crustacean

File:Crab from Long Island.jpg of a lady crab, part of the hard exoskeleton|alt=A convex oval-shaped piece of shell, covered with fine orange-pink markings: the front edge is lined with 13 coarse serrations, while the rear edge is smooth.]]

File:Krillanatomykils.jpg

The body of a crustacean is composed of segments, which are grouped into three regions: the cephalon or head,{{cite web |url=http://crustacea.nhm.org/glossary/define.html?term=Cephalon |title=Cephalon |work=Crustacean Glossary |publisher=Natural History Museum of Los Angeles County |access-date=2016-09-10 |archive-url=https://web.archive.org/web/20110727124332/http://crustacea.nhm.org/glossary/define.html?term=Cephalon |archive-date=2011-07-27 |url-status=dead }} the pereon or thorax,{{cite web |url=http://crustacea.nhm.org/glossary/define.html?term=Thorax |title=Thorax |work=Crustacean Glossary |publisher=Natural History Museum of Los Angeles County |access-date=2016-09-10 |archive-url=https://web.archive.org/web/20110727124551/http://crustacea.nhm.org/glossary/define.html?term=Thorax |archive-date=2011-07-27 |url-status=dead }} and the pleon or abdomen.{{cite web |url=http://crustacea.nhm.org/glossary/define.html?term=Abdomen |title=Abdomen |work=Crustacean Glossary |publisher=Natural History Museum of Los Angeles County |access-date=2016-09-10 |archive-url=https://web.archive.org/web/20110727123419/http://crustacea.nhm.org/glossary/define.html?term=Abdomen |archive-date=2011-07-27 |url-status=dead }} The head and thorax may be fused together to form a cephalothorax,{{cite web |url=http://crustacea.nhm.org/glossary/define.html?term=Cephalothorax |title=Cephalothorax |work=Crustacean Glossary |publisher=Natural History Museum of Los Angeles County |access-date=2016-09-10 |archive-url=https://web.archive.org/web/20110727123552/http://crustacea.nhm.org/glossary/define.html?term=Cephalothorax |archive-date=2011-07-27 |url-status=dead }} which may be covered by a single large carapace.{{cite web |url=http://crustacea.nhm.org/glossary/define.html?term=Carapace |title=Carapace |work=Crustacean Glossary |publisher=Natural History Museum of Los Angeles County |access-date=2016-09-10 |archive-url=https://web.archive.org/web/20110727123601/http://crustacea.nhm.org/glossary/define.html?term=Carapace |archive-date=2011-07-27 |url-status=dead }} The crustacean body is protected by the hard exoskeleton, which must be moulted for the animal to grow. The shell around each somite can be divided into a dorsal tergum, ventral sternum and a lateral pleuron. Various parts of the exoskeleton may be fused together.{{cite book |author1=P. J. Hayward |author2=J. S. Ryland |year=1995 |title=Handbook of the marine fauna of north-west Europe |publisher=Oxford University Press |isbn=978-0-19-854055-7 |url={{google books |plainurl=y |id=sZ8mdRT4jbIC}} |access-date=2016-09-10}}{{rp|289}}

Each somite, or body segment can bear a pair of appendages: on the segments of the head, these include two pairs of antennae, the mandibles and maxillae; the thoracic segments bear legs, which may be specialised as pereiopods (walking legs) and maxillipeds (feeding legs). Malacostraca and Remipedia (and the hexapods) have abdominal appendages. All other classes of crustaceans have a limbless abdomen, except from a telson and caudal rami which is present in many groups.{{Cite journal|title=How body patterning might have worked in the evolution of arthropods—A case study of the mystacocarid Derocheilocaris remanei (Crustacea, Oligostraca)|first1=Martin|last1=Fritsch|first2=Stefan|last2=Richter|date=September 5, 2022|journal=Journal of Experimental Zoology Part B: Molecular and Developmental Evolution|volume=338|issue=6|pages=342–359|doi=10.1002/jez.b.23140|pmid=35486026 |bibcode=2022JEZB..338..342F |s2cid=248430846 |doi-access=free}}[https://rosdok.uni-rostock.de/file/rosdok_disshab_0000001592/rosdok_derivate_0000034811/Dissertation_Stegner_2016.pdf Morphology of the brain in Hutchinsoniella macracantha (Cephalocarida, Crustacea) – page 290]

The abdomen in malacostracans bears pleopods, and ends in a telson, which bears the anus, and is often flanked by uropods to form a tail fan.{{cite web |url=http://crustacea.nhm.org/glossary/define.html?term=Telson |title=Telson |work=Crustacean Glossary |publisher=Natural History Museum of Los Angeles County |access-date=2016-09-10 |archive-url=https://web.archive.org/web/20110727123649/http://crustacea.nhm.org/glossary/define.html?term=Telson |archive-date=2011-07-27 |url-status=dead }} The number and variety of appendages in different crustaceans may be partly responsible for the group's success.{{cite journal |journal=Science |date=July 4, 1997 |volume=277 |issue=5322 |pages=36 |doi=10.1126/science.277.5322.36 |title=Crab legs and lobster claws |author=Elizabeth Pennisi |author-link=Elizabeth Pennisi |s2cid=83148200 }}

Crustacean appendages are typically biramous, meaning they are divided into two parts; this includes the second pair of antennae, but not the first, which is usually uniramous, the exception being in the Class Malacostraca where the antennules may be generally biramous or even triramous.{{cite web |url=http://crustacea.nhm.org/glossary/define.html?term=Antennule |title=Antennule |work=Crustacean Glossary |publisher=Natural History Museum of Los Angeles County |access-date=2016-09-10 |archive-date=2013-11-05 |archive-url=https://web.archive.org/web/20131105090306/http://crustacea.nhm.org/glossary/define.html?term=Antennule |url-status=dead }}{{cite web |url=http://www.ucmp.berkeley.edu/arthropoda/crustacea/appendages.html |title=Crustaceamorpha: appendages |publisher=University of California, Berkeley |access-date=2016-09-10}} It is unclear whether the biramous condition is a derived state which evolved in crustaceans, or whether the second branch of the limb has been lost in all other groups. Trilobites, for instance, also possessed biramous appendages.{{cite journal |title=Trilobite tagmosis and body patterning from morphological and developmental perspectives |author=N. C. Hughes |date=February 2003 |journal=Integrative and Comparative Biology |volume=43 |issue=1 |pages=185–206 |doi=10.1093/icb/43.1.185|pmid=21680423|doi-access=free }}

{{See also|Hemolymph}}

The main body cavity is an open circulatory system, where blood is pumped into the haemocoel by a heart located near the dorsum.{{cite web |url=http://www2.gsu.edu/~bioasx/closeopen.html |title=Closed and Open Circulatory System |publisher=Georgia State University |author=Akira Sakurai |access-date=2016-09-10 |archive-date=2016-09-17 |archive-url=https://web.archive.org/web/20160917173108/http://www2.gsu.edu/~bioasx/closeopen.html |url-status=dead }} Malacostraca have haemocyanin as the oxygen-carrying pigment, while copepods, ostracods, barnacles and branchiopods have haemoglobins.{{cite book |author=Klaus Urich |year=1994 |title=Comparative Animal Biochemistry |publisher=Springer |isbn=978-3-540-57420-0 |chapter=Respiratory pigments |pages=249–287 |chapter-url={{google books |plainurl=y |id=GLbcWyeaCGQC|page=272}}}} The alimentary canal consists of a straight tube that often has a gizzard-like "gastric mill" for grinding food and a pair of digestive glands that absorb food; this structure goes in a spiral format.{{cite book |author=H. J. Ceccaldi |title=Anatomy and physiology of digestive tract of Crustaceans Decapods reared in aquaculture |pages=243–259 |series=AQUACOP, IFREMER. Actes de Colloque 9 |url=http://www.ifremer.fr/docelec/doc/1989/acte-1486.pdf |work=Advances in Tropical Aquaculture. Tahiti Feb. 20 – March 4, 1989 }}{{Dead link|date=March 2022 |bot=InternetArchiveBot |fix-attempted=yes }} Structures that function as kidneys are located near the antennae. A brain exists in the form of ganglia close to the antennae, and a collection of major ganglia is found below the gut.{{cite book |title=Encarta |year=2005 |chapter=Crustacean |first=Michael T. |last=Ghiselin |author-link=Michael Ghiselin |publisher=Microsoft|title-link=Encarta }}

In many decapods, the first (and sometimes the second) pair of pleopods are specialised in the male for sperm transfer. Many terrestrial crustaceans (such as the Christmas Island red crab) mate seasonally and return to the sea to release the eggs. Others, such as woodlice, lay their eggs on land, albeit in damp conditions. In most decapods, the females retain the eggs until they hatch into free-swimming larvae.{{cite journal|author=Burkenroad, M. D.|year=1963|title=The evolution of the Eucarida (Crustacea, Eumalacostraca), in relation to the fossil record|journal=Tulane Studies in Geology|issue=1|pages=1–17 | volume=2}}

File:Abludomelita obtusata.jpg, an amphipod]]

Most crustaceans are aquatic, living in either marine or freshwater environments, but a few groups have adapted to life on land, such as terrestrial crabs, terrestrial hermit crabs, and woodlice. Marine crustaceans are as ubiquitous in the oceans as insects are on land.{{cite web |url=https://australian.museum/learn/animals/crustaceans/ |title=Crabs, lobsters, prawns and other crustaceans |publisher=Australian Museum |access-date=2016-09-10 |date=January 5, 2010}}{{cite web |url=http://www.fisheries.is/ecosystem/marine-life/benthic-animals/ |title=Benthic animals |publisher=Icelandic Ministry of Fisheries and Agriculture |access-date=2016-09-10 |url-status=dead |archive-url=https://web.archive.org/web/20140511155231/http://www.fisheries.is/ecosystem/marine-life/benthic-animals/ |archive-date=2014-05-11 }} Most crustaceans are also motile, moving about independently, although a few taxonomic units are parasitic and live attached to their hosts (including sea lice, fish lice, whale lice, tongue worms, and Cymothoa exigua, all of which may be referred to as "crustacean lice"), and adult barnacles live a sessile life – they are attached headfirst to the substrate and cannot move independently. Some branchiurans are able to withstand rapid changes of salinity and will also switch hosts from marine to non-marine species.{{cite book |editor1=James H. Thorp |editor2=Alan P. Covich |year=1991 |title=Ecology and Classification of North American Freshwater Invertebrates |publisher=Academic Press |isbn=978-0-12-690645-5 |chapter-url={{google books |plainurl=y |id=hA9VWcz1lwsC}} |access-date=2016-09-10 |edition=1st |author1=Alan P. Covich |author2=James H. Thorp |chapter=Crustacea: Introduction and Peracarida |pages=665–722}}{{rp|672}} Krill are the bottom layer and most important part of the food chain in Antarctic animal communities.{{cite book |first1=P. D. |last1=Virtue |first2=P. D. |last2=Nichols |first3=S. |last3=Nicols |chapter=Dietary-related mechanisms of survival in Euphasia superba: biochemical changes during long term starvation and bacteria as a possible source of nutrition. |editor1=Bruno Battaglia |editor-first2=José |editor-last2=Valencia |editor-first3=D. W. H. |editor-last3=Walton |year=1997 |title=Antarctic communities: species, structure, and survival |publisher=Cambridge University Press |isbn=978-0-521-48033-8 |chapter-url={{google books |plainurl=y |id=P_DMEiH6AM4C}}|access-date=2016-09-10}}{{rp|64}} Some crustaceans are significant invasive species, such as the Chinese mitten crab, Eriocheir sinensis,{{cite web |work=Global Invasive Species Database |title=Eriocheir sinensis |url=http://www.europe-aliens.org/pdf/Eriocheir_sinensis.pdf |access-date=2016-09-10 |first=Stephan |last=Gollasch |date=October 30, 2006 |publisher=Invasive Species Specialist Group |archive-date=2017-12-24 |archive-url=https://web.archive.org/web/20171224080706/http://www.europe-aliens.org/pdf/Eriocheir_sinensis.pdf |url-status=dead }} and the Asian shore crab, Hemigrapsus sanguineus.{{cite book |editor-first= Frederick R. |editor-last=Schram |editor-link= Frederick Schram |editor2-first=J. C. von Vaupel |editor2-last= Klein |year=1999 |title= Crustaceans and the biodiversity crisis: Proceedings of the Fourth International Crustacean Congress, Amsterdam, the Netherlands, July 20–24, 1998 |chapter-url={{google books |plainurl=y |id= msw2BgcugkcC}} |access-date=2016-09-10 |author= John J. McDermott |chapter=The western Pacific brachyuran Hemigrapsus sanguineus (Grapsidae) in its new habitat along the Atlantic coast of the United States: feeding, cheliped morphology and growth |pages= 425–444 |publisher= Koninklijke Brill |isbn= 978-90-04-11387-9}} Since the opening of the Suez Canal, close to 100 species of crustaceans from the Red Sea and the Indo-Pacific realm have established themselves in the eastern Mediterranean sub-basin, with often significant impact on local ecosystems.{{cite book |editor-last1=Briand |editor-first1=Frederic | last1=Galil |first1=Bella | last2=Froglia |first2=Carlo | last3=Noël | first3=Pierre |title=CIESM Atlas of Exotic Species in the Mediterranean: Vol 2 Crustaceans |date=2002 |publisher=CIESM Publishers |location=Paris, Monaco |isbn=92-990003-2-8 |page=192 |url=https://ciesm.org/catalog/index.php?article=2002}}

File:Potamon fluviatile9.jpg, a freshwater crab|alt=Seven round translucent spheres: inside some of them, a pair of compound eyes can be seen.]]

File:Homarus gammarus zoea.jpg larva of the European lobster, Homarus gammarus|alt=A grey-green translucent animal is seen from the side. The eye is large and shining and is in a recess of the large carapace and its long rostrum. An abdomen, similar in length to the carapace, projects from the rear, and below the carapace, there is a mass of legs, some with small claws.]]

Most crustaceans have separate sexes, and reproduce sexually. In fact, a recent study explains how the male T. californicus decide which females to mate with by dietary differences, preferring when the females are algae-fed instead of yeast-fed.{{cite book |chapter-url=https://www.britannica.com/EBchecked/topic/144848/crustacean/33799/Natural-history# |chapter=Crustacean (arthropod) |title=Encyclopædia Britannica |title-link=Encyclopædia Britannica |date=5 May 2023 }} A small number are hermaphrodites, including barnacles, remipedes,{{Cite web |url=http://www.luciopesce.net/gw/remi.html |title=Remipedia Yager, 1981 |author=G. L. Pesce}} and Cephalocarida.{{cite encyclopedia |url=http://www.thecanadianencyclopedia.com/index.cfm?PgNm=TCE&Params=A1ARTA0002059 |title=Crustacean |encyclopedia=The Canadian Encyclopedia |access-date=2016-09-10 |author1=D. E. Aiken |author2=V. Tunnicliffe |author3=C. T. Shih |author4=L. D. Delorme |archive-date=2011-06-07 |archive-url=https://web.archive.org/web/20110607134942/http://www.thecanadianencyclopedia.com/index.cfm?PgNm=TCE&Params=A1ARTA0002059 |url-status=dead }} Some may even change sex during the course of their life. Parthenogenesis is also widespread among crustaceans, where viable eggs are produced by a female without needing fertilisation by a male. This occurs in many branchiopods, some ostracods, some isopods, and certain "higher" crustaceans, such as the Marmorkrebs crayfish.

In many crustaceans, the fertilised eggs are released into the water column, while others have developed a number of mechanisms for holding on to the eggs until they are ready to hatch. Most decapods carry the eggs attached to the pleopods, while peracarids, notostracans, anostracans, and many isopods form a brood pouch from the carapace and thoracic limbs. Female Branchiura do not carry eggs in external ovisacs but attach them in rows to rocks and other objects.{{cite book |editor1=James H. Thorp |editor2=Alan P. Covich |year=2001 |title=Ecology and classification of North American freshwater invertebrates |publisher=Academic Press |isbn=978-0-12-690647-9 |chapter-url={{google books |plainurl=y |id=aj2ZMSekmHEC}}|access-date=2016-09-10|edition=2nd |author1=Alan P. Covich |author2=James H. Thorp |chapter=Introduction to the Subphylum Crustacea |pages=777–798}}{{rp|788}} Most leptostracans and krill carry the eggs between their thoracic limbs; some copepods carry their eggs in special thin-walled sacs, while others have them attached together in long, tangled strings.

{{main article|Crustacean larvae}}

Crustaceans exhibit a number of larval forms, of which the earliest and most characteristic is the nauplius. This has three pairs of appendages, all emerging from the young animal's head, and a single naupliar eye. In most groups, there are further larval stages, including the zoea (pl. zoeæ or zoeas{{OED|Zoea}}). This name was given to it when naturalists believed it to be a separate species.{{Cite EB1911 |wstitle=Crab |volume=7 |page=356|first=William Thomas |last=Calman |author-link=William Thomas Calman}} It follows the nauplius stage and precedes the post-larva. Zoea larvae swim with their thoracic appendages, as opposed to nauplii, which use cephalic appendages, and megalopa, which use abdominal appendages for swimming. It often has spikes on its carapace, which may assist these small organisms in maintaining directional swimming.{{cite journal |author=W. F. R. Weldon |author-link=Raphael Weldon |date=July 1889 |title=Note on the function of the spines of the Crustacean zoœa |journal=Journal of the Marine Biological Association of the United Kingdom |volume=1 |issue=2 |pages=169–172 |url=http://sabella.mba.ac.uk/16/01/Note_on_the_Function_of_the_Spines_of_the_Crustacean_Zooea.pdf |doi=10.1017/S0025315400057994 |doi-broken-date=1 November 2024 |bibcode=1889JMBUK...1..169W |s2cid=54759780 |url-status=dead |archive-url=https://web.archive.org/web/20110717055819/http://sabella.mba.ac.uk/16/01/Note_on_the_Function_of_the_Spines_of_the_Crustacean_Zooea.pdf |archive-date=2011-07-17 }} In many decapods, due to their accelerated development, the zoea is the first larval stage. In some cases, the zoea stage is followed by the mysis stage, and in others, by the megalopa stage, depending on the crustacean group involved.

Providing camouflage against predators, the otherwise black eyes in several forms of swimming larvae are covered by a thin layer of crystalline isoxanthopterin that gives their eyes the same color as the surrounding water, while tiny holes in the layer allow light to reach the retina.Shavit, Keshet, et al, [https://www.science.org/doi/10.1126/science.add4099 A tunable reflector enabling crustaceans to see but not be seen], Science, February 16, 2023, and published in volume 379, issue 6633, February 17, 2023 As the larvae mature into adults, the layer migrates to a new position behind the retina where it works as a backscattering mirror that increases the intensity of light passing through the eyes, as seen in many nocturnal animals.{{Cite journal|url=https://slate.com/technology/2023/02/shrimp-larvae-eye-glitter-nanotechnology.html|title="Disco Eye-Glitter" Makes Baby Crustaceans Invisible|first=Meg|last=Duff|journal=Slate |date=February 16, 2023|via=slate.com}}

In an effort to understand whether DNA repair processes can protect crustaceans against DNA damage, basic research was conducted to elucidate the repair mechanisms used by Penaeus monodon (black tiger shrimp).{{cite journal |last1=Srivastava |first1=Shikha |last2=Dahal |first2=Sumedha |last3=Naidu |first3=Sharanya J. |last4=Anand |first4=Deepika |last5=Gopalakrishnan |first5=Vidya |last6=Kooloth Valappil |first6=Rajendran |last7=Raghavan |first7=Sathees C. |title=DNA double-strand break repair in Penaeus monodon is predominantly dependent on homologous recombination |journal=DNA Research |date=24 January 2017 |volume=24 |issue=2 |pages=117–128 |doi=10.1093/dnares/dsw059 |pmid=28431013 |pmc=5397610 }} Repair of DNA double-strand breaks was found to be predominantly carried out by accurate homologous recombinational repair. Another, less accurate process, microhomology-mediated end joining, is also used to repair such breaks. The expression pattern of DNA repair related and DNA damage response genes in the intertidal copepod Tigriopus japonicus was analyzed after ultraviolet irradiation.{{cite journal|pmid=22051804 |date=2012 |last1=Rhee |first1=J. S. |last2=Kim |first2=B. M. |last3=Choi |first3=B. S. |last4=Lee |first4=J. S. |title=Expression pattern analysis of DNA repair-related and DNA damage response genes revealed by 55K oligomicroarray upon UV-B irradiation in the intertidal copepod, Tigriopus japonicus |journal=Comparative Biochemistry and Physiology. Toxicology & Pharmacology|volume=155 |issue=2 |pages=359–368 |doi=10.1016/j.cbpc.2011.10.005 }} This study revealed increased expression of proteins associated with the DNA repair processes of non-homologous end joining, homologous recombination, base excision repair and DNA mismatch repair.

File:Haeckel Copepoda.jpg's 1904 work Kunstformen der Natur]]

File:Haeckel Decapoda.jpg

The name "crustacean" dates from the earliest works to describe the animals, including those of Pierre Belon and Guillaume Rondelet, but the name was not used by some later authors, including Carl Linnaeus, who included crustaceans among the "Aptera" in his {{lang|la|Systema Naturae}}.{{cite book |chapter-url=http://nlbif.eti.uva.nl/bis/lobsters.php?selected=foto&menuentry=inleiding&record=Classification |chapter=Introduction |pages=1–2 |series=FAO Species Catalogue, Volume 13 |title=Marine Lobsters of the World |author=Lipke B. Holthuis |author-link=Lipke Holthuis |publisher=Food and Agriculture Organization |year=1991 |isbn=978-92-5-103027-1 }}{{Dead link|date=October 2022 |bot=InternetArchiveBot |fix-attempted=yes }} The earliest nomenclatural valid work to use the name "Crustacea" was Morten Thrane Brünnich's {{lang|la|Zoologiæ Fundamenta}} in 1772,{{cite book |author=M. T. Brünnich |author-link=Morten Thrane Brünnich |year=1772 |title=Zoologiæ fundamenta prælectionibus academicis accomodata. Grunde i Dyrelaeren |language=la, da |publisher=Fridericus Christianus Pelt |location=Copenhagen & Leipzig |pages=[https://archive.org/details/mthbrnnichiizo00br/page/n8 1]–254 |url=https://archive.org/details/mthbrnnichiizo00br}} although he also included chelicerates in the group.

The subphylum Crustacea comprises almost 67,000 described species,{{cite journal |author=Zhi-Qiang Zhang |year=2011 |editor=Z.-Q. Zhang |title=Animal biodiversity: an outline of higher-level classification and survey of taxonomic richness - Phylum Arthropoda von Siebold, 1848|journal=Zootaxa |volume=4138 |pages=99–103 |url=http://www.mapress.com/zootaxa/2011/f/zt03148p103.pdf }} which is thought to be just {{frac|1|10}} to {{frac|1|100}} of the total number as most species remain as yet undiscovered.{{cite book |chapter-url=http://www.teara.govt.nz/en/crabs-crayfish-and-other-crustaceans/1 |title=Te Ara: The Encyclopedia of New Zealand |chapter=Crustaceans — bugs of the sea |publisher=Ministry for Culture and Heritage |access-date=2016-09-10|title-link=Te Ara: The Encyclopedia of New Zealand }} Although most crustaceans are small, their morphology varies greatly and includes both the largest arthropod in the world – the Japanese spider crab with a leg span of {{convert|12|ft|order=flip}}{{cite web |url=http://www.aquarium.org/JapaneseSpiderCrabs.asp |title=Japanese Spider Crabs Arrive at Aquarium |work=Oregon Coast Aquarium |access-date=2016-09-10 |url-status=dead |archive-url=https://web.archive.org/web/20100323043021/http://www.aquarium.org/JapaneseSpiderCrabs.asp |archive-date=2010-03-23 }} – and the smallest, the 100-micrometre-long (0.004 in) Stygotantulus stocki.{{cite journal |title=Biodiversity and body size are linked across metazoans |author1=Craig R. McClain |author2=Alison G. Boyer |doi=10.1098/rspb.2009.0245 |pmc=2677615 |journal=Proceedings of the Royal Society B: Biological Sciences |pmid=19324730 |date=June 22, 2009 |volume= 276|issue=1665 |pages=2209–2215 |url=}} Despite their diversity of form, crustaceans are united by the special larval form known as the nauplius.

The exact relationships of the Crustacea to other taxa are not completely settled {{as of|2012|April|lc=yes}}. Studies based on morphology led to the Pancrustacea hypothesis,{{cite journal|author1=J. Zrzavý |author2=P. Štys |title=The basic body plan of arthropods: insights from evolutionary morphology and developmental biology |journal=Journal of Evolutionary Biology |date=May 1997 |volume=10 |issue=3 |pages=353–367 |doi=10.1046/j.1420-9101.1997.10030353.x|s2cid=84906139 |doi-access=free }} in which Crustacea and Hexapoda (insects and allies) are sister groups. More recent studies using DNA sequences suggest that Crustacea is paraphyletic, with the hexapods nested within a larger Pancrustacea clade.{{cite journal |author1=Jerome C. Regier |author2=Jeffrey W. Shultz |author3=Andreas Zwick |author4=April Hussey |author5=Bernard Ball |author6=Regina Wetzer |author7=Joel W. Martin |author8=Clifford W. Cunningham |date=February 25, 2010 |title=Arthropod relationships revealed by phylogenomic analysis of nuclear protein-coding sequences |journal=Nature |volume=463 |pages=1079–1083 |doi=10.1038/nature08742 |pmid=20147900 |issue=7284|bibcode = 2010Natur.463.1079R |s2cid=4427443 }}{{cite journal |author1=Björn M. von Reumont |author2=Ronald A. Jenner |author3=Matthew A. Wills |author4=Emiliano Dell'Ampio |author5=Günther Pass |author6=Ingo Ebersberger |author7=Benjamin Meyer |author8=Stefan Koenemann |author9=Thomas M. Iliffe |author10=Alexandros Stamatakis |author11=Oliver Niehuis |author12=Karen Meusemann |author13=Bernhard Misof |date=March 2012 |title=Pancrustacean phylogeny in the light of new phylogenomic data: support for Remipedia as the possible sister group of Hexapoda |journal=Molecular Biology and Evolution |volume=29 |issue=3 |pages=1031–1045 |doi=10.1093/molbev/msr270 |pmid=22049065|doi-access=free }}

The traditional classification of Crustacea based on morphology recognised four to six classes.{{cite book |url=http://atiniui.nhm.org/pdfs/3839/3839.pdf |title=An Updated Classification of the Recent Crustacea |author1=Joel W. Martin |author2=George E. Davis |year=2001 |pages=1–132 |publisher=Natural History Museum of Los Angeles County |access-date=2009-12-14 |archive-date=2013-05-12 |archive-url=https://web.archive.org/web/20130512091254/http://atiniui.nhm.org/pdfs/3839/3839.pdf |url-status=dead }}

Bowman and Abele (1982) recognised 652 extant families and 38 orders, organised into six classes: Branchiopoda, Remipedia, Cephalocarida,

Maxillopoda, Ostracoda, and Malacostraca. Martin and Davis (2001) updated this classification, retaining the six classes but including 849 extant families in 42 orders. Despite outlining the evidence that Maxillopoda was non-monophyletic, they retained it as one of the six classes, although did suggest that Maxillipoda could be replaced by elevating its subclasses to classes.{{cite journal |first=Rony |last=Huys |title=An Updated Classification of the Recent Crustacea |journal=Journal of Crustacean Biology |volume=23 |issue=2 |year=2003 |pages=495–497 |doi=10.1163/20021975-99990355 |department=review|doi-access=free }} Since then phylogenetic studies have confirmed the polyphyly of Maxillipoda and the paraphyletic nature of Crustacea with respect to Hexapoda.{{cite journal |first1=Todd H. |last1=Oakley |first2=Joanna M. |last2=Wolfe |first3=Annie R. |last3=Lindgren |first4=Alexander K. |last4=Zaharoff |date=January 2013 |title=Phylotranscriptomics to bring the understudied into the fold: monophyletic ostracoda, fossil placement, and pancrustacean phylogeny |journal=Molecular Biology and Evolution |volume=30 |pages=215–233 |doi=10.1093/molbev/mss216 |pmid=22977117 |issue=1|doi-access=free }}{{cite journal|last1=Schwentner |first1=M |last2=Combosch |first2=DJ |last3=Nelson |first3=JP |last4=Giribet |first4=G |date=2017|title=A Phylogenomic Solution to the Origin of Insects by Resolving Crustacean-Hexapod Relationships|journal=Current Biology|volume=27|issue=12|pages=1818–1824.e5|doi=10.1016/j.cub.2017.05.040 |pmid=28602656|doi-access=free|bibcode=2017CBio...27E1818S }}{{cite journal |doi=10.1093/gbe/evz097|title=Pancrustacean Evolution Illuminated by Taxon-Rich Genomic-Scale Data Sets with an Expanded Remipede Sampling |year=2019 |last1=Lozano-Fernandez |first1=Jesus |last2=Giacomelli |first2=Mattia |last3=Fleming |first3=James F. |last4=Chen |first4=Albert |last5=Vinther |first5=Jakob |last6=Thomsen |first6=Philip Francis |last7=Glenner |first7=Henrik |last8=Palero |first8=Ferran |last9=Legg |first9=David A. |last10=Iliffe |first10=Thomas M. |last11=Pisani |first11=Davide |last12=Olesen |first12=Jørgen |journal=Genome Biology and Evolution |volume=11 |issue=8 |pages=2055–2070 |pmid=31270537 |pmc=6684935 }}{{cite journal |last1=Bernot |first1=James P. |last2=Owen |first2=Christopher L. |last3=Wolfe |first3=Joanna M. |last4=Meland |first4=Kenneth |last5=Olesen |first5=Jørgen |last6=Crandall |first6=Keith A. |title=Major Revisions in Pancrustacean Phylogeny and Evidence of Sensitivity to Taxon Sampling |year=2023 |journal=Molecular Biology and Evolution |volume=40 |issue=8 |page=msad175 |doi=10.1093/molbev/msad175 |pmid=37552897 |pmc=10414812 }} Recent classifications recognise ten to twelve classes in Crustacea or Pancrustacea, with several former maxillopod subclasses now recognised as classes (e.g. Thecostraca, Tantulocarida, Mystacocarida, Copepoda, Branchiura and Pentastomida).{{Cite book|last=Brusca|first=Richard C. |title=Invertebrates |edition= 3rd |publisher=Sinauer Associates |year=2016 |isbn=9781605353753 |location=Sunderland, MA |pages=222}}{{cite book |last1=Giribet |first1=G. |last2=Edgecombe |first2=G.D. |title=The Invertebrate Tree of Life |year=2020 |publisher=Princeton University Press |url={{GBurl|id=YHetDwAAQBAJ|p=21}} |page=21 |isbn=978-0-6911-7025-1 |access-date=27 May 2023 }}

The following cladogram shows the updated relationships between the different extant groups of the paraphyletic Crustacea in relation to the class Hexapoda.

{{clade

|grouplabel1=Crustacea

|label1=Pancrustacea

|1={{clade

|label1=Oligostraca

|1={{clade

|1=Ostracoda |barbegin1=green

|2={{clade

|1=Mystacocarida|bar1=green

|label2=Ichthyostraca

|2={{clade

|1=Branchiura|bar1=green

|2=Pentastomida|bar2=green

}}

}}

}}

|label2=Altocrustacea

|2={{clade

|label1=Multicrustacea

|1={{clade

|1=Malacostraca |bar1=green

|2={{clade

|1=Copepoda|bar1=green

|2={{clade

|1=Tantulocarida|bar1=green

|2=Thecostraca|bar2=green

}}

}}

}}

|label2=Allotriocarida

|2={{clade

|1=Cephalocarida|bar1=green

|label2=Athalassocarida

|2={{clade

|1=Branchiopoda  |bar1=green

|label2=Labiocarida

|2={{clade

|1=Remipedia|barend1=green

|2=Hexapoda

}}

}}

}}

}}

}}

}}

According to this diagram, the Hexapoda are deep in the Crustacea tree, and any of the Hexapoda is distinctly closer to e.g. a Multicrustacean than an Oligostracan is.

File:Eryma mandelslohi (Krebs) - Oberer Brauner Jura - Bissingen unter Teck.jpg, a fossil decapod from the Jurassic of Bissingen an der Teck, Germany|alt=In a smooth grey block of stone, there is a brown fossil similar to a crayfish. Two long legs, each with a large claw extend forwards from the animal; one of the claws is held open.]]

Crustaceans have a rich and extensive fossil record, most of the major groups of crustaceans appear in the fossil record before the end of the Cambrian, namely the Branchiopoda, Maxillopoda (including barnacles and tongue worms) and Malacostraca; there is some debate as to whether or not Cambrian animals assigned to Ostracoda are truly ostracods, which would otherwise start in the Ordovician.{{cite web |url=http://www.ucl.ac.uk/GeolSci/micropal/ostracod.html |title=Ostracods |publisher=University College, London |work=An insight into micropalaeontology |first= Matthew |last= Olney |access-date=2016-09-10}} The only classes to appear later are the Cephalocarida,{{cite book |first=R. R. |last=Hessler |year=1984 |chapter=Cephalocarida: living fossil without a fossil record |editor1=N. Eldredge |editor2=S. M. Stanley |title=Living Fossils |publisher=Springer Verlag |location=New York |pages=181–186 |chapter-url={{google books |plainurl=y |id=yeU1zmMkAtgC}}|isbn=978-3-540-90957-6}} which have no fossil record, and the Remipedia, which were first described from the fossil Tesnusocaris goldichi, but do not appear until the Carboniferous.{{cite journal |first1= Stefan |last1= Koenemann |first2= Frederick R.| last2= Schram |authorlink= Frederick Schram |first3= Mario |last3= Hönemann |first4= Thomas M.| last4= Iliffe |date=12 April 2007 |title=Phylogenetic analysis of Remipedia (Crustacea) |journal=Organisms Diversity & Evolution |volume=7 |issue=1 |pages=33–51 |doi=10.1016/j.ode.2006.07.001|bibcode= 2007ODivE...7...33K }} Most of the early crustaceans are rare, but fossil crustaceans become abundant from the Carboniferous period onwards.{{cite web |url= http://palaeo.gly.bris.ac.uk/Palaeofiles/Fossilgroups/Crustacea/fossils.html |title=Fossil Record |work=Fossil Groups: Crustacea |publisher= University of Bristol |access-date=2016-09-10 |archive-url= https://web.archive.org/web/20160907045729/http://palaeo.gly.bris.ac.uk/Palaeofiles/Fossilgroups/Crustacea/fossils.html |archive-date=2016-09-07 |url-status=dead }}

Within the Malacostraca, no fossils are known for krill,{{cite web |url=http://www.aad.gov.au/default.asp?casid=1680 |title=Antarctic Prehistory |publisher=Australian Antarctic Division |date=July 29, 2008 |access-date=February 25, 2010 |archive-url=https://web.archive.org/web/20090930064352/http://www.aad.gov.au/default.asp?casid=1680 |archive-date=September 30, 2009 |url-status=dead }} while both Hoplocarida and Phyllopoda contain important groups that are now extinct as well as extant members (Hoplocarida: mantis shrimp are extant, while Aeschronectida are extinct;{{cite journal |journal=Contributions to Zoology |volume=67 |issue=3 |pages=155–186 |year=1998 |title=Palaeo- and archaeostomatopods (Hoplocarida: Crustacea) from the Bear Gulch Limestone, Mississippian (Namurian), of central Montana |first1= Ronald A.| last1= Jenner |first2= Cees H. J.| last2= Hof |first3= Frederick R. |last3= Schram |doi=10.1163/18759866-06703001 |doi-access=free }} Phyllopoda: Canadaspidida are extinct, while Leptostraca are extant{{cite journal |first= Derek |last= Briggs |author-link=Derek Briggs |date=January 23, 1978 |title=The morphology, mode of life, and affinities of Canadaspis perfecta (Crustacea: Phyllocarida), Middle Cambrian, Burgess Shale, British Columbia |journal=Philosophical Transactions of the Royal Society B |volume=281 |issue=984 |pages=439–487 |doi=10.1098/rstb.1978.0005|bibcode = 1978RSPTB.281..439B |doi-access= }}). Cumacea and Isopoda are both known from the Carboniferous,{{cite journal |first1= Frederick |last1= Schram |first2= Cees H. J.|last2= Hof |first3= Royal H.| last3= Mapes |first4= Polly |last4= Snowdon |name-list-style=amp |year=2003 |url= http://dpc.uba.uva.nl/ctz/vol72/nr01/art01 |title=Paleozoic cumaceans (Crustacea, Malacostraca, Peracarida) from North America |journal=Contributions to Zoology |volume=72 |issue=1 |pages=1–16|doi=10.1163/18759866-07201001 |doi-access=free }}{{cite journal |first= Frederick R. |last= Schram |date=August 28, 1970 |title=Isopod from the Pennsylvanian of Illinois |journal=Science |volume=169 |issue=3948 |pages=854–855 |doi=10.1126/science.169.3948.854 |pmid=5432581 |bibcode= 1970Sci...169..854S|s2cid=31851291 }} as are the first true mantis shrimp.{{cite journal |title=Fossil stomatopods (Crustacea: Malacostraca) and their phylogenetic impact |first= Cees H. J.| last= Hof |doi=10.1080/00222939800771101 |journal=Journal of Natural History |volume=32 |issue=10 & 11 |year=1998 |pages=1567–1576|bibcode= 1998JNatH..32.1567H}} In the Decapoda, prawns and polychelids appear in the Triassic,{{cite web |url= http://palaeo.gly.bris.ac.uk/Palaeofiles/Fossilgroups/Decapoda/dendrobranchiata.html |title=Dendrobranchiata |publisher=University of Bristol |work=Order Decapoda |first= Robert P. D.| last= Crean |date=November 14, 2004 |access-date=February 25, 2010 |archive-url= https://web.archive.org/web/20120229170005/http://palaeo.gly.bris.ac.uk/Palaeofiles/Fossilgroups/Decapoda/dendrobranchiata.html |archive-date=February 29, 2012 |url-status=dead }}{{cite journal |url=http://joi.jlc.jst.go.jp/JST.JSTAGE/prpsj/7.357 |first1= Hiroaki |last1= Karasawa |first2=Fumio |last2= Takahashi |first3= Eiji |last3= Doi |first4= Hideo |last4= Ishida |year=2003 |title=First notice of the family Coleiidae Van Straelen (Crustacea: Decapoda: Eryonoides) from the upper Triassic of Japan |journal=Paleontological Research |volume=7 |issue=4 |pages=357–362 |doi=10.2517/prpsj.7.357|s2cid=129330859 |doi-access=free }} and shrimp and crabs appear in the Jurassic.{{cite journal |title=Two new caridean shrimps, one representing a new family, from marine pools on Ascension Island (Crustacea: Decapoda: Natantia) |first1= Fenner A. Jr.| last1= Chace | first2= Raymond B.|last2= Manning |s2cid=53067015 |author2-link= Raymond B. Manning |journal=Smithsonian Contributions to Zoology |year=1972 |volume=131 |issue=131 |pages=1–18 |doi= 10.5479/si.00810282.131}} The fossil burrow Ophiomorpha is attributed to ghost shrimps, whereas the fossil burrow Camborygma is attributed to crayfishes. The Permian–Triassic deposits of Nurra preserve the oldest (Permian: Roadian) fluvial burrows ascribed to ghost shrimps (Decapoda: Axiidea, Gebiidea) and crayfishes (Decapoda: Astacidea, Parastacidea), respectively.{{cite journal| last1= Baucon| first1= A.| last2= Ronchi| first2= A.| last3= Felletti| first3= F.| last4= Neto de Carvalho| first4= C.| year= 2014| title= Evolution of Crustaceans at the edge of the end-Permian crisis: ichnonetwork analysis of the fluvial succession of Nurra (Permian-Triassic, Sardinia, Italy)| journal= Palaeogeography, Palaeoclimatology, Palaeoecology| volume= 410| page= 74| doi= 10.1016/j.palaeo.2014.05.034| bibcode= 2014PPP...410...74B| url= http://www.tracemaker.com/portfolio/evolution-of-crustaceans-at-the-edge-of-the-end-permian-crisis-ichnonetwork-analysis-of-the-fluvial-succession-of-nurra-permian-triassic-sardinia-italy/| access-date= May 19, 2022| archive-date= July 6, 2022| archive-url= https://web.archive.org/web/20220706111428/https://www.tracemaker.com/portfolio/evolution-of-crustaceans-at-the-edge-of-the-end-permian-crisis-ichnonetwork-analysis-of-the-fluvial-succession-of-nurra-permian-triassic-sardinia-italy/| url-status= dead}}

However, the great radiation of crustaceans occurred in the Cretaceous, particularly in crabs, and may have been driven by the adaptive radiation of their main predators, bony fish.{{cite journal |first= J. W.| last= Wägele |date=December 1989 |title=On the influence of fishes on the evolution of benthic crustaceans |journal=Zeitschrift für Zoologische Systematik und Evolutionsforschung |volume=27 |issue=4 |pages=297–309 |doi=10.1111/j.1439-0469.1989.tb00352.x|doi-access=free }} The first true lobsters also appear in the Cretaceous.{{cite journal |journal=Journal of Paleontology |year=2005 |volume=79 |issue=5 |pages=961–968 |doi= 10.1666/0022-3360(2005)079[0961:HAANSO]2.0.CO;2 |title=Hoploparia albertaensis, a new species of clawed lobster (Nephropidae) from the Late Coniacean, shallow-marine Bad Heart Formation of northwestern Alberta, Canada |first1=Dale |last1= Tshudy |first2= W. Steven |last2= Donaldson |first3= Christopher |last3= Collom |first4= Rodney M. |last4= Feldmann |first5= Carrie E.| last5= Schweitzer |s2cid=131067067 | display-authors= 3}}

File:Shrimps at market in Valencia.jpgs on sale at a Spanish market|alt=A heap of small pink lobsters on their sides, with their claws extended forwards towards the camera.]]

Many crustaceans are consumed by humans, and nearly 10,700,000 tons were harvested in 2007; the vast majority of this output is of decapod crustaceans: crabs, lobsters, shrimp, crawfish, and prawns.{{cite web |url=http://www.fao.org/figis/servlet/TabLandArea?tb_ds=Production&tb_mode=TABLE&tb_act=SELECT&tb_grp=COUNTRY |publisher=Food and Agriculture Organization |access-date=2016-09-10|title=FIGIS: Global Production Statistics 1950–2007}} Over 60% by weight of all crustaceans caught for consumption are shrimp and prawns, and nearly 80% is produced in Asia, with China alone producing nearly half the world's total. Non-decapod crustaceans are not widely consumed, with only 118,000 tons of krill being caught, despite krill having one of the greatest biomasses on the planet.{{cite book |first1=Steven |last1=Nicol |first2=Yoshinari |last2=Endo |url=http://www.fao.org/DOCREP/003/W5911E/W5911E00.HTM |series=Fisheries Technical Paper |volume=367 |title=Krill Fisheries of the World |publisher=Food and Agriculture Organization |year=1997 |isbn=978-92-5-104012-6}}

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{{Portal|Crustaceans}}

• Pain in crustaceans

{{Reflist}}

• {{cite book |first = Frederick |last=Schram |author-link= Frederick Schram |title=Crustaceans|url={{google books |plainurl=y |id=tm8WAQAAIAAJ}}|isbn=978-0-19-503742-5 |publisher = Oxford University Press |year=1986}}
• Powers, M., Hill, G., Weaver, R., & Goymann, W. (2020). An experimental test of mate choice for red carotenoid coloration in the marine copepod Tigriopus californicus. Ethology., 126(3), 344–352. [https://doi.org/10.1111/eth.12976 An experimental test of mate choice for red carotenoid coloration in the marine copepod Tigriopus californicus]

{{Commons category|Crustacea}}

{{Wikispecies|Crustacea}}

• {{Wikisource-inline|list=
• {{cite Americana|Crustacea |title=Encyclopedia Americana - Wikipedia |volume=8 |noicon=x |short=x}}
• {{Cite NIE |last=Clark |first=Hubert Lyman |last2=Ingersoll |first2=Ernest |author-link=Hubert Lyman Clark |author2-link=Ernest Ingersoll |wstitle=Crustacea |volume=5 |noicon=x |short=x}}

}}

• [http://www.crustacea.net Crustacea.net, an online resource on the biology of crustaceans]
• [http://crustacea.nhm.org/ Crustacea] {{Webarchive|url=https://web.archive.org/web/20071009123630/http://crustacea.nhm.org/|date=October 9, 2007}}: Natural History Museum of Los Angeles County
• [http://www.tolweb.org/Crustacea/2529 Crustacea]: Tree of Life Web Project
• [http://web.vims.edu/tcs/?svr=www The Crustacean Society] {{Webarchive|url=https://web.archive.org/web/20111110110156/http://web.vims.edu/tcs/?svr=www |date=2011-11-10 }}
• [http://www.nhc.ed.ac.uk/index.php?page=24.25.312.330.363 Natural History Collections: Crustacea]: University of Edinburgh
• [http://www.wildsingapore.com/wildfacts/crustacea/crustacea.htm Crustaceans (Crustacea) on the shore of Singapore]
• [http://www.biodiversityexplorer.org/mm/crustacea/ Crustacea(crabs, lobsters, shrimps, prawns, barnacles)] {{Webarchive|url=https://web.archive.org/web/20120111120955/http://www.biodiversityexplorer.org/mm/crustacea/ |date=2012-01-11 }}: Biodiversity Explorer

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Category:Cambrian Series 2 first appearances

Category:Extant Cambrian first appearances

Category:Paraphyletic groups

Category:Pancrustacea