journal | vauthors = Pani AM, Mullarkey EE, Aronowicz J, Assimacopoulos S, Grove EA, Lowe CJ | title = Ancient deuterostome origins of vertebrate brain signalling centres | journal = Nature | volume = 483 | issue = 7389 | pages = 289–94 | date = March 2012 | pmid = 22422262 | pmc = 3719855 | doi = 10.1038/nature10838 | publisher = ScienceLife | bibcode = 2012Natur.483..289P }}

[[File:Figure 29 01 04.jpg|thumb| The lancelet, like all cephalochordates, has a head. Adult lancelets retain the four key features of chordates: a

notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail. Water from the mouth enters the pharyngeal slits, which filter out food particles. The filtered water then collects in the atrium and exits through the atriopore.{{cite web | url = https://cnx.org/contents/VB2yhrAh@8/Chordates | title = Chordates | work = OpenStax | date = 9 May 2019 }} ]]

The chordate phylum has three subphyla, one of which is the vertebrates (see below). The other two subphyla are marine invertebrates: the tunicates (salps and sea squirts) and the cephalochordates (such as lancelets). Invertebrate chordates are close relatives to vertebrates. In particular, there has been discussion about how closely some extinct marine species, such as Pikaiidae, Palaeospondylus, Zhongxiniscus and Vetulicolia, might relate ancestrally to vertebrates.

File:Amphioxus.png|The lancelet, a small translucent fish-like cephalochordate, is one of the closest living invertebrate relative of the vertebrates.{{cite journal| vauthors = Gewin V |year = 2005|title = Functional genomics thickens the biological plot|journal = PLOS Biology|volume = 3|issue = 6|page = e219|doi = 10.1371/journal.pbio.0030219|pmid=15941356|pmc=1149496 | doi-access=free }}{{cite web | vauthors = Timmer J | url = https://arstechnica.com/science/2008/06/lancelet-amphioxus-genome-and-the-origin-of-vertebrates/ | title = Lancelet (amphioxus) genome and the origin of vertebrates | work = Ars Technica | date = 19 June 2008 }}

File:Ascidian (Rhopalaea Crassa) (4 cm).png| Tunicates, like these fluorescent-colored sea squirts, may provide clues to vertebrate and therefore human ancestry.{{cite journal| vauthors = Lemaire P |year = 2011|title = Evolutionary crossroads in developmental biology: the tunicates|journal = Development|volume = 138|issue = 11|pages = 2143–2152|doi = 10.1242/dev.048975|pmid=21558365|s2cid = 40452112}}

File:Tunicate off Atauro island.jpg| Pyrosomes are free-floating bioluminescent tunicates made up of hundreds of individuals.

File:23 salpchain frierson odfw (8253212250).jpg|Salp chain

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File:Figure 29 01 02.png Modified text was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License]]]

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{{multiple image

| align = right

| direction = vertical

| width = 300

| footer = Skeletal structures showing the vertebral column and internal skeleton running from the head to the tail.

| image1 = Skeleton of a bass.jpg

| caption1 = Ray-finned fish

| image2 = SpermWhaleLyd3.jpg

| caption2 = Marine tetrapod (sperm whale)

}}

{{main|Marine vertebrate}}

Vertebrates (Latin for joints of the spine) are a subphylum of chordates. They are chordates that have a vertebral column (backbone). The vertebral column provides the central support structure for an internal skeleton which gives shape, support, and protection to the body and can provide a means of anchoring fins or limbs to the body. The vertebral column also serves to house and protect the spinal cord that lies within the vertebral column.

Marine vertebrates can be divided into marine fish and marine tetrapods.

{{Further|Fish|diversity of fish|evolution of fish}}

Fish typically breathe by extracting oxygen from water through gills and have a skin protected by scales and mucous. They use fins to propel and stabilise themselves in the water, and usually have a two-chambered heart and eyes well adapted to seeing underwater, as well as other sensory systems. Over 33,000 species of fish have been described as of 2017,{{cite web|url=http://www.fishbase.org/home.htm|title=FishBase: A Global Information System on Fishes|publisher=FishBase|access-date=17 January 2017}} of which about 20,000 are marine fish.{{cite web|title=How Many Fish In The Sea? Census Of Marine Life Launches First Report|url=https://www.sciencedaily.com/releases/2003/10/031024064333.htm|website=Science Daily|access-date=17 January 2017}}

{{clade

|label1= ← vertebrates

|sublabel1= (extant)

|1={{clade |style = float:right;

|label1= jawless fish

|1={{clade

|1= hagfish 70 px

|2={{clade

|1= lampreys 70 px

}}

}}

|label2=jawed fish

|sublabel3=? mya

|2={{clade

|label1=

|1={{clade

|1= cartilaginous fish 70 px

}}

|2={{clade

|1= bony fish 70px →

}}

}}

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}}

File:Tullimonstrum NT small.jpg protruding from its back, may be an early jawless fish.]]

Early fish had no jaws. Most went extinct when they were outcompeted by jawed fish (below), but two groups survived: hagfish and lampreys. Hagfish form a class of about 20 species of eel-shaped, slime-producing marine fish. They are the only known living animals that have a skull but no vertebral column. Lampreys form a superclass containing 38 known extant species of jawless fish.{{cite journal | vauthors = Docker MF | date = November 2006 | url = https://journal.lib.uoguelph.ca/index.php/gir/article/view/61/128 | title = Bill Beamish's Contributions to Lamprey Research and Recent Advances in the Field | journal = Guelph Ichthyology Reviews | volume = 7 }} The adult lamprey is characterized by a toothed, funnel-like sucking mouth. Although they are well known for boring into the flesh of other fish to suck their blood,{{cite book | vauthors = Hardisty MW, Potter IC | veditors = Hardisty MW, Potter IC |year=1971 |title=The Biology of Lampreys |edition=1st |publisher=Academic Press |isbn=978-0-123-24801-5 |url-access=registration |url=https://archive.org/details/biologyoflamprey0001hard }} only 18 species of lampreys are actually parasitic.{{cite journal | vauthors = Gill HS, Renaud CB, Chapleau F, Mayden RL, Potter IC |title=Phylogeny of Living Parasitic Lampreys (Petromyzontiformes) Based on Morphological Data|journal=Copeia|volume=2003|issue=4|pages=687–703|doi=10.1643/IA02-085.1|year=2003|s2cid=85969032}} Together hagfish and lampreys are the sister group to vertebrates. Living hagfish remain similar to hagfish from around 300 million years ago.{{cite web|url=http://www.ucmp.berkeley.edu/vertebrates/basalfish/myxini.html|title=Myxini|publisher=University of California Museum of Paleontology|access-date=17 January 2017|archive-url=https://web.archive.org/web/20171215173214/http://www.ucmp.berkeley.edu/vertebrates/basalfish/myxini.html|archive-date=15 December 2017|url-status=dead}} The lampreys are a very ancient lineage of vertebrates, though their exact relationship to hagfishes and jawed vertebrates is still a matter of dispute.{{cite journal | vauthors = Green SA, Bronner ME | title = The lamprey: a jawless vertebrate model system for examining origin of the neural crest and other vertebrate traits | journal = Differentiation; Research in Biological Diversity | volume = 87 | issue = 1–2 | pages = 44–51 | year = 2014 | pmid = 24560767 | pmc = 3995830 | doi = 10.1016/j.diff.2014.02.001 }} Molecular analysis since 1992 has suggested that hagfish are most closely related to lampreys,{{cite journal | vauthors = Stock DW, Whitt GS | title = Evidence from 18S ribosomal RNA sequences that lampreys and hagfishes form a natural group | journal = Science | volume = 257 | issue = 5071 | pages = 787–9 | date = August 1992 | pmid = 1496398 | doi = 10.1126/science.1496398 | bibcode = 1992Sci...257..787S }} and so also are vertebrates in a monophyletic sense. Others consider them a sister group of vertebrates in the common taxon of craniata.{{cite journal | vauthors = Nicholls H | title = Evolution: Mouth to mouth | journal = Nature | volume = 461 | issue = 7261 | pages = 164–6 | date = September 2009 | pmid = 19741680 | doi = 10.1038/461164a | doi-access = free }}

The Tully monster is an extinct genus of soft-bodied bilaterians that lived in tropical estuaries about 300 million years ago. Since 2016 there has been controversy over whether this animal was a vertebrate or an invertebrate.{{cite journal | vauthors = McCoy VE, Saupe EE, Lamsdell JC, Tarhan LG, McMahon S, Lidgard S, Mayer P, Whalen CD, Soriano C, Finney L, Vogt S, Clark EG, Anderson RP, Petermann H, Locatelli ER, Briggs DE | display-authors = 6 | title = The 'Tully monster' is a vertebrate | journal = Nature | volume = 532 | issue = 7600 | pages = 496–9 | date = April 2016 | pmid = 26982721 | doi = 10.1038/nature16992 | s2cid = 205247805 | bibcode = 2016Natur.532..496M | url = https://ora.ox.ac.uk/objects/uuid:a6b3721a-819f-4bb6-9e78-e219362fcae9 }}{{cite journal |title=The 'Tully Monster' is not a vertebrate: characters, convergence and taphonomy in Palaeozoic problematic animals |journal=Palaeontology | vauthors = Sallan L, Giles S, Sansom RS, Clarke JT, Johanson Z, Sansom IJ, Janvier P |display-authors=6 |date=February 20, 2017 |doi=10.1111/pala.12282 |volume=60 |issue=2 |pages=149–157|bibcode=2017Palgy..60..149S |url=http://pure-oai.bham.ac.uk/ws/files/39492257/Tullimonstrum_Palaeo_Revision_FINAL.pdf |doi-access=free }} In 2020 researchers found "strong evidence" that the Tully monster was a vertebrate, and was a jawless fish in the lineage of the lamprey,{{cite web | vauthors = Geggel L | url = https://www.livescience.com/ancient-tully-monster-vertebrate.html| title = Ancient 'Tully monster' was a vertebrate, not a spineless blob, study claims | work = Live Science | date = 4 May 2020 }}{{cite journal | vauthors = McCoy VE, Wiemann J, Lamsdell JC, Whalen CD, Lidgard S, Mayer P, Petermann H, Briggs DE | display-authors = 6 | title = Chemical signatures of soft tissues distinguish between vertebrates and invertebrates from the Carboniferous Mazon Creek Lagerstätte of Illinois | journal = Geobiology | volume = 18 | issue = 5 | pages = 560–565 | date = September 2020 | pmid = 32347003 | doi = 10.1111/gbi.12397 | bibcode = 2020Gbio...18..560M | s2cid = 216646333 }} while in 2023 other researchers found 3D fossils scans did not support those conclusions.{{Cite journal |last1=Mikami |first1=Tomoyuki |last2=Ikeda |first2=Takafumi |last3=Muramiya |first3=Yusuke |last4=Hirasawa |first4=Tatsuya |last5=Iwasaki |first5=Wataru |date=2023 |editor-last=Cherns |editor-first=Lesley |title=Three-dimensional anatomy of the Tully monster casts doubt on its presumed vertebrate affinities |url=https://onlinelibrary.wiley.com/doi/10.1111/pala.12646 |journal=Palaeontology |language=en |volume=66 |issue=2 |page=12646 |doi=10.1111/pala.12646 |bibcode=2023Palgy..6612646M |s2cid=258198566 |issn=0031-0239}}

File:Eptatretus polytrema.jpg|Hagfish are the only known living animals with a skull but no vertebral column.

File:Eudontomyzon mariae Dunai ingola.jpg|Lampreys are often parasitic and have a toothed, funnel-like sucking mouth.

File:Pteraspidomorphi.gif|The extinct Pteraspidomorphi, ancestral to jawed vertebrates

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Pteraspidomorphi is an extinct class of early jawless fish ancestral to jawed vertebrates. The few characteristics they share with the latter are now considered as primitive for all vertebrates.

Around the start of the Devonian, fish started appearing with a deep remodelling of the vertebrate skull that resulted in a jaw.{{cite journal | vauthors = Kimmel CB, Miller CT, Keynes RJ | title = Neural crest patterning and the evolution of the jaw | journal = Journal of Anatomy | volume = 199 | issue = Pt 1-2 | pages = 105–20 | date = 2001 | pmid = 11523812 | pmc = 1594948 | doi = 10.1017/S0021878201008068 }}

All vertebrate jaws, including the human jaw, have evolved from these early fish jaws. The appearance of the early vertebrate jaw has been described as "perhaps the most profound and radical evolutionary step in vertebrate history".{{cite journal | vauthors = Gai Z, Zhu M | date = 2012 | title = The origin of the vertebrate jaw: Intersection between developmental biology-based model and fossil evidence | journal = Chinese Science Bulletin | volume = 57 | issue = 30| pages = 3819–3828 | doi = 10.1007/s11434-012-5372-z | bibcode = 2012ChSBu..57.3819G | doi-access = free }}{{cite book | vauthors = Maisey JG | date = 2000 | url = {{google books|id=gAiAPwAACAAJ|plainurl=yes}} | title = Discovering Fossil Fishes | publisher = Westview Press | pages = 1–223 | isbn = 978-0-8133-3807-1 }} Jaws make it possible to capture, hold, and chew prey. Fish without jaws had more difficulty surviving than fish with jaws, and most jawless fish became extinct during the Triassic period.

{{main|Cartilaginous fish}}

Jawed fish fall into two main groups: fish with bony internal skeletons and fish with cartilaginous internal skeletons. Cartilaginous fish, such as sharks and rays, have jaws and skeletons made of cartilage rather than bone. Megalodon is an extinct species of shark that lived about 28 to 1.5 Ma. It may looked much like a stocky version of the great white shark, but was much larger with estimated lengths reaching {{convert|20.3|m|ft}}. Found in all oceans{{cite journal | vauthors = Pimiento C, Ehret DJ, Macfadden BJ, Hubbell G | title = Ancient nursery area for the extinct giant shark megalodon from the Miocene of Panama | journal = PLOS ONE | volume = 5 | issue = 5 | pages = e10552 | date = May 2010 | pmid = 20479893 | pmc = 2866656 | doi = 10.1371/journal.pone.0010552 | veditors = Stepanova A | bibcode = 2010PLoSO...510552P | doi-access = free }} it was one of the largest and most powerful predators in vertebrate history,{{cite journal | vauthors = Wroe S, Huber DR, Lowry M, McHenry C, Moreno K, Clausen P, Ferrara TL, Cunningham E, Dean MN, Summers AP | display-authors = 6 |title=Three-dimensional computer analysis of white shark jaw mechanics: how hard can a great white bite? |url=http://www.bio-nica.info/Biblioteca/Wroe2008GreatWhiteSharkBiteForce.pdf |journal=Journal of Zoology |volume=276 |issue=4 |pages=336–342 |year=2008 |doi=10.1111/j.1469-7998.2008.00494.x}} and probably had a profound impact on marine life.{{cite journal | vauthors = Lambert O, Bianucci G, Post K, de Muizon C, Salas-Gismondi R, Urbina M, Reumer J | title = The giant bite of a new raptorial sperm whale from the Miocene epoch of Peru | journal = Nature | volume = 466 | issue = 7302 | pages = 105–8 | date = July 2010 | pmid = 20596020 | doi = 10.1038/nature09067 | s2cid = 4369352 | bibcode = 2010Natur.466..105L }} The Greenland shark has the longest known lifespan of all vertebrates, about 400 years.{{cite journal | vauthors = Nielsen J, Hedeholm RB, Heinemeier J, Bushnell PG, Christiansen JS, Olsen J, Ramsey CB, Brill RW, Simon M, Steffensen KF, Steffensen JF | display-authors = 6 | title = Eye lens radiocarbon reveals centuries of longevity in the Greenland shark (Somniosus microcephalus) | journal = Science | volume = 353 | issue = 6300 | pages = 702–4 | date = August 2016 | pmid = 27516602 | doi = 10.1126/science.aaf1703 | s2cid = 206647043 | bibcode = 2016Sci...353..702N | hdl = 2022/26597 | hdl-access = free }}

• {{cite magazine |author=Enrico de Lazaro |date=12 August 2016 |title=Greenland Sharks are Longest-Lived Vertebrates on Earth, Marine Biologists Say |magazine=Science News |url=http://www.sci-news.com/biology/greenland-sharks-longest-lived-vertebrates-04099.html}} Some sharks such as the great white are partially warm blooded and give live birth. The manta ray, largest ray in the world, has been targeted by fisheries and is now vulnerable.{{Cite iucn | vauthors = Marshall A, Bennett MB, Kodja G, Hinojosa-Alvarez S, Galvan-Magana F, Harding M, Stevens G, Kashiwagi T | display-authors = 6 | title = Manta birostris | volume = 2011 | page = e.T198921A9108067 | date = 2011 | doi = 10.2305/IUCN.UK.2011-2.RLTS.T198921A9108067.en }}

File:Acanthodes BW spaced.jpg|Cartilaginous fishes may have evolved from spiny sharks.

File:Myliobatis aquila sasrája.jpg|Stingray

File:MantaAlfrediLCouterier.jpg| Manta ray, the largest ray

File:Pristis clavata 2.jpg|Sawfish, rays with long rostrums resembling a saw. All species are now endangered.{{cite news | vauthors = Black R |date=11 June 2007|title=Sawfish protection acquires teeth|work=BBC News|url=http://news.bbc.co.uk/2/hi/science/nature/6740609.stm}}

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File:Megalodon_size_chart.png|The extinct megalodon resembled a giant great white shark.

File:Somniosus microcephalus1.jpg|The Greenland shark lives longer than any other vertebrate.

File:Rhincodon typus (recropped).jpg| The largest extant fish, the whale shark, is now a vulnerable species.

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File:Guiyu BW.jpg, the earliest-known bony fish lived during the Late Silurian 419 million years ago.]]

File:Coelacanth-bgiu.png

File:Carassius wild golden fish 2013 G1 (2).jpg

{{Further|Bony fish}}

Bony fish have jaws and skeletons made of bone rather than cartilage. Bony fish also have hard, bony plates called operculum which help them respire and protect their gills, and they often possess a swim bladder which they use for better control of their buoyancy. Bony fish can be further divided into those with lobe fins and those with ray fins. The approximate dates in the phylogenetic tree are from Near et al., 2012{{cite journal | vauthors = Near TJ, Eytan RI, Dornburg A, Kuhn KL, Moore JA, Davis MP, Wainwright PC, Friedman M, Smith WL | display-authors = 6 | title = Resolution of ray-finned fish phylogeny and timing of diversification | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 109 | issue = 34 | pages = 13698–703 | date = August 2012 | pmid = 22869754 | pmc = 3427055 | doi = 10.1073/pnas.1206625109 | bibcode = 2012PNAS..10913698N | doi-access = free }} and Zhu et al., 2009.

{{clade

|label1= ← bony fish

|sublabel1= (extant)

|1={{clade

|label1=Sarcopterygii

|sublabel1= 419 mya

|1={{clade

|1=coelacanths 70 px

|2={{clade

|1=lungfish 70 px

|2={{clade

|1=tetrapods →

}}

}}

}}

|label2=Actinopterygii

|sublabel2= 400 mya

|2={{clade

|label1=chondrosteans

|1=70 px (sturgeon, paddlefish, bichir, reedfish)

|label2=neopterygians

|sublabel2= 360 mya

|2={{clade

|1=70 px (bowfin, gars)

|label1 =holosteans

|sublabel1= 275 mya

|2=all remaining fish (about 14,000 marine species)

|label2=teleosts

|sublabel2= 310 mya

}}

}}

}}

}}

Lobe fins have the form of fleshy lobes supported by bony stalks which extend from the body.Clack, J. A. (2002) Gaining Ground. Indiana University Guiyu oneiros, the earliest-known bony fish, lived during the Late Silurian 419 million years ago. It has the combination of both ray-finned and lobe-finned features, although analysis of the totality of its features place it closer to lobe-finned fish.{{cite journal | vauthors = Zhu M, Zhao W, Jia L, Lu J, Qiao T, Qu Q | title = The oldest articulated osteichthyan reveals mosaic gnathostome characters | journal = Nature | volume = 458 | issue = 7237 | pages = 469–74 | date = March 2009 | pmid = 19325627 | doi = 10.1038/nature07855 | s2cid = 669711 | bibcode = 2009Natur.458..469Z }} Lobe fins evolved into the legs of the first tetrapod land vertebrates, so by extension an early ancestor of humans was a lobe-finned fish. Apart from the coelacanths and the lungfishes, lobe-finned fishes are now extinct.

The remaining bony fish have ray fins. These are made of webs of skin supported by bony or horny spines (rays) which can be erected to control the fin stiffness.

• The main distinguishing feature of the chondrosteans (sturgeon, paddlefish, bichir and reedfish) is the cartilaginous nature of their skeletons. The ancestors of the chondrosteans are thought to be bony fish, but the characteristic of an ossified skeleton was lost in later evolutionary development, resulting in a lightening of the frame.{{cite web |url=http://www.palaeos.com/Vertebrates/Units/090Teleostomi/090.300.html |title=Chondrosteans: Sturgeon Relatives |publisher=paleos.com |url-status=dead |archive-url=https://web.archive.org/web/20101225152809/http://www.palaeos.com/Vertebrates/Units/090Teleostomi/090.300.html |archive-date=2010-12-25 }}
• Neopterygians (from Greek for new fins) appeared sometime in the Late Permian, before dinosaurs. They were a very successful group of fish, because they could move more rapidly than their ancestors. Their scales and skeletons began to lighten during their evolution, and their jaws became more powerful and efficient.{{cite journal | vauthors = López-Arbarello A | title = Phylogenetic interrelationships of ginglymodian fishes (Actinopterygii: Neopterygii) | journal = PLOS ONE | volume = 7 | issue = 7 | pages = e39370 | year = 2012 | pmid = 22808031 | pmc = 3394768 | doi = 10.1371/journal.pone.0039370 | bibcode = 2012PLoSO...739370L | doi-access = free }}

File:Barb gonio 080525 9610 ltn Cf.jpg.]]

{{main|Teleost}}

About 96% of all modern fish species are teleosts,{{cite book| vauthors = Berra TM |title=Freshwater Fish Distribution|url=https://books.google.com/books?id=K-1Ygw6XwFQC&pg=PA55|year=2008|publisher=University of Chicago Press|isbn=978-0-226-04443-9|page=55}} of which about 14,000 are marine species.{{cite journal | vauthors = Lackmann AR, Andrews AH, Butler MG, Bielak-Lackmann ES, Clark ME | title = Bigmouth Buffalo Ictiobus cyprinellus sets freshwater teleost record as improved age analysis reveals centenarian longevity | language = En | journal = Communications Biology | volume = 2 | issue = 1 | pages = 197 | date = 2019-05-23 | pmid = 31149641 | pmc = 6533251 | doi = 10.1038/s42003-019-0452-0 }} Teleosts can be distinguished from other bony fish by their possession of a homocercal tail, a tail where the upper half mirrors the lower half. Another difference lies in their jaw bones – teleosts have modifications in the jaw musculature which make it possible for them to protrude their jaws. This enables them to grab prey and draw it into their mouth.{{cite book| vauthors = Benton M |title=Vertebrate Palaeontology|chapter-url=https://books.google.com/books?id=VThUUUtM8A4C&pg=PA175|year=2005|publisher=John Wiley & Sons|edition=3rd|chapter=The Evolution of Fishes After the Devonian|isbn=978-1-4051-4449-0|pages=175–84}} In general, teleosts tend to be quicker and more flexible than more basal bony fishes. Their skeletal structure has evolved towards greater lightness. While teleost bones are well calcified, they are constructed from a scaffolding of struts, rather than the dense cancellous bones of holostean fish.{{cite book| vauthors = Bone Q, Moore R |year=2008|title=Biology of Fishes|publisher=Garland Science|page=29|isbn=978-0-415-37562-7}}

Teleosts are found in almost all marine habitats.{{cite book|title=Zoology| vauthors = Dorit R, Walker WF, Barnes RD |year=1991|publisher=Saunders College Publishing|isbn=978-0-03-030504-7|pages=[https://archive.org/details/zoology0000dori/page/67 67–69]|url=https://archive.org/details/zoology0000dori/page/67}} They have enormous diversity, and range in size from adult gobies 8mm long {{cite web|url=https://scripps.ucsd.edu/news/2645|title=Scientists Describe the World's Smallest, Lightest Fish|date=20 July 2004|publisher=Scripps Institution of Oceanography|access-date=9 April 2016|archive-date=5 March 2016|archive-url=https://web.archive.org/web/20160305095456/https://scripps.ucsd.edu/news/2645|url-status=dead}} to ocean sunfish weighing over 2,000 kg.{{cite news|title=World's Heaviest Bony Fish Discovered? | vauthors = Roach J |url= http://news.nationalgeographic.com/news/2003/05/0513_030513_sunfish.html |archive-url= https://web.archive.org/web/20030517062722/http://news.nationalgeographic.com/news/2003/05/0513_030513_sunfish.html |url-status= dead |archive-date= 17 May 2003 |newspaper=National Geographic News|date=13 May 2003|access-date=9 January 2016}} The following images show something of the diversity in the shape and colour of modern marine teleosts...

File:Istiophorus platypterus.jpg|Sailfish

File:Anguilla japonica 1856.jpg|Eel

File:Seepferdlein.jpg|Seahorse

File:Sunfish.jpg|Ocean sunfish

File:Humpback anglerfish.png|Anglerfish

File:Tetraodon-hispidus.jpg|Pufferfish

File:Synchiropus splendidus 2 Luc Viatour cropped.png|Mandarin dragonet

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Nearly half of all extant vertebrate species are teleosts.{{cite web | publisher = The World Conservation Union | work = IUCN Red List of Threatened Species | date = Autumn 2014 | title = Summary Statistics for Globally Threatened Species | url = http://cmsdocs.s3.amazonaws.com/summarystats/2014_3_Summary_Stats_Page_Documents/2014_3_RL_Stats_Table_1.pdf | quote = Table 1: Numbers of threatened species by major groups of organisms (1996–2014) }}

{{See also|Tetrapods|evolution of tetrapods}}

File:Tiktaalik BW flopped.jpg, an extinct lobe-finned fish, developed limb-like fins that could take it onto land.]]

A tetrapod (Greek for four feet) is a vertebrate with limbs (feet). Tetrapods evolved from ancient lobe-finned fishes about 400 million years ago during the Devonian Period when their earliest ancestors emerged from the sea and adapted to living on land.{{cite journal| vauthors = Narkiewicz K, Narkiewicz M |title=The age of the oldest tetrapod tracks from Zachełmie, Poland|journal=Lethaia|volume=48|issue=1|date=January 2015|pages=10–12|issn=0024-1164|doi=10.1111/let.12083|bibcode=2015Letha..48...10N }} This change from a body plan for breathing and navigating in gravity-neutral water to a body plan with mechanisms enabling the animal to breathe in air without dehydrating and move on land is one of the most profound evolutionary changes known.{{cite journal | vauthors = Long JA, Gordon MS | title = The greatest step in vertebrate history: a paleobiological review of the fish-tetrapod transition | journal = Physiological and Biochemical Zoology | volume = 77 | issue = 5 | pages = 700–19 | date = Sep–Oct 2004 | pmid = 15547790 | doi = 10.1086/425183 | s2cid = 1260442 | url = http://usf.usfca.edu/fac_staff/dever/tetrapod_review.pdf }}{{cite book | vauthors = Shubin N |author-link=Neil Shubin |title=Your Inner Fish: A Journey Into the 3.5-Billion-Year History of the Human Body |publisher=Pantheon Books |location=New York |year=2008 |isbn=978-0-375-42447-2 |url=https://archive.org/details/yourinnerfishjou00shub_0 }} Tetrapods can be divided into four classes: amphibians, reptiles, birds and mammals.

{{clade

|label1=← tetrapods

|1={{clade

|1=amphibians (there are no true marine amphibians)

|label2=amniotes

|2={{clade

|1=mammals 40 px

|label2=sauropsids

|2={{clade

|1= lepidosaurs 20px (lizards, including snakes)

|2= archosaurs 20px (turtles, crocodiles & birds)

}}

}}

}}

}}

Marine tetrapods are tetrapods that returned from land back to the sea again. The first returns to the ocean may have occurred as early as the Carboniferous Period{{cite book | vauthors = Laurin M |title=How Vertebrates Left the Water |publisher=University of California Press |location=Berkeley, California, USA. |year=2010 |isbn=978-0-520-26647-6 |author-link=Michel Laurin }} whereas other returns occurred as recently as the Cenozoic, as in cetaceans, pinnipeds,{{cite journal | vauthors = Canoville A, Laurin M |year=2010 |title=Evolution of humeral microanatomy and lifestyle in amniotes, and some comments on paleobiological inferences |journal=Biological Journal of the Linnean Society |volume=100 |pages=384–406 |doi=10.1111/j.1095-8312.2010.01431.x |issue=2 |doi-access=free }} and several modern amphibians.{{cite journal | vauthors = Laurin M, Canoville A, Quilhac A | title = Use of paleontological and molecular data in supertrees for comparative studies: the example of lissamphibian femoral microanatomy | journal = Journal of Anatomy | volume = 215 | issue = 2 | pages = 110–23 | date = August 2009 | pmid = 19508493 | pmc = 2740958 | doi = 10.1111/j.1469-7580.2009.01104.x | author-link = Michel Laurin }} Amphibians (from Greek for both kinds of life) live part of their life in water and part on land. They mostly require fresh water to reproduce. A few inhabit brackish water, but there are no true marine amphibians.{{cite journal | vauthors = Hopkins GR, Brodie Jr ED | year = 2015 | title = Occurrence of Amphibians in Saline Habitats: A Review and Evolutionary Perspective | journal = Herpetological Monographs | volume = 29 | issue = 1| pages = 1–27 | doi = 10.1655/HERPMONOGRAPHS-D-14-00006 |s2cid=83659304 }} There have been reports, however, of amphibians invading marine waters, such as a Black Sea invasion by the natural hybrid Pelophylax esculentus reported in 2010.{{cite journal |vauthors=Natchev N, Tzankov N, Geme R |year=2011 |title=Green frog invasion in the Black Sea: habitat ecology of the Pelophylax esculentus complex (Anura, Amphibia) population in the region of Shablenska Tuzla lagoon in Bulgaria |journal=Herpetology Notes |volume=4 |pages=347–351 |url=http://www.herpetologynotes.seh-herpetology.org/Volume4_PDFs/Natchev_et_al_Herpetology_Notes_Volume4_pages347-351.pdf |access-date=11 August 2016 |archive-date=24 September 2015 |archive-url=https://web.archive.org/web/20150924025946/http://www.herpetologynotes.seh-herpetology.org/Volume4_PDFs/Natchev_et_al_Herpetology_Notes_Volume4_pages347-351.pdf |url-status=dead }}

{{Main|Marine reptile}}

{{See also|Evolution of reptiles}}

Reptiles (Late Latin for creeping or crawling) do not have an aquatic larval stage, and in this way are unlike amphibians. Most reptiles are oviparous, although several species of squamates are viviparous, as were some extinct aquatic clades{{cite journal | vauthors = Sander PM | title = Paleontology. Reproduction in early amniotes | journal = Science | volume = 337 | issue = 6096 | pages = 806–8 | date = August 2012 | pmid = 22904001 | doi = 10.1126/science.1224301 | s2cid = 7041966 | bibcode = 2012Sci...337..806S }} — the fetus develops within the mother, contained in a placenta rather than an eggshell. As amniotes, reptile eggs are surrounded by membranes for protection and transport, which adapt them to reproduction on dry land. Many of the viviparous species feed their fetuses through various forms of placenta analogous to those of mammals, with some providing initial care for their hatchlings.

Some reptiles are more closely related to birds than other reptiles, and many scientists prefer to make Reptilia a monophyletic group which includes the birds.{{cite journal | vauthors = Modesto SP, Anderson JS | title = The phylogenetic definition of reptilia | journal = Systematic Biology | volume = 53 | issue = 5 | pages = 815–21 | date = October 2004 | pmid = 15545258 | doi = 10.1080/10635150490503026 | doi-access = free }}{{open access}}{{cite book | vauthors = Gauthier JA, Kluge AG, Rowe T |year=1988 |chapter=The early evolution of the Amniota |title=The Phylogeny and Classification of the Tetrapods |volume=1 | veditors = Benton MJ |publisher=Clarendon Press |location=Oxford |isbn=978-0-19-857705-8 |pages=103–155}}{{Cite journal | vauthors = Laurin M, Reisz RR |year=1995 |title=A reevaluation of early amniote phylogeny |journal=Zoological Journal of the Linnean Society |volume=113 |issue=2 |pages=165–223 |doi=10.1111/j.1096-3642.1995.tb00932.x |url=http://www.iucn-tftsg.org/wp-content/uploads/file/Articles/Laurin_and_Reisz_1995.pdf |access-date=14 August 2016 |archive-url=https://web.archive.org/web/20190608150549/http://www.iucn-tftsg.org/wp-content/uploads/file/Articles/Laurin_and_Reisz_1995.pdf |archive-date=8 June 2019 |url-status=dead }}{{open access}}{{cite journal | vauthors = Modesto SP |year=1999 |title=Observations of the structure of the Early Permian reptile Stereosternum tumidum Cope |journal=Palaeontologia Africana |volume=35 |pages=7–19 }} Extant non-avian reptiles which inhabit or frequent the sea include sea turtles, sea snakes, terrapins, the marine iguana, and the saltwater crocodile. Currently, of the approximately 12,000 extant reptile species and sub-species, only about 100 of are classed as marine reptiles.{{cite journal | vauthors = Rasmussen AR, Murphy JC, Ompi M, Gibbons JW, Uetz P | title = Marine reptiles | journal = PLOS ONE | volume = 6 | issue = 11 | pages = e27373 | date = 2011-11-08 | pmid = 22087300 | pmc = 3210815 | doi = 10.1371/journal.pone.0027373 | bibcode = 2011PLoSO...627373R | doi-access = free }}

Except for some sea snakes, most extant marine reptiles are oviparous and need to return to land to lay their eggs. Apart from sea turtles, the species usually spend most of their lives on or near land rather than in the ocean. Sea snakes generally prefer shallow waters nearby land, around islands, especially waters that are somewhat sheltered, as well as near estuaries.{{cite book | vauthors = Stidworthy J | date = 1974 | title = Snakes of the World | publisher = Grosset & Dunlap Inc. | pages = 160 | isbn = 978-0-448-11856-7}}{{cite web | url = http://www.fao.org/3/y0870e/y0870e65.pdf | title = Sea snakes | publisher = Food and Agriculture Organization of the United Nations | access-date = 22 August 2020 }} Unlike land snakes, sea snakes have evolved flattened tails which help them swim.{{cite journal | vauthors = Rasmussen AR, Murphy JC, Ompi M, Gibbons JW, Uetz P | title = Marine reptiles | journal = PLOS ONE | volume = 6 | issue = 11 | pages = e27373 | year = 2011 | pmid = 22087300 | pmc = 3210815 | doi = 10.1371/journal.pone.0027373 | bibcode = 2011PLoSO...627373R | doi-access = free }}

File:Marine-Iguana-Espanola.jpg|Marine iguana

File:Leatherback sea turtle Tinglar, USVI (5839996547).jpg|Leatherback sea turtle

File:SaltwaterCrocodile('Maximo').jpg|Saltwater crocodile

File:Micrurus fulviusHolbrookV3P10AA.jpg|Marine snakes have flattened tails.

File:Ichthyosaurus BW.jpg|The ancient Ichthyosaurus communis independently evolved flippers similar to dolphins.

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Some extinct marine reptiles, such as ichthyosaurs, evolved to be viviparous and had no requirement to return to land. Ichthyosaurs resembled dolphins. They first appeared about 245 million years ago and disappeared about 90 million years ago. The terrestrial ancestor of the ichthyosaur had no features already on its back or tail that might have helped along the evolutionary process. Yet the ichthyosaur developed a dorsal and tail fin which improved its ability to swim.Martill D.M. (1993). "Soupy Substrates: A Medium for the Exceptional Preservation of Ichthyosaurs of the Posidonia Shale (Lower Jurassic) of Germany". Kaupia - Darmstädter Beiträge zur Naturgeschichte, 2 : 77-97. The biologist Stephen Jay Gould said the ichthyosaur was his favourite example of convergent evolution.{{cite book | vauthors = Gould SJ | date = 1993 | chapter-url = https://archive.org/details/eightlittlepiggi00goul | chapter = Bent Out of Shape | title = Eight Little Piggies: Reflections in Natural History | publisher = Norton | pages = 179–94 | isbn = 978-0-393-31139-6 }} The earliest marine reptiles arose in the Permian. During the Mesozoic many groups of reptiles became adapted to life in the seas, including ichthyosaurs, plesiosaurs, mosasaurs, nothosaurs, placodonts, sea turtles, thalattosaurs and thalattosuchians. Marine reptiles were less numerous after mass extinction at the end of the Cretaceous.

{{Main|Seabird}}

File:Chesapeake Waterbird Food Web.jpg]]

Marine birds are adapted to life within the marine environment. They are often called seabirds. While marine birds vary greatly in lifestyle, behaviour and physiology, they often exhibit striking convergent evolution, as the same environmental problems and feeding niches have resulted in similar adaptations. Examples include albatross, penguins, gannets, and auks.

In general, marine birds live longer, breed later and have fewer young than terrestrial birds do, but they invest a great deal of time in their young. Most species nest in colonies, which can vary in size from a few dozen birds to millions. Many species are famous for undertaking long annual migrations, crossing the equator or circumnavigating the Earth in some cases. They feed both at the ocean's surface and below it, and even feed on each other. Marine birds can be highly pelagic, coastal, or in some cases spend a part of the year away from the sea entirely. Some marine birds plummet from heights, plunging through the water leaving vapour-like trails, similar to that of fighter planes.{{cite web|url=http://www.lifeinthefastlane.ca/sardine-run-shark-feeding-frenzy-phenomenon-in-africa/miraculous-things |title=Sardine Run Shark Feeding Frenzy Phenomenon in Africa |url-status=dead |archive-url=https://web.archive.org/web/20081202092952/http://www.lifeinthefastlane.ca/sardine-run-shark-feeding-frenzy-phenomenon-in-africa/miraculous-things |archive-date=2 December 2008 }} Gannets plunge into the water at up to 100 kilometres per hour (60 mph). They have air sacs under their skin in their face and chest which act like bubble-wrap, cushioning the impact with the water.

File:Goéland argenté - Julien Salmon.jpg|European herring gull attack herring schools from above.

File:Pygoscelis papua -Nagasaki Penguin Aquarium -swimming underwater-8a.jpg|Gentoo penguin swimming underwater

File:Royal Albatross - east of the Tasman Peninsula, Tasmania.jpg|Albatrosses range over huge areas of ocean and some even circle the globe.

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The first marine birds evolved in the Cretaceous period, and modern marine bird families emerged in the Paleogene.

File:sea otter with sea urchin.jpg, a classic keystone species which controls sea urchin numbers]]

{{main| Marine mammal}}

{{See also|Evolution of cetaceans|Evolution of sirenians|List of marine mammal species}}

Mammals (from Latin for breast) are characterised by the presence of mammary glands which in females produce milk for feeding (nursing) their young. There are about 130 living and recently extinct marine mammal species such as seals, dolphins, whales, manatees, sea otters and polar bears.{{cite web|url=https://www.marinemammalscience.org/species-information/list-of-marine-mammal-species-subspecies |title=The Society for Marine Mammalogy's Taxonomy Committee List of Species and subspecies |publisher=Society for Marine Mammalogy |date=October 2015 |access-date=23 November 2015 |url-status=dead |archive-url=https://web.archive.org/web/20150106152733/https://www.marinemammalscience.org/species-information/list-of-marine-mammal-species-subspecies/ |archive-date=6 January 2015 }} They do not represent a distinct taxon or systematic grouping, but are instead unified by their reliance on the marine environment for feeding. Both cetaceans and sirenians are fully aquatic and therefore are obligate water dwellers. Seals and sea-lions are semiaquatic; they spend the majority of their time in the water, but need to return to land for important activities such as mating, breeding and molting. In contrast, both otters and the polar bear are much less adapted to aquatic living. Their diet varies considerably as well: some may eat zooplankton; others may eat fish, squid, shellfish, and sea-grass; and a few may eat other mammals.

In a process of convergent evolution, marine mammals, especially cetaceans such as dolphins and whales, redeveloped their body plan to parallel the streamlined fusiform body plan of pelagic fish. Front legs became flippers and back legs disappeared, a dorsal fin reappeared and the tail morphed into a powerful horizontal fluke. This body plan is an adaptation to being an active predator in a high drag environment. A parallel convergence occurred with the now extinct marine reptile ichthyosaur.{{cite book | vauthors = Romer AS, Parsons TS | date = 1986 | title = The Vertebrate Body | page = 96 | publisher = Sanders College Publishing | isbn = 978-0-03-058446-6 }}

File:Bluewhale2 noaa.jpg|Endangered blue whale, the largest living animal{{cite web|url=http://wwf.panda.org/what_we_do/endangered_species/cetaceans/about/blue_whale/|title=Blue whale|publisher=World Wide Fund For Nature|access-date=15 August 2016}}

File:Tursiops truncatus 01.jpg|The bottlenose dolphin has the highest encephalization of any animal after humans{{cite journal| vauthors = Marino L |title=Cetacean Brain Evolution: Multiplication Generates Complexity|journal=International Society for Comparative Psychology|issue=17|pages=1–16 |year=2004 |url=http://www.cogs.indiana.edu/spackled/2005readings/CetaceanBrainEvolution.pdf |access-date=15 August 2016|archive-url= https://web.archive.org/web/20180916132752/http://www.cogs.indiana.edu/spackled/2005readings/CetaceanBrainEvolution.pdf |archive-date=16 September 2018|url-status=dead}}

File:Beluga03.jpg|Beluga whale

File:Noaa-walrus22.jpg|Walrus

File:Polar Bear - Alaska (cropped).jpg|Polar bear

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File:Seawifs global biosphere.jpg and terrestrial vegetation. Dark red and blue-green indicate regions of high photosynthetic activity in the ocean and on land, respectively.]]

{{main|marine primary production}}

{{See also|evolution of photosynthesis}}

Primary producers are the autotroph organisms that make their own food instead of eating other organisms. This means primary producers become the starting point in the food chain for heterotroph organisms that do eat other organisms. Some marine primary producers are specialised bacteria and archaea which are chemotrophs, making their own food by gathering around hydrothermal vents and cold seeps and using chemosynthesis. However most marine primary production comes from organisms which use photosynthesis on the carbon dioxide dissolved in the water. This process uses energy from sunlight to convert water and carbon dioxide{{cite book | vauthors = Campbell NA, Reece JB, Urry LA, Cain ML, Wasserman SA, Minorsky PV, Jackson RB | title = Biology | url = https://archive.org/details/essentialbiology00camp_0 | url-access = registration | edition = 8 | year = 2008 | publisher = Pearson – Benjamin Cummings | location = San Francisco | isbn = 978-0-321-54325-7 }}{{rp|186–187}} into sugars that can be used both as a source of chemical energy and of organic molecules that are used in the structural components of cells.{{rp|1242}} Marine primary producers are important because they underpin almost all marine animal life by generating most of the oxygen and food that provide other organisms with the chemical energy they need to exist.

The principal marine primary producers are cyanobacteria, algae and marine plants. The oxygen released as a by-product of photosynthesis is needed by nearly all living things to carry out cellular respiration. In addition, primary producers are influential in the global carbon and water cycles. They stabilize coastal areas and can provide habitats for marine animals. The term division has been traditionally used instead of phylum when discussing primary producers, but the International Code of Nomenclature for algae, fungi, and plants now accepts both terms as equivalents.{{Cite book |year=2012 | veditors = Barrie FR, Buck WR, Demoulin V, Greuter W, Hawksworth DL, Herendeen PS, Knapp S, Marhold K, Prado J, Prud'homme Van Reine WF | display-editors = 6 |title=International Code of Nomenclature for algae, fungi, and plants (Melbourne Code), Adopted by the Eighteenth International Botanical Congress Melbourne, Australia, July 2011 |edition=electronic |publisher=International Association for Plant Taxonomy |url=http://www.iapt-taxon.org/nomen/main.php?page=art3 |access-date=2017-05-14 }}

{{multiple image

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| width = 180

| header = Cyanobacteria

| image1 = Cyanobacteria guerrero negro.jpg

| caption1 = Cyanobacteria from a microbial mat. Cyanobacteria were the first organisms to release oxygen via photosynthesis.

| image2 = Prochlorococcus marinus (cropped).jpg

| caption2 = The cyanobacterium genus Prochlorococcus is a major contributor to atmospheric oxygen.

}}

Cyanobacteria were the first organisms to evolve an ability to turn sunlight into chemical energy. They form a phylum (division) of bacteria which range from unicellular to filamentous and include colonial species. They are found almost everywhere on earth: in damp soil, in both freshwater and marine environments, and even on Antarctic rocks.{{cite book| veditors = Walsh PJ, Smith S, Fleming L, Solo-Gabriele H, Gerwick WH |title=Oceans and Human Health: Risks and Remedies from the Seas|chapter-url={{google books |plainurl=y |id=LMZPqW-PmFYC|page=271}}|date=2 September 2011|chapter=Cyanobacteria and cyanobacterial toxins|pages=271–296|publisher=Academic Press|isbn=978-0-08-087782-2}} In particular, some species occur as drifting cells floating in the ocean, and as such were amongst the first of the phytoplankton.

The first primary producers that used photosynthesis were oceanic cyanobacteria about 2.3 billion years ago.{{Cite web|url= http://www.astrobio.net/news-exclusive/the-rise-of-oxygen/|title= The Rise of Oxygen - Astrobiology Magazine|website= Astrobiology Magazine|date= 30 July 2003|language= en-US|access-date= 2016-04-06}}{{cite journal| vauthors = Flannery DT, Walter RM |title= Archean tufted microbial mats and the Great Oxidation Event: new insights into an ancient problem|journal= Australian Journal of Earth Sciences|date= 2012|volume= 59|issue= 1|pages= 1–11|doi= 10.1080/08120099.2011.607849 |bibcode = 2012AuJES..59....1F |s2cid= 53618061}} The release of molecular oxygen by cyanobacteria as a by-product of photosynthesis induced global changes in the Earth's environment. Because oxygen was toxic to most life on Earth at the time, this led to the near-extinction of oxygen-intolerant organisms, a dramatic change which redirected the evolution of the major animal and plant species.{{cite web |url=http://astrobiology.arc.nasa.gov/roadmap/g5.html |archive-url=https://web.archive.org/web/20120329092237/http://astrobiology.arc.nasa.gov/roadmap/g5.html |archive-date=29 March 2012 |title=Understand the evolutionary mechanisms and environmental limits of life |access-date=13 July 2009 | vauthors = Rothschild L |date=September 2003 |url-status=dead |publisher=NASA}}

The tiny marine cyanobacterium Prochlorococcus, discovered in 1986, forms today part of the base of the ocean food chain and accounts for much of the photosynthesis of the open ocean{{cite journal | vauthors = Nadis S | title = The cells that rule the seas | journal = Scientific American | volume = 289 | issue = 6 | pages = 52–3 | date = December 2003 | pmid = 14631732 | doi = 10.1038/scientificamerican1203-52 | url = http://guowei.ccps.tp.edu.tw/nc/UploadDocument/255_02%20The%20Cells%20that%20Rules%20the%20Sea.pdf | access-date = 2 June 2019 | url-status = dead | bibcode = 2003SciAm.289f..52N | archive-url = https://web.archive.org/web/20140419222251/http://guowei.ccps.tp.edu.tw/nc/UploadDocument/255_02%20The%20Cells%20that%20Rules%20the%20Sea.pdf | archive-date = 19 April 2014 }} and an estimated 20% of the oxygen in the Earth's atmosphere.{{cite news|url=https://www.npr.org/templates/story/story.php?storyId=91448837|title=The Most Important Microbe You've Never Heard Of|website=npr.org}} It is possibly the most plentiful genus on Earth: a single millilitre of surface seawater may contain 100,000 cells or more.{{cite journal | vauthors = Flombaum P, Gallegos JL, Gordillo RA, Rincón J, Zabala LL, Jiao N, Karl DM, Li WK, Lomas MW, Veneziano D, Vera CS, Vrugt JA, Martiny AC | display-authors = 6 | title = Present and future global distributions of the marine Cyanobacteria Prochlorococcus and Synechococcus | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 110 | issue = 24 | pages = 9824–9 | date = June 2013 | pmid = 23703908 | pmc = 3683724 | doi = 10.1073/pnas.1307701110 | bibcode = 2013PNAS..110.9824F | doi-access = free }}

Originally, biologists classified cyanobacteria as algae, and referred to it as "blue-green algae". The more recent view is that cyanobacteria are bacteria, and hence are not even in the same Kingdom as algae. Most authorities today exclude all prokaryotes, and hence cyanobacteria from the definition of algae.{{cite book | vauthors = Nabors MW |title= Introduction to Botany |year=2004 |publisher=Pearson Education, Inc |location=San Francisco, CA |isbn=978-0-8053-4416-5}}{{cite encyclopedia | veditors = Allaby M |year=1992 |encyclopedia=The Concise Dictionary of Botany |publisher=Oxford University Press |location=Oxford |title=Algae}}

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{{multiple image

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| header = Diatoms

| footer = Diatoms have a silica shell (frustule) with radial (centric) or bilateral (pennate) symmetry.

| image1 = Centric diatom (38258532722).jpg

| caption1 = Centric

| image2 = Pennate diatoms (3075304186).jpg

| caption2 = Pennate

}}

{{multiple image

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| header = Dinoflagellates

| footer = Traditionally dinoflagellates have been presented as armoured or unarmoured.

| image1 = Peridinium digitale.jpg

| caption1 = Armoured

| image2 = Gymnodinium agile sp.jpg

| caption2 = Unarmoured

}}

Algae is an informal term for a widespread and diverse group of photosynthetic protists which are not necessarily closely related and are thus polyphyletic. Marine algae can be divided into six groups:

• green algae, an informal group containing about 8,000 recognised species.{{cite journal | vauthors = Guiry MD | title = How Many Species of Algae Are There? | journal = Journal of Phycology | volume = 48 | issue = 5 | pages = 1057–63 | date = October 2012 | pmid = 27011267 | doi = 10.1111/j.1529-8817.2012.01222.x | bibcode = 2012JPcgy..48.1057G | s2cid = 30911529 }} Many species live most of their lives as single cells or are filamentous, while others form colonies made up from long chains of cells, or are highly differentiated macroscopic seaweeds.
• red algae, a (disputed) phylum containing about 7,000 recognised species,{{Cite web |url= http://www.algaebase.org/browse/taxonomy/?id=97240 |title=Algaebase| vauthors = Guiry MD, Guiry GM |date=2016 |website=www.algaebase.org |access-date=November 20, 2016}} mostly multicellular and including many notable seaweeds.{{cite book|title=Seaweeds|publisher=Life Series. Natural History Museum, London|year=2002|isbn=978-0-565-09175-0| vauthors = Thomas D }}
• brown algae, a class containing about 2,000 recognised species,{{Cite book|url=https://books.google.com/books?id=s1P855ZWc0kC&pg=166|title=Algae: an introduction to phycology| vauthors = Hoek C, Mann D, Jahns HM, Jahns M |date=1995|publisher=Cambridge University Press|isbn=9780521316873|oclc=443576944|page=166}} mostly multicellular and including many seaweeds, including kelp
• diatoms, a (disputed) phylum containing about 100,000 recognised species of mainly unicellular algae. Diatoms generate about 20 percent of the oxygen produced on the planet each year, take in over 6.7 billion metric tons of silicon each year from the waters in which they live,{{cite journal | vauthors = Tréguer P, Nelson DM, Van Bennekom AJ, Demaster DJ, Leynaert A, Quéguiner B | title = The silica balance in the world ocean: a reestimate | journal = Science | volume = 268 | issue = 5209 | pages = 375–9 | date = April 1995 | pmid = 17746543 | doi = 10.1126/science.268.5209.375 | s2cid = 5672525 | bibcode = 1995Sci...268..375T }} and contribute nearly half of the organic material found in the oceans. The shells (frustules) of dead diatoms can reach as much as half a mile deep on the ocean floor.{{Cite web|url=https://www.kcl.ac.uk/sspp/departments/geography/people/academic/drake/Research/The-Sahara-Megalakes-Project/Lake-Megachad.aspx|title=King's College London - Lake Megachad|website=www.kcl.ac.uk|language=en-GB|access-date=2018-05-05}}
• dinoflagellates, a phylum of unicellular flagellates with about 2,000 marine species.{{cite journal| vauthors = Gómez F |title=A checklist and classification of living dinoflagellates (Dinoflagellata, Alveolata) |journal=CICIMAR Oceánides |volume=27 |issue=1 |pages=65–140 |year=2012 |doi=10.37543/oceanides.v27i1.111 |doi-access=free }} Many dinoflagellates are known to be photosynthetic, but a large fraction of these are in fact mixotrophic, combining photosynthesis with ingestion of prey (phagotrophy).{{Cite journal | vauthors = Stoecker DK | title = Mixotrophy among Dinoflagellates | doi = 10.1111/j.1550-7408.1999.tb04619.x | journal = The Journal of Eukaryotic Microbiology | volume = 46 | issue = 4 | pages = 397–401 | year = 1999 | s2cid = 83885629 | name-list-style = vanc}} Some species are endosymbionts of marine animals and play an important part in the biology of coral reefs. Others predate other protozoa, and a few forms are parasitic.
• euglenophytes, a phylum of unicellular flagellates with only a few marine members

Unlike higher plants, algae lack roots, stems, or leaves. They can be classified by size as microalgae or macroalgae.

Microalgae are the microscopic types of algae, not visible to the naked eye. They are mostly unicellular species which exist as individuals or in chains or groups, though some are multicellular. Microalgae are important components of the marine protists (discussed above), as well as the phytoplankton (discussed below). They are very diverse. It has been estimated there are 200,000-800,000 species of which about 50,000 species have been described.{{cite web | vauthors = Starckx S | date = 31 October 2012 | url = http://www.flanderstoday.eu/current-affairs/place-sun | archive-url = https://web.archive.org/web/20160304105525/http://www.flanderstoday.eu/current-affairs/place-sun | archive-date = 4 March 2016 | title = A place in the sun - Algae is the crop of the future, according to researchers in Geel] Flanders Today | access-date = 8 December 2012 }} Depending on the species, their sizes range from a few micrometers (μm) to a few hundred micrometers. They are specially adapted to an environment dominated by viscous forces.

File:Chlamydomonas globosa - 400x (13263097835).jpg|Chlamydomonas globosa, a unicellular green alga with two flagella just visible at bottom left

File:Инфузории Ophridium versatile.jpg|Chlorella vulgaris, a common green microalgae, in endosymbiosis with a ciliate{{cite journal | vauthors = Duval B, Margulis L | year = 1995 | title = The microbial community of Ophrydium versatile colonies: endosymbionts, residents, and tenants | journal = Symbiosis | volume = 18 | pages = 181–210 | pmid = 11539474 }}

File:Centric diatom.jpg|Centric diatom

File:Dinoflagellates.jpg|Dinoflagellates

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Macroalgae are the larger, multicellular and more visible types of algae, commonly called seaweeds. Seaweeds usually grow in shallow coastal waters where they are anchored to the seafloor by a holdfast. Seaweed that becomes adrift can wash up on beaches. Kelp is a large brown seaweed that forms large underwater forests covering about 25% of the world coastlines.Wernberg, T., Krumhansl, K., Filbee-Dexter, K. and Pedersen, M.F. (2019) "Status and trends for the world's kelp forests". In: World seas: an environmental evaluation, pages 57–78). Academic Press. {{doi|10.1016/B978-0-12-805052-1.00003-6}}. They are among the most productive and dynamic ecosystems on Earth.{{cite journal | vauthors = Mann KH | title = Seaweeds: Their Productivity and Strategy for Growth: The role of large marine algae in coastal productivity is far more important than has been suspected | journal = Science | volume = 182 | issue = 4116 | pages = 975–81 | date = December 1973 | pmid = 17833778 | doi = 10.1126/science.182.4116.975 | s2cid = 26764207 | bibcode = 1973Sci...182..975M }} Some Sargassum seaweeds are planktonic (free-floating). Like microalgae, macroalgae (seaweeds) are technically marine protists since they are not true plants.

File:Algae Pengo.svg| A seaweed is a macroscopic form of red or brown or green algae.

File:Sargassum on the beach, Cuba.JPG| Sargassum seaweed is a planktonic brown alga with air bladders that help it float.

File:Histrio histrio by A. H. Baldwin.jpg| Sargassum fish are camouflaged to live among drifting Sargassum seaweed.

File:Kelp forest.jpgs are among the most productive ecosystems on the planet.]]

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{{anchor|Unicellular macroalgae}}

File:Ventricaria ventricosa.JPG| The unicellular bubble algae lives in tidal zones. It can have a 4 cm diameter.{{cite book | vauthors = Tunnell JW, Chávez EA, Withers K |year=2007 |title=Coral reefs of the southern Gulf of Mexico |publisher=Texas A&M University Press |isbn=978-1-58544-617-9 |page=91 |url=https://books.google.com/books?id=tu0sqBp8eAAC&pg=PA91}}

File:Acetabularia sp.jpg| The unicellular mermaid's wineglass are mushroom-shaped algae that grow up to 10 cm high.

File:CaulerpaTaxifolia.jpg| Killer algae are single-celled organisms, but look like ferns and grow stalks up to 80 cm long.{{cite web | url = https://www.cabi.org/isc/datasheet/29292 | work = Invasive Species Compendium | title = Caulerpa taxifolia (killer algae) | publisher = Centre for Agriculture and Bioscience International | date = 6 November 2018 }}

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Unicellular organisms are usually microscopic, less than one tenth of a millimeter long. There are exceptions. Mermaid's wineglass, a genus of subtropical green algae, is single-celled but remarkably large and complex in form with a single large nucleus, making it a model organism for studying cell biology.{{cite journal | vauthors = Mandoli DF | title = Elaboration of Body Plan and Phase Change During Development of Acetabularia: How Is the Complex Architecture of a Giant Unicell Built? | journal = Annual Review of Plant Physiology and Plant Molecular Biology | volume = 49 | pages = 173–198 | date = June 1998 | pmid = 15012232 | doi = 10.1146/annurev.arplant.49.1.173 | s2cid = 6241264 }} Another single celled algae, Caulerpa taxifolia, has the appearance of a vascular plant including "leaves" arranged neatly up stalks like a fern. Selective breeding in aquariums to produce hardier strains resulted in an accidental release into the Mediterranean where it has become an invasive species known colloquially as killer algae.{{cite journal | vauthors = Madl P, Yip M | title = Literature Review of Caulerpa taxifolia | journal = BUFUS-Info | volume = 19 | issue = 31 | year = 2004 | url = http://biophysics.sbg.ac.at/ct/caulerpa.htm | access-date = 18 July 2017 | archive-date = 8 October 2022 | archive-url = https://web.archive.org/web/20221008200554/http://biophysics.sbg.ac.at/ct/caulerpa.htm | url-status = dead }}

File:Evolution of seagrasses Pengo 8.png

Back in the Silurian, some phytoplankton evolved into red, brown and green algae. These algae then invaded the land and started evolving into the land plants we know today. Later, in the Cretaceous, some of these land plants returned to the sea as marine plants, such as mangroves and seagrasses.{{cite journal | vauthors = Orth RJ, Carruthers TJ, Dennison WC, Duarte CM, Fourqurean JW, Heck KL, Hughes AR, Kendrick GA, Kenworthy WJ, Olyarnik S, Short FT | display-authors = 6 | year = 2006 | title = A global crisis for seagrass ecosystems | journal = BioScience | volume = 56 | issue = 12| pages = 987–996 | doi = 10.1641/0006-3568(2006)56[987:AGCFSE]2.0.CO;2 | hdl = 10261/88476 | s2cid = 4936412 | doi-access = free }}

Marine plants can be found in intertidal zones and shallow waters, such as seagrasses like eelgrass and turtle grass, Thalassia. These plants have adapted to the high salinity of the ocean environment. Plant life can also flourish in the brackish waters of estuaries, where mangroves or cordgrass or beach grass beach grass might grow.

File:Mangrove-Keti Bundar.jpg|Mangroves

File:Floridian seagrass bed.jpg|Seagrass meadow

File:Leafy Sea Dragon SA.jpg| Sea dragons camouflaged to look like floating seaweed live in kelp forests and seagrass meadows.{{FishBase |genus=Phycodurus |species=eques |year=2009 |month=July}}

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The total world area of mangrove forests was estimated in 2010 as {{convert|134,257|km2}} (based on satellite data).{{cite journal | vauthors = Giri C, Ochieng E, Tieszen LL, Zhu Z, Singh A, Loveland T, Masek J, Duke N | display-authors = 6 | year = 2011 | title = Status and distribution of mangrove forests of the world using earth observation satellite data | journal = Global Ecology and Biogeography | volume = 20 | issue = 1| pages = 154–159 | doi = 10.1111/j.1466-8238.2010.00584.x | bibcode = 2011GloEB..20..154G }}{{cite journal | vauthors = Thomas N, Lucas R, Bunting P, Hardy A, Rosenqvist A, Simard M | title = Distribution and drivers of global mangrove forest change, 1996-2010 | journal = PLOS ONE | volume = 12 | issue = 6 | pages = e0179302 | year = 2017 | pmid = 28594908 | pmc = 5464653 | doi = 10.1371/journal.pone.0179302 | bibcode = 2017PLoSO..1279302T | doi-access = free }} The total world area of seagrass meadows is more difficult to determine, but was conservatively estimated in 2003 as {{convert|177000|km2}}.{{cite book |vauthors=Short FT, Frederick T |title=World atlas of seagrasses |date=2003 |publisher=University of California Press |location=Berkeley, Calif. |isbn=978-0-520-24047-6 |url=https://www.unep-wcmc.org/resources-and-data/world-atlas-of-seagrasses |page=24 |access-date=10 July 2019 |archive-date=10 July 2019 |archive-url=https://web.archive.org/web/20190710075836/https://www.unep-wcmc.org/resources-and-data/world-atlas-of-seagrasses |url-status=dead }}

Mangroves and seagrasses provide important nursery habitats for marine life, acting as hiding and foraging places for larval and juvenile forms of larger fish and invertebrates.{{cite book | vauthors = Spalding M | date = 2010 | url = https://www.taylorfrancis.com/books/9781849776608 | title = World atlas of mangroves | publisher = Routledge | isbn = 978-1-84977-660-8 | doi = 10.4324/9781849776608}}

File:Plankton collage.jpg, archaea, algae, protozoa and animals.|alt=Six relatively large variously shaped organisms with dozens of small light-colored dots all against a dark background. Some of the organisms have antennae that are longer than their bodies.]]

{{Further|Plankton|Bacterioplankton|Ichthyoplankton|Mycoplankton}}

Plankton (from Greek for wanderers) are a diverse group of organisms that live in the water column of large bodies of water but cannot swim against a current. As a result, they wander or drift with the currents.{{cite book | vauthors = Lalli C, Parsons T | year = 1993 | title = Biological Oceanography: An Introduction | publisher = Butterworth-Heinemann | isbn = 0-7506-3384-0 }} Plankton are defined by their ecological niche, not by any phylogenetic or taxonomic classification. They are a crucial source of food for many marine animals, from forage fish to whales. Plankton can be divided into a plant-like component and an animal component.

Phytoplankton are the plant-like components of the plankton community ("phyto" comes from the Greek for plant). They are autotrophic (self-feeding), meaning they generate their own food and do not need to consume other organisms.

Phytoplankton consist mainly of microscopic photosynthetic eukaryotes which inhabit the upper sunlit layer in all oceans. They need sunlight so they can photosynthesize. Most phytoplankton are single-celled algae, but other phytoplankton are bacteria and some are protists.{{cite web | vauthors = Lindsey R, Scott M, Simmon R | date = 2010 | url = https://earthobservatory.nasa.gov/features/Phytoplankton | title = What are phytoplankton | work = NASA Earth Observatory }} Phytoplankton groups include cyanobacteria (above), diatoms, various other types of algae (red, green, brown, and yellow-green), dinoflagellates, euglenoids, coccolithophorids, cryptomonads, chrysophytes, chlorophytes, prasinophytes, and silicoflagellates. They form the base of the primary production that drives the ocean food web, and account for half of the current global primary production, more than the terrestrial forests.{{cite journal | vauthors = Field CB, Behrenfeld MJ, Randerson JT, Falkowski P | title = Primary production of the biosphere: integrating terrestrial and oceanic components | journal = Science | volume = 281 | issue = 5374 | pages = 237–40 | date = July 1998 | pmid = 9657713 | doi = 10.1126/science.281.5374.237 | bibcode = 1998Sci...281..237F | url = https://escholarship.org/uc/item/9gm7074q }}

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| footer = Coccolithophores build calcite skeletons important to the marine carbon cycle.{{cite book | vauthors = Rost B, Riebesell U | date = 2004 | chapter = Coccolithophores and the biological pump: responses to environmental changes | title = Coccolithophores: From Molecular Processes to Global Impact | pages = 99–125 | publisher = Springer | isbn = 978-3-662-06278-4 }}

| image1 = 9Calcidiscus leptoporus, diploid, SEM, showing coccoliths.tif

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File:Phytoplankton Lake Chuzenji.jpg|Phytoplankton are the foundation of the ocean food chain.

File:Phytopla.jpg|Phytoplankton come in many shapes and sizes.

File:Diatoms (248 05) Various diatoms.jpg|Diatoms are one of the most common types of phytoplankton.

File:Prochlorococcus marinus 2.jpg|The cyanobacterium Prochlorococcus accounts for much of the ocean's primary production.

File:Cyanobacterial Scum.JPG|Green cyanobacteria scum washed up on a rock in California

File:Stichotricha secunda - 400x (14974779356).jpg|Zoochlorellae (green) living inside the ciliate Stichotricha secunda

File:Pinnularia major.jpgs which account for 50% of the ocean's primary production.]]

File:Super Blooms.ogv

File:JRYSEM-247-05-azurapl.jpg|Coccolithophores named after the BBC documentary series. The Blue Planet

File:Emiliania huxleyi.jpg|The coccolithophore Emiliania huxleyi

File:Cwall99 lg.jpg|Algae bloom of Emiliania huxleyi off the southern coast of England

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Zooplankton are the animal component of the planktonic community ("zoo" comes from the Greek for animal). They are heterotrophic (other-feeding), meaning they cannot produce their own food and must consume instead other plants or animals as food. In particular, this means they eat phytoplankton.

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| footer = Foraminiferans are important unicellular zooplankton protists, with calcium shells.

| image1 = Foram-globigerina hg.jpg

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| footer = Computer simulations of Turing patterns on a sphere closely replicate some radiolarian shell patterns.{{cite journal | vauthors = Varea C, Aragón JL, Barrio RA | title = Turing patterns on a sphere | journal = Physical Review E | volume = 60 | issue = 4 Pt B | pages = 4588–92 | date = October 1999 | pmid = 11970318 | doi = 10.1103/PhysRevE.60.4588 | bibcode = 1999PhRvE..60.4588V }}

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Zooplankton are generally larger than phytoplankton, mostly still microscopic but some can be seen with the naked eye. Many protozoans (single-celled protists that prey on other microscopic life) are zooplankton, including zooflagellates, foraminiferans, radiolarians and some dinoflagellates. Other dinoflagellates are mixotrophic and could also be classified as phytoplankton; the distinction between plants and animals often breaks down in very small organisms. Other zooplankton include pelagic cnidarians, ctenophores, molluscs, arthropods and tunicates, as well as planktonic arrow worms and bristle worms.

Radiolarians are unicellular protists with elaborate silica shells

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Microzooplankton: major grazers of the plankton

File:Mikrofoto.de-Radiolarien 6.jpg|Radiolarians come in many shapes.

File:Planktic Foraminifera of the northern Gulf of Mexico.jpg| Group of planktic foraminiferans

File:Copepod 2 with eggs.jpg|Copepods eat phytoplankton. This one is carrying eggs.

File:Protoperidinium dinoflagellate.jpg| The dinoflagellate, Protoperidinium extrudes a large feeding veil to capture prey.

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Larger zooplankton can be predatory on smaller zooplankton.

Macrozooplankton

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File:Aurelia aurita (Cnidaria) Luc Viatour.jpg| Moon jellyfish

File:Cestum veneris in Hawaii.png| Venus girdle, a ctenophore

File:Chaetoblack 2.png| Arrow worm

File:Tomopteriskils.jpg|Tomopteris, a planktonic segmented worm with unusual yellow bioluminescence{{cite book| vauthors = Harvey EN |title = Bioluminescence|publisher = Academic Press|year = 1952 }}

File:Amphipodredkils.jpg|Marine amphipod

File:Antarctic krill (Euphausia superba).jpg| Krill

File:Enypniastes sp.jpg| Pelagic sea cucumber

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Many marine animals begin life as zooplankton in the form of eggs or larvae, before they develop into adults. These are meroplanktic, that is, they are planktonic for only part of their life.

File:Salmonlarvakils 2.jpg| Salmon larva hatching from its egg

File:Molalavdj.jpg|Ocean sunfish larva

File:Squidu.jpg|Juvenile planktonic squid

File:Larva de phyllosoma.jpg| Larva stage of a spiny lobster

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| image1 = Dinoflagellate lumincescence 2.jpg

| caption1 = A surf wave at night sparkles with blue light due to the presence of a bioluminescent dinoflagellate, such as Lingulodinium polyedrum

| image2 = Potential Mechanism for Dazzling Blue Flashes of Light in Oceans Identified (6300345394).jpg

| caption2 = A suggested explanation for glowing seas{{cite web | url = https://www.nsf.gov/news/news_summ.jsp?org=NSF&cntn_id=122037 | title = Suggested Explanation for Glowing Seas--Including Currently Glowing California Seas | work = National Science Foundation | date = 18 October 2011 }}

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{{See also|Mixotrophic dinoflagellate}}

Dinoflagellates are often mixotrophic or live in symbiosis with other organisms.

File:Tintinnid ciliate Favella.jpg| Tintinnid ciliate Favella

File:Euglena mutabilis - 400x - 1 (10388739803) (cropped).jpg|Euglena mutabilis, a photosynthetic flagellate

File:Noctiluca scintillans unica.jpg| Noctiluca scintillans, a bioluminescence dinoflagellate

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Some dinoflagellates are bioluminescent. At night, ocean water can light up internally and sparkle with blue light because of these dinoflagellates.{{cite book | vauthors = Castro P, Huber ME |title=Marine Biology |url=https://archive.org/details/marinebiology00cast_419 |url-access=limited |publisher=McGraw Hill |year=2010 |isbn=978-0071113021 |pages=[https://archive.org/details/marinebiology00cast_419/page/n110 95] |edition=8th }}{{cite journal | vauthors = Hastings JW | title = Chemistries and colors of bioluminescent reactions: a review | journal = Gene | volume = 173 | issue = 1 Spec No | pages = 5–11 | year = 1996 | pmid = 8707056 | doi = 10.1016/0378-1119(95)00676-1 }} Bioluminescent dinoflagellates possess scintillons, individual cytoplasmic bodies which contain dinoflagellate luciferase, the main enzyme involved in the luminescence. The luminescence, sometimes called the phosphorescence of the sea, occurs as brief (0.1 sec) blue flashes or sparks when individual scintillons are stimulated, usually by mechanical disturbances from, for example, a boat or a swimmer or surf.{{cite journal | vauthors = Haddock SH, Moline MA, Case JF | title = Bioluminescence in the sea | journal = Annual Review of Marine Science | volume = 2 | pages = 443–93 | date = 2009 | pmid = 21141672 | doi = 10.1146/annurev-marine-120308-081028 | s2cid = 3872860 | bibcode = 2010ARMS....2..443H }}

{{See also|Marine food web}}

File:Oceanic Food Web.jpg]]

Compared to terrestrial environments, marine environments have biomass pyramids which are inverted at the base. In particular, the biomass of consumers (copepods, krill, shrimp, forage fish) is larger than the biomass of primary producers. This happens because the ocean's primary producers are tiny phytoplankton which tend to be r-strategists that grow and reproduce rapidly, so a small mass can have a fast rate of primary production. In contrast, terrestrial primary producers, such as mature forests, are often K-strategists that grow and reproduce slowly, so a much larger mass is needed to achieve the same rate of primary production.

Because of this inversion, it is the zooplankton that make up most of the marine animal biomass. As primary consumers, they are the crucial link between the primary producers (mainly phytoplankton) and the rest of the marine food web (secondary consumers).{{cite web | date = 2008 | url = https://public.ornl.gov/site/gallery/detail.cfm?id=326 | title = Carbon Cycling and Biosequestration | page = 81 | quote = Workshop report DOE/SC-108 | publisher = US Department of Energy Department of Energy Office of Science }}

If phytoplankton dies before it is eaten, it descends through the euphotic zone as part of the marine snow and settles into the depths of sea. In this way, phytoplankton sequester about 2 billion tons of carbon dioxide into the ocean each year, causing the ocean to become a sink of carbon dioxide holding about 90% of all sequestered carbon.{{cite web|url=http://www.earthtimes.org/scitech/role-marine-plankton-sequestration-carbon/1062/|title=The role of marine plankton in sequestration of carbon | vauthors = Campbell M |date=22 June 2011 |website=EarthTimes|access-date=22 August 2014}}

In 2010 researchers found whales carry nutrients from the depths of the ocean back to the surface using a process they called the whale pump.{{cite journal | vauthors = Roman J, McCarthy JJ | title = The whale pump: marine mammals enhance primary productivity in a coastal basin | journal = PLOS ONE | volume = 5 | issue = 10 | pages = e13255 | date = October 2010 | pmid = 20949007 | pmc = 2952594 | doi = 10.1371/journal.pone.0013255 | bibcode = 2010PLoSO...513255R | id = e13255 | doi-access = free }} Whales feed at deeper levels in the ocean where krill is found, but return regularly to the surface to breathe. There whales defecate a liquid rich in nitrogen and iron. Instead of sinking, the liquid stays at the surface where phytoplankton consume it. In the Gulf of Maine the whale pump provides more nitrogen than the rivers.{{cite web|url=https://www.sciencedaily.com/releases/2010/10/101012101255.htm|title=Whale poop pumps up ocean health| vauthors = Brown JE |date=12 Oct 2010|website=Science Daily|access-date=18 August 2014}}

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{{further|Marine biogeochemical cycles|biological pump|blue carbon}}

Taken as a whole, the oceans form a single marine system where water – the "universal solvent" {{cite web |title=Water, the Universal Solvent |url=http://water.usgs.gov/edu/solvent.html |website=USGS |access-date=27 June 2017 |archive-url=https://web.archive.org/web/20170709141251/https://water.usgs.gov/edu/solvent.html |archive-date=9 July 2017 |url-status=live }} – dissolves nutrients and substances containing elements such as oxygen, carbon, nitrogen and phosphorus. These substances are endlessly cycled and recycled, chemically combined and then broken down again, dissolved and then precipitated or evaporated, imported from and exported back to the land and the atmosphere and the ocean floor. Powered both by the biological activity of marine organisms and by the natural actions of the sun and tides and movements within the Earth's crust, these are the marine biogeochemical cycles.{{cite conference | vauthors = Brum JR, Morris JJ, Décima M, Stukel MR | date = 2014 | title = Chapter 2: Mortality in the oceans: Causes and consequences | conference = Eco-DAS IX Symposium Proceedings | pages = 16–48 | publisher = Association for the Sciences of Limnology and Oceanography | isbn = 978-0-9845591-3-8}}.{{Cite book |title=Campbell Biology | vauthors = Reece JB |year=2013 |publisher=Pearson |isbn=978-0-321-77565-8 |edition=10th }}

File:OceanCarbonCycle.jpg|Marine carbon cycle{{Cite web| vauthors = Prentice IC |title=The carbon cycle and atmospheric carbon dioxide|url =http://ir.anet.ua.ac.be/irua/handle/10067/381670151162165141?show=full|publisher=Climate change 2001: the scientific basis: contribution of Working Group I to the Third Assessment Report of the Intergouvernmental Panel on Climate Change / Houghton, J.T. [edit.]|year=2001|access-date=31 May 2012 }}

File:Oxygen cycle.svg|Oxygen cycle

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File:Marine sediment thickness (cropped).jpg

{{see also|Marine sediment|Protist shells}}

Sediments at the bottom of the ocean have two main origins, terrigenous and biogenous. Terrigenous sediments account for about 45% of the total marine sediment, and originate in the erosion of rocks on land, transported by rivers and land runoff, windborne dust, volcanoes, or grinding by glaciers.

Biogenous sediments account for the other 55% of the total sediment, and originate in the skeletal remains of marine protists (single-celled plankton and benthos organisms). Much smaller amounts of precipitated minerals and meteoric dust can also be present. Ooze, in the context of a marine sediment, does not refer to the consistency of the sediment but to its biological origin. The term ooze was originally used by John Murray, the "father of modern oceanography", who proposed the term radiolarian ooze for the silica deposits of radiolarian shells brought to the surface during the Challenger Expedition.{{cite book | vauthors = Thomson CW | date = 2014 | url = https://books.google.com/books?id=zcFkAwAAQBAJ&q=radiolarian+ooze | title = Voyage of the Challenger: The Atlantic | publisher = Cambridge University Press | page = 235 | isbn = 978-1-108-07475-9 }} A biogenic ooze is a pelagic sediment containing at least 30 percent from the skeletal remains of marine organisms.

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class="wikitable" ! colspan=8 |{{centre|Main types of biogenic ooze}}
type ! mineral forms ! protist responsible ! ! name of skeleton ! description
width=90px rowspan=2 | Siliceous ooze | rowspan=2 align=center | SiO2 quartz glass opal chert | diatoms | style="background:#000000;"| 90px | frustule | Individual diatoms range in size from 0.002 to 0.2 mm.{{cite book| vauthors = Hasle GR, Syvertsen EE, Steidinger, Tangen K | veditors = Tomas CR |title=Identifying Marine Diatoms and Dinoflagellates|chapter-url=https://books.google.com/books?id=KQxPtwonlqoC|access-date=2013-11-13|date=1996-01-25|publisher=Academic Press|isbn=978-0-08-053441-1|pages=5–385|chapter=Marine Diatoms}}
radiolarians | style="background:#000000;"| 90px | skeleton | Radiolarians are protozoa with diameters typically between 0.1 and 0.2 mm that produce intricate mineral skeletons, usually made of silica
rowspan=3 | Calcareous ooze | rowspan=3 align=center | CaCO3 calcite aragonite limestone chalk | foraminiferans | style="background:#000000;"| 90px | test | There are about 10,000 living species of foraminiferans,{{cite journal | vauthors = Adl SM, Leander BS, Simpson AG, Archibald JM, Anderson OR, Bass D, Bowser SS, Brugerolle G, Farmer MA, Karpov S, Kolisko M, Lane CE, Lodge DJ, Mann DG, Meisterfeld R, Mendoza L, Moestrup Ø, Mozley-Standridge SE, Smirnov AV, Spiegel F | display-authors = 6 | title = Diversity, nomenclature, and taxonomy of protists | journal = Systematic Biology | volume = 56 | issue = 4 | pages = 684–9 | date = August 2007 | pmid = 17661235 | doi = 10.1080/10635150701494127 | doi-access = free }} usually under 1 mm in size.
coccolithophores | style="background:#000000;"| 90px | coccolith | Coccolithophores are spherical cells usually less than 0.1 mm across, enclosed by calcareous plates called coccoliths.{{cite journal | vauthors = Moheimani NR, Webb JP, Borowitzka MA | title = Bioremediation and other potential applications of coccolithophorid algae: a review. | journal = Algal Research | date = October 2012 | volume = 1 | issue = 2 | pages = 120–33 | doi = 10.1016/j.algal.2012.06.002}} Coccoliths are important microfossils. They are the largest global source of biogenic calcium carbonate, and make significant contributions to the global carbon cycle.{{cite journal | vauthors = Taylor AR, Chrachri A, Wheeler G, Goddard H, Brownlee C | title = A voltage-gated H+ channel underlying pH homeostasis in calcifying coccolithophores | journal = PLOS Biology | volume = 9 | issue = 6 | pages = e1001085 | date = June 2011 | pmid = 21713028 | pmc = 3119654 | doi = 10.1371/journal.pbio.1001085 | doi-access = free }} They are the main constituent of chalk deposits such as the white cliffs of Dover.

File:Radiolarian - Heliodiscus umbonatus (Ehr.), Haeckel (28187768550).jpg|An elaborate mineral skeleton of a radiolarian made of silica.

File:Marine diatoms SEM2.jpg|Diatoms, major components of marine plankton, also have silica skeletons called frustules.

File:CSIRO ScienceImage 7202 SEM Coccolithophorid.jpg|Coccolithophores have plates or scales made with calcium carbonate called coccoliths

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File:Stephanopyxis grunowii.jpg|A diatom microfossil from 40 million years ago

File:Diatomaceous Earth BrightField.jpg|Diatomaceous earth is a soft, siliceous, sedimentary rock made up of microfossils in the form of the frustules (shells) of single cell diatoms (click to magnify).

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File:Ocean drainage.pngs. The grey areas are endorheic basins that do not drain to the ocean.]]

{{See also|Freshwater ecosystem|Continental shelf pump}}

Land interactions impact marine life in many ways. Coastlines typically have continental shelves extending some way from the shore. These provide extensive shallows sunlit down to the seafloor, allowing for photosynthesis and enabling habitats for seagrass meadows, coral reefs, kelp forests and other benthic life. Further from shore the continental shelf slopes towards deep water. Wind blowing at the ocean surface or deep ocean currents can result in cold and nutrient rich waters from abyssal depths moving up the continental slopes. This can result in upwellings along the outer edges of continental shelves, providing conditions for phytoplankton blooms.

Water evaporated by the sun from the surface of the ocean can precipitate on land and eventually return to the ocean as runoff or discharge from rivers, enriched with nutrients as well as pollutants. As rivers discharge into estuaries, freshwater mixes with saltwater and becomes brackish. This provides another shallow water habitat where mangrove forests and estuarine fish thrive. Overall, life in inland lakes can evolve with greater diversity than happens in the sea, because freshwater habitats are themselves diverse and compartmentalised in a way marine habitats are not. Some aquatic life, such as salmon and eels, migrate back and forth between freshwater and marine habitats. These migrations can result in exchanges of pathogens and have impacts on the way life evolves in the ocean.

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File:Global cumulative human impact on the ocean.png

{{main|Human impact on marine life}}

Human activities affect marine life and marine habitats through overfishing, pollution, acidification and the introduction of invasive species. These impact marine ecosystems and food webs and may result in consequences as yet unrecognised for the biodiversity and continuation of marine life forms.{{cite web | url = https://www.geomar.de/en/research/human-impacts-on-marine-ecosystems/ | title = Human impacts on marine ecosystems | work = GEOMAR Helmholtz Centre for Ocean Research | access-date = 22 October 2019 | archive-date = 31 October 2020 | archive-url = https://web.archive.org/web/20201031110957/https://www.geomar.de/en/research/human-impacts-on-marine-ecosystems/ | url-status = dead }}

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File:Phanerozoic Biodiversity.svg

{{annotated image/Extinction|caption= Apparent extinction intensity, i.e. the fraction of genera going extinct at any given time as reconstructed from the fossil record (excluding the current Holocene extinction event)}}

Biodiversity is the result of over three billion years of evolution. Until approximately 600 million years ago, all life consisted of archaea, bacteria, protozoans and similar single-celled organisms. The history of biodiversity during the Phanerozoic (the last 540 million years), starts with rapid growth during the Cambrian explosion – a period during which nearly every phylum of multicellular organisms first appeared. Over the next 400 million years or so, invertebrate diversity showed little overall trend and vertebrate diversity shows an overall exponential trend.{{cite journal | vauthors = Sahney S, Benton MJ, Ferry PA | title = Links between global taxonomic diversity, ecological diversity and the expansion of vertebrates on land | journal = Biology Letters | volume = 6 | issue = 4 | pages = 544–7 | date = August 2010 | pmid = 20106856 | pmc = 2936204 | doi = 10.1098/rsbl.2009.1024 }}

However, more than 99 percent of all species that ever lived on Earth, amounting to over five billion species,{{cite book | vauthors = McKinney ML | chapter = How do rare species avoid extinction? A paleontological view | title = The Biology of Rarity | date = 1997 | pages = 110–29 | doi = 10.1007/978-94-011-5874-9_7 | isbn = 978-94-010-6483-5 | chapter-url = https://books.google.com/books?id=4LHnCAAAQBAJ&pg=PA110 }} are estimated to be extinct.{{cite book | vauthors = Stearns BP, Stearns SC |title=Watching, from the Edge of Extinction |date=1999 |publisher=Yale University Press |location=New Haven, CT |isbn=978-0-300-08469-6 | url = https://books.google.com/books?id=0BHeC-tXIB4C&pg=PA1921 | page = x }}{{cite news | vauthors = Novacek MJ |date=8 November 2014 |title=Prehistory's Brilliant Future |url=https://www.nytimes.com/2014/11/09/opinion/sunday/prehistorys-brilliant-future.html |newspaper=The New York Times |location=New York |issn=0362-4331 |access-date=2014-12-25}} These extinctions occur at an uneven rate. The dramatic rise in diversity has been marked by periodic, massive losses of diversity classified as mass extinction events. Mass extinction events occur when life undergoes precipitous global declines. Most diversity and biomass on earth is found among the microorganisms, which are difficult to measure. Recorded extinction events are therefore based on the more easily observed changes in the diversity and abundance of larger multicellular organisms, rather than the total diversity and abundance of life.{{cite journal | vauthors = Nee S | title = Extinction, slime, and bottoms | journal = PLOS Biology | volume = 2 | issue = 8 | pages = E272 | date = August 2004 | pmid = 15314670 | pmc = 509315 | doi = 10.1371/journal.pbio.0020272 | doi-access = free }} Marine fossils are mostly used to measure extinction rates because of their superior fossil record and stratigraphic range compared to land organisms.

Based on the fossil record, the background rate of extinctions on Earth is about two to five taxonomic families of marine animals every million years. The Great Oxygenation Event was perhaps the first major extinction event. Since the Cambrian explosion five major mass extinctions have significantly exceeded the background extinction rate.{{cite journal | vauthors = Ward PD | title = Impact from the deep | journal = Scientific American | volume = 295 | issue = 4 | pages = 64–71 | date = October 2006 | pmid = 16989482 | doi = 10.1038/scientificamerican1006-64 | doi-broken-date = 1 November 2024 | bibcode = 2006SciAm.295d..64W }} The worst was the Permian-Triassic extinction event, 251 million years ago. One generally estimates that the Big Five mass extinctions of the Phanerozoic (the last 540 million years) wiped out more than 40% of marine genera and probably more than 70% of marine species.Marine Extinctions: Patterns and Processes - an overview. 2013. CIESM Monograph 45 [https://www.researchgate.net/publication/271767063] The current Holocene extinction caused by human activity, and now referred to as the "sixth extinction", may prove ultimately more devastating.

In order to perform research and enrich Marine Life knowledge, Scientists use various methods in-order to reach and explore the depths of the ocean. several Hi-tech instruments and vehicles are used for this purpose.{{Cite web |title=Investigating Marine Life {{!}} Census of Marine Life |url=http://www.coml.org/investigating/home.html |access-date=2023-12-27 |website=www.coml.org}}

• Autonomous Underwater Vehicles (AUVs)- Underwater robots used to explore the ocean. AUVs are independent robots and can explore unmanned. They are released from a ship and are operated from the surface.{{Cite web |title=Exploration Tools: AUVs: NOAA Office of Ocean Exploration and Research |url=https://oceanexplorer.noaa.gov/technology/subs/auvs/auvs.html |access-date=2023-12-27 |website=oceanexplorer.noaa.gov |language=en-US}}
• Deep-Towed Vehicles (DTVs)- vehicles towed behind research vessels, offering a simpler alternative to more advanced underwater vehicles. They serve as versatile platforms for deploying oceanographic instruments to measure various ocean parameters, with specific models like the DTV BRIDGET used for studying hydrothermal vent plumes by moving near the ocean floor.{{Cite web |title=Deep-Towed Vehicles (DTVs) {{!}} Census of Marine Life |url=http://www.coml.org/investigating/transport/dtvs.html |access-date=2023-12-27 |website=www.coml.org}}
• Manned Submersibles- an manned underwater vehicle used for exploring, experimenting and is often used by army.
• Research vessels (R/Vs)- a boat or ship used to conduct research over a ling period of time. It is capable of transporting a diverse range of sampling and surveying equipment. Research vessels typically feature on-board laboratory space, allowing researchers to promptly analyze the materials collected during cruises.
• Remotely Operated Vehicles (ROVs)- unmanned vehicles. able to reach greater depths under water in order to collect a wider variety of information.{{Cite web |title=The Drone Revolution underwater |url=https://www.advancednavigation.com/robotics/micro-auv/hydrus/ |access-date=2023-12-27 |website=Advanced Navigation |language=en-AU}}

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{{Portal|Marine life|Oceans}}

• {{annotated link|The Blue Planet|Blue Planet}} - David Attenborough
• {{annotated link|Blue Planet II}}
• {{annotated link|Census of Marine Life}}
• {{annotated link|Colonization of land}}
• Marine larval ecology
• {{annotated link|Taxonomy of invertebrates (Brusca & Brusca, 2003)|Taxonomy of invertebrates}}

{{Reflist|30em

| group = note

| refs =

Myxozoa were thought to be an exception, but are now thought to be heavily modified members of the Cnidaria. {{cite journal | vauthors = Jiménez-Guri E, Philippe H, Okamura B, Holland PW | title = Buddenbrockia is a cnidarian worm | journal = Science | volume = 317 | issue = 5834 | pages = 116–8 | date = July 2007 | pmid = 17615357 | doi = 10.1126/science.1142024 | bibcode = 2007Sci...317..116J | s2cid = 5170702 }}

This is the measurement taken by the vessel Kaikō in March 1995 and is considered the most accurate measurement to date. See the Challenger Deep article for more details.

}}

{{Notelist}}

{{Reflist|colwidth=30em|

| refs =

{{cite web |title=7,000 m Class Remotely Operated Vehicle KAIKO 7000 |url=http://www.jamstec.go.jp/e/about/equipment/ships/kaiko7000.html |publisher=Japan Agency for Marine-Earth Science and Technology (JAMSTEC) |access-date=7 June 2008 |archive-date=10 April 2020 |archive-url=https://web.archive.org/web/20200410211118/http://www.jamstec.go.jp/e/about/equipment/ships/kaiko7000.html |url-status=dead }}

{{cite book | vauthors = Kennish KJ |date=2001 |title=Practical handbook of marine science |page=35 |edition=3rd |publisher=CRC Press |series=Marine science series |isbn=978-0-8493-2391-1}}

{{cite journal | vauthors = Le Calvez T, Burgaud G, Mahé S, Barbier G, Vandenkoornhuyse P | title = Fungal diversity in deep-sea hydrothermal ecosystems | journal = Applied and Environmental Microbiology | volume = 75 | issue = 20 | pages = 6415–21 | date = October 2009 | pmid = 19633124 | pmc = 2765129 | doi = 10.1128/AEM.00653-09 | bibcode = 2009ApEnM..75.6415L }}

{{cite web | vauthors = Scott M |date=24 April 2006 |url=http://earthobservatory.nasa.gov/Study/HeatBucket/ |title=Earth's Big heat Bucket |publisher=NASA Earth Observatory |access-date=14 March 2007}}

{{cite web| vauthors = Mullen L |date=11 June 2002 |url=http://www.astrobio.net/news/article223.html |archive-url=https://web.archive.org/web/20070630122335/http://www.astrobio.net/news/article223.html |archive-date=30 June 2007 |title=Salt of the Early Earth |publisher=NASA Astrobiology Magazine |access-date=14 March 2007 |url-status=dead }}

{{Cite journal| vauthors = Charette MA, Smith WH |title=The Volume of Earth's Ocean |journal=Oceanography |volume=23 |issue=2 |pages=112–14 |date=June 2010 |doi=10.5670/oceanog.2010.51 |doi-access=free |bibcode=2010Ocgpy..23b.112C |hdl=1912/3862 |hdl-access=free }}

{{cite web | vauthors = Morris RM |url=http://seis.natsci.csulb.edu/rmorris/oxy/oxy4.html |title=Oceanic Processes |publisher=NASA Astrobiology Magazine |access-date=14 March 2007 |url-status=dead |archive-url=https://web.archive.org/web/20090415082741/http://seis.natsci.csulb.edu/rmorris/oxy/oxy4.html |archive-date=15 April 2009}}

{{cite web | vauthors = Sample S |date=21 June 2005 |url=http://science.hq.nasa.gov/oceans/physical/SST.html |title=Sea Surface Temperature |publisher=NASA |access-date=21 April 2007 |url-status=dead |archive-url=https://web.archive.org/web/20130406163412/http://science.nasa.gov/earth-science/oceanography/ |archive-date=6 April 2013 |df=dmy-all }}

}}

{{Wikivoyage|Marine life}}

{{refbegin}}

• {{cite journal | vauthors = Halpern BS, Walbridge S, Selkoe KA, Kappel CV, Micheli F, D'Agrosa C, Bruno JF, Casey KS, Ebert C, Fox HE, Fujita R, Heinemann D, Lenihan HS, Madin EM, Perry MT, Selig ER, Spalding M, Steneck R, Watson R | display-authors = 6 | title = A global map of human impact on marine ecosystems | journal = Science | volume = 319 | issue = 5865 | pages = 948–52 | date = February 2008 | pmid = 18276889 | doi = 10.1126/science.1149345 | s2cid = 26206024 | bibcode = 2008Sci...319..948H }}
• {{cite journal | vauthors = Paleczny M, Hammill E, Karpouzi V, Pauly D | title = Population Trend of the World's Monitored Seabirds, 1950-2010 | journal = PLOS ONE | volume = 10 | issue = 6 | pages = e0129342 | year = 2015 | pmid = 26058068 | pmc = 4461279 | doi = 10.1371/journal.pone.0129342 | bibcode = 2015PLoSO..1029342P | doi-access = free }}
• {{Cite book | ref = Ruppert | vauthors = Ruppert EE, Fox RS, Barnes RD | title = Invertebrate Zoology | publisher = Brooks / Cole | edition = 7th | isbn = 978-0-03-025982-1 | year = 2004 | url = https://archive.org/details/isbn_9780030259821 }}
• {{cite web | url = https://www.theguardian.com/environment/radical-conservation/2015/sep/22/after-60-million-years-of-extreme-living-seabirds-are-crashing | title = After 60 million years of extreme living, seabirds are crashing | work = The Guardian | date = 22 September 2015 }}

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{{Aquatic ecosystem topics|expanded=marine}}

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Category:Fisheries science