Sea urchin#Feeding and digestion

{{See also|List of echinodermata orders}}

Sea urchins are members of the phylum Echinodermata, which also includes starfish, sea cucumbers, sand dollars, brittle stars, and crinoids. Like other echinoderms, they have five-fold symmetry (called pentamerism) and move by means of hundreds of tiny, transparent, adhesive "tube feet". The symmetry is not obvious in the living animal, but is easily visible in the dried test.

Specifically, the term "sea urchin" refers to the "regular echinoids", which are symmetrical and globular, and includes several different taxonomic groups, with two subclasses: Euechinoidea ("modern" sea urchins, including irregular ones) and Cidaroidea, or "slate-pencil urchins", which have very thick, blunt spines, with algae and sponges growing on them. The "irregular" sea urchins are an infra-class inside the Euechinoidea, called Irregularia, and include Atelostomata and Neognathostomata. Irregular echinoids include flattened sand dollars, sea biscuits, and heart urchins.{{cite WoRMS |author=Kroh, A. |author2=Hansson, H. |year=2013 |title=Echinoidea (Leske, 1778) |id=123082 |access-date=2014-01-04}}

Together with sea cucumbers (Holothuroidea), they make up the subphylum Echinozoa, which is characterized by a globoid shape without arms or projecting rays. Sea cucumbers and the irregular echinoids have secondarily evolved diverse shapes. Although many sea cucumbers have branched tentacles surrounding their oral openings, these have originated from modified tube feet and are not homologous to the arms of the crinoids, sea stars, and brittle stars.

File:Paracentrotus lividus profil.JPG|Paracentrotus lividus, a regular sea urchin (Euechinoidea, infraclass Carinacea)

File:Live Sand Dollar trying to bury itself in beach sand.jpg|A sand dollar, an irregular sea urchin (Irregularia)

File:Phyllacanthus.jpg|Phyllacanthus imperialis, a cidaroid sea urchin (Cidaroidea)

File:Urchin9b.jpg sp.]]

Urchins typically range in size from {{convert|3|to|10|cm|in|0|abbr=on}}, but the largest species can reach up to {{convert|36|cm|in|abbr=on}}.{{cite book |last=Barnes |first=Robert D. |year=1982 |title= Invertebrate Zoology |publisher=Holt-Saunders International |location= Philadelphia, PA|pages= 961–981|isbn= 0-03-056747-5}} They have a rigid, usually spherical body bearing moveable spines, which give the class the name Echinoidea (from the Greek {{wikt-lang|grc|ἐχῖνος}} {{lang|grc-la|ekhinos}} 'spine').{{cite web |url=http://www.tulane.edu/~bwee/eeb111/GreekLatin%20Roots.html |title=Taxonomic Etymologies EEOB 111 |author=Guill, Michael |access-date=13 March 2018 |archive-date=17 May 2024 |archive-url=https://web.archive.org/web/20240517161239/https://www2.tulane.edu/~bwee/eeb111/GreekLatin%20Roots.html |url-status=live }} The name urchin is an old word for hedgehog, which sea urchins resemble; they have archaically been called sea hedgehogs.Wright, Anne. 1851. The Observing Eye, Or, Letters to Children on the Three Lowest Divisions of Animal Life. London: Jarrold and Sons, p. 107.Soyer, Alexis. 1853. The Pantropheon Or History of Food, And Its Preparation: From The Earliest Ages Of The World. Boston: Ticknor, Reed, and Fields, p. 245. The name is derived from the Old French {{wikt-lang|fro|herichun}}, from Latin {{wikt-lang|la|ericius}} ('hedgehog').{{cite encyclopedia |title=urchin (n.) |url=https://www.etymonline.com/word/urchin |dictionary=Online Etymology Dictionary |access-date=13 March 2018 |archive-date=15 March 2018 |archive-url=https://web.archive.org/web/20180315070539/https://www.etymonline.com/word/urchin |url-status=live }}

Like other echinoderms, sea urchin early larvae have bilateral symmetry,Stachan and Read, Human Molecular Genetics, [https://books.google.com/books?id=8U0VAAAAIAAJ&pg=PA381 "What Makes Us Human", p. 381]. but they develop five-fold symmetry as they mature. This is most apparent in the "regular" sea urchins, which have roughly spherical bodies with five equally sized parts radiating out from their central axes. The mouth is at the base of the animal and the anus at the top; the lower surface is described as "oral" and the upper surface as "aboral".{{efn|The tube feet are present in all parts of the animal except around the anus, so technically, the whole surface of the body should be considered to be the oral surface, with the aboral (non-mouth) surface limited to the immediate vicinity of the anus.}}{{Cite book |last1=Ruppert |first1=Edward E. |last2=Fox |first2=Richard S. |last3=Barnes |first3=Robert D. |year=2004 |title=Invertebrate Zoology: A Functional Evolutionary Approach |url=https://archive.org/details/isbn_9780030259821 |url-access=registration |edition=7th |location=Belmont, CA |publisher=Thomson-Brooks/Cole |pages=[https://archive.org/details/isbn_9780030259821/page/896/mode/2up 896–906] |isbn=0030259827 |oclc=53021401}}

Several sea urchins, however, including the sand dollars, are oval in shape, with distinct front and rear ends, giving them a degree of bilateral symmetry. In these urchins, the upper surface of the body is slightly domed, but the underside is flat, while the sides are devoid of tube feet. This "irregular" body form has evolved to allow the animals to burrow through sand or other soft materials.

{{Further|Test (biology)|Tube feet}}

File:Seeigel-Saugfuesse(Galicien2005).jpg of a purple sea urchin]]

The internal organs are enclosed in a hard shell or test composed of fused plates of calcium carbonate covered by a thin dermis and epidermis. The test is referred to as an endoskeleton rather than exoskeleton even though it encloses almost all of the urchin. This is because it is covered with a thin layer of muscle and skin; sea urchins also do not need to molt the way invertebrates with true exoskeletons do, instead the plates forming the test grow as the animal does.

The test is rigid, and divides into five ambulacral grooves separated by five wider interambulacral areas. Each of these ten longitudinal columns consists of two sets of plates (thus comprising 20 columns in total). The ambulacral plates have pairs of tiny holes through which the tube feet extend.{{cite web |title=The Echinoid Directory – Natural History Museum |url=https://www.nhm.ac.uk/our-science/data/echinoid-directory/morphology/regulars/intro.html |website=www.nhm.ac.uk |publisher=Natural History Museum, UK |access-date=28 December 2022 |archive-date=8 December 2023 |archive-url=https://web.archive.org/web/20231208134929/https://www.nhm.ac.uk/our-science/data/echinoid-directory/morphology/regulars/intro.html |url-status=live }}

All of the plates are covered in rounded tubercles to which the spines are attached. The spines are used for defence and for locomotion and come in a variety of forms.{{cite web |title=The Echinoid Directory – Natural History Museum |url=https://www.nhm.ac.uk/our-science/data/echinoid-directory/morphology/regulars/intro.html |website=www.nhm.ac.uk |publisher=Natural History Museum, UK. |access-date=28 December 2022 |archive-date=8 December 2023 |archive-url=https://web.archive.org/web/20231208134929/https://www.nhm.ac.uk/our-science/data/echinoid-directory/morphology/regulars/intro.html |url-status=live }} The inner surface of the test is lined by peritoneum. Sea urchins convert aqueous carbon dioxide using a catalytic process involving nickel into the calcium carbonate portion of the test.{{cite web|url=http://www.gizmag.com/carbon-capture-calcium-carbonate/26101/|title=Sea urchins reveal promising carbon capture alternative|publisher=Gizmag|date=4 February 2013|access-date=2013-02-05|archive-date=2016-07-14|archive-url=https://web.archive.org/web/20160714210351/http://www.gizmag.com/carbon-capture-calcium-carbonate/26101/|url-status=live}}

File:Ricci di mare luminosi.jpg

Most species have two series of spines, primary (long) and secondary (short), distributed over the surface of the body, with the shortest at the poles and the longest at the equator. The spines are usually hollow and cylindrical. Contraction of the muscular sheath that covers the test causes the spines to lean in one direction or another, while an inner sheath of collagen fibres can reversibly change from soft to rigid which can lock the spine in one position. Located among the spines are several types of pedicellaria, moveable stalked structures with jaws.

Sea urchins move by walking, using their many flexible tube feet in a way similar to that of starfish; regular sea urchins do not have any favourite walking direction.Kazuya Yoshimura, Tomoaki Iketani et Tatsuo Motokawa, "Do regular sea urchins show preference in which part of the body they orient forward in their walk ?", Marine Biology, vol. 159, no 5, 2012, p. 959–965. The tube feet protrude through pairs of pores in the test, and are operated by a water vascular system; this works through hydraulic pressure, allowing the sea urchin to pump water into and out of the tube feet. During locomotion, the tube feet are assisted by the spines which can be used for pushing the body along or to lift the test off the substrate. Movement is generally related to feeding, with the red sea urchin (Mesocentrotus franciscanus) managing about {{convert|7.5|cm|in|0|abbr=on}} a day when there is ample food, and up to {{convert|50|cm|in|0|abbr=on}} a day where there is not. An inverted sea urchin can right itself by progressively attaching and detaching its tube feet and manipulating its spines to roll its body upright. Some species bury themselves in soft sediment using their spines, and Paracentrotus lividus uses its jaws to burrow into soft rocks.{{cite book |last1=Boudouresque |first1=Charles F. |last2=Verlaque |first2=Marc |editor-last=Lawrence |editor-first=John M. |title=Edible Sea Urchins: Biology and Ecology |publisher=Elsevier |year=2006 |page=243 |chapter=13: Ecology of Paracentrotus lividus |chapter-url=https://books.google.com/books?id=6T2JomruARoC&pg=PA243 |isbn=978-0-08-046558-6 }}

File:Sea Urchin test 5629 03 22.jpg|Test of an Echinus esculentus, a regular sea urchin

File:BlackSeaUrchinTest.jpg|Test of black sea urchin, showing tubercles and ambulacral plates (on right)

File:Inner surface of black sea urchin test.jpg|Inner surface of test, showing pentagonal interambulacral plates on right, and holes for tube feet on left.

File:Echinodiscus2.jpg|Test of an Echinodiscus tenuissimus, an irregular sea urchin ("sand dollar")

File:Phyllacanthus imperialis test.JPG|Test of a Phyllacanthus imperialis, a cidaroid sea urchin. These are characterised by their big tubercles, bearing large radiola.

File:Sea urchin shell - pattern (6658690371).jpg|Close-up of the test showing an ambulacral groove with its two rows of pore-pairs, between two interambulacra areas (green). The tubercles are non-perforated.

File:Sea Urchin Shell detail.jpg|Close-up of a cidaroid sea urchin apical disc: the 5 holes are the gonopores, and the central one is the anus ("periproct"). The biggest genital plate is the madreporite.{{cite web |title=Apical disc and periproct |url=https://www.nhm.ac.uk/our-science/data/echinoid-directory/morphology/regulars/discmorph.html |publisher=Natural History Museum, London |access-date=2 November 2019 |archive-date=2 March 2024 |archive-url=https://web.archive.org/web/20240302122236/https://www.nhm.ac.uk/our-science/data/echinoid-directory/morphology/regulars/discmorph.html |url-status=live }}

File:Seeigel Gebiss.jpg

The mouth lies in the centre of the oral surface in regular urchins, or towards one end in irregular urchins. It is surrounded by lips of softer tissue, with numerous small, embedded bony pieces. This area, called the peristome, also includes five pairs of modified tube feet and, in many species, five pairs of gills. The jaw apparatus consists of five strong arrow-shaped plates known as pyramids, the ventral surface of each of which has a toothband with a hard tooth pointing towards the centre of the mouth. Specialised muscles control the protrusion of the apparatus and the action of the teeth, and the animal can grasp, scrape, pull and tear. The structure of the mouth and teeth have been found to be so efficient at grasping and grinding that similar structures have been tested for use in mechanical applications.{{Cite web |url=https://www.engadget.com/2016/05/04/sea-urchin-inspired-rover-claw/ |title=Claw inspired by sea urchins' mouth can scoop up Martian soil |access-date=2017-09-11 |archive-date=2019-10-12 |archive-url=https://web.archive.org/web/20191012025318/https://www.engadget.com/2016/05/04/sea-urchin-inspired-rover-claw/ |url-status=live }}

On the upper surface of the test at the aboral pole is a membrane, the periproct, which surrounds the anus. The periproct contains a variable number of hard plates, five of which, the genital plates, contain the gonopores, and one is modified to contain the madreporite, which is used to balance the water vascular system.

File:Lanterne d'Aristote.jpg

The mouth of most sea urchins is made up of five calcium carbonate teeth or plates, with a fleshy, tongue-like structure within. The entire chewing organ is known as Aristotle's lantern from Aristotle's description in his History of Animals (translated by D'Arcy Thompson):

{{Blockquote|... the urchin has what we mainly call its head and mouth down below, and a place for the issue of the residuum up above. The urchin has, also, five hollow teeth inside, and in the middle of these teeth a fleshy substance serving the office of a tongue. Next to this comes the esophagus, and then the stomach, divided into five parts, and filled with excretion, all the five parts uniting at the anal vent, where the shell is perforated for an outlet ... In reality the mouth-apparatus of the urchin is continuous from one end to the other, but to outward appearance it is not so, but looks like a horn lantern with the panes of horn left out.}}

However, this has recently been proven to be a mistranslation. Aristotle's lantern is actually referring to the whole shape of sea urchins, which look like the ancient lamps of Aristotle's time.{{cite journal|last1=Voultsiadou|first1=Eleni|last2=Chintiroglou|first2=Chariton|title=Aristotle's lantern in echinoderms: an ancient riddle|url=http://users.auth.gr/~elvoults/pdf/Aristotle%27s%20lantern%2008.pdf|year=2008|volume=49|pages=299–302|journal=Cahiers de Biologie Marine|publisher=Station Biologique de Roscoff|issue=3|access-date=2020-12-23|archive-date=2020-12-23|archive-url=https://web.archive.org/web/20201223165338/http://users.auth.gr/~elvoults/pdf/Aristotle%27s%20lantern%2008.pdf|url-status=live}}{{cite web|last1=Choi|first1=Charles Q.|title=Rock-Chewing Sea Urchins Have Self-Sharpening Teeth|url=https://news.nationalgeographic.com/news/2010/12/101228-sea-urchin-teeth-self-sharpening-tools-science-animals/|archive-url=https://web.archive.org/web/20101231160917/http://news.nationalgeographic.com/news/2010/12/101228-sea-urchin-teeth-self-sharpening-tools-science-animals/|url-status=dead|archive-date=December 31, 2010|website=National Geographic News|access-date=2017-11-12|date=29 December 2010}}

Heart urchins are unusual in not having a lantern. Instead, the mouth is surrounded by cilia that pull strings of mucus containing food particles towards a series of grooves around the mouth.

File:Echinoidea anatomie.svg; m = madreporite; s = aquifer canal; r = radial canal; p = podial ampulla; k = test wall; i = intestine; b = mouth]]

The lantern, where present, surrounds both the mouth cavity and the pharynx. At the top of the lantern, the pharynx opens into the esophagus, which runs back down the outside of the lantern, to join the small intestine and a single caecum. The small intestine runs in a full circle around the inside of the test, before joining the large intestine, which completes another circuit in the opposite direction. From the large intestine, a rectum ascends towards the anus. Despite the names, the small and large intestines of sea urchins are in no way homologous to the similarly named structures in vertebrates.

Digestion occurs in the intestine, with the caecum producing further digestive enzymes. An additional tube, called the siphon, runs beside much of the intestine, opening into it at both ends. It may be involved in resorption of water from food.

File:Diadema setosum (Kenya).JPG]]

The water vascular system leads downwards from the madreporite through the slender stone canal to the ring canal, which encircles the oesophagus. Radial canals lead from here through each ambulacral area to terminate in a small tentacle that passes through the ambulacral plate near the aboral pole. Lateral canals lead from these radial canals, ending in ampullae. From here, two tubes pass through a pair of pores on the plate to terminate in the tube feet.

Sea urchins possess a hemal system with a complex network of vessels in the mesenteries around the gut, but little is known of the functioning of this system. However, the main circulatory fluid fills the general body cavity, or coelom. This coelomic fluid contains phagocytic coelomocytes, which move through the vascular and hemal systems and are involved in internal transport and gas exchange. The coelomocytes are an essential part of blood clotting, but also collect waste products and actively remove them from the body through the gills and tube feet.

Most sea urchins possess five pairs of external gills attached to the peristomial membrane around their mouths. These thin-walled projections of the body cavity are the main organs of respiration in those urchins that possess them. Fluid can be pumped through the gills' interiors by muscles associated with the lantern, but this does not provide a continuous flow, and occurs only when the animal is low in oxygen. Tube feet can also act as respiratory organs, and are the primary sites of gas exchange in heart urchins and sand dollars, both of which lack gills. The inside of each tube foot is divided by a septum which reduces diffusion between the incoming and outgoing streams of fluid.

The nervous system of sea urchins has a relatively simple layout. With no true brain, the neural center is a large nerve ring encircling the mouth just inside the lantern. From the nerve ring, five nerves radiate underneath the radial canals of the water vascular system, and branch into numerous finer nerves to innervate the tube feet, spines, and pedicellariae.

Sea urchins are sensitive to touch, light, and chemicals. There are numerous sensitive cells in the epithelium, especially in the spines, pedicellaria and tube feet, and around the mouth. Although they do not have eyes or eye spots (except for diadematids, which can follow a threat with their spines), the entire body of most regular sea urchins might function as a compound eye.{{cite journal |last1=Knight |first1=K. |title=Sea Urchins Use Whole Body As Eye |journal=Journal of Experimental Biology |volume=213 |pages=i–ii |year=2009 |doi=10.1242/jeb.041715 |issue=2|doi-access=free }}

• {{cite press release |author=Charles Q. Choi |date=December 28, 2009 |title=Body of Sea Urchin is One Big Eye |website=LiveScience |url=http://www.livescience.com/animals/091228-sea-urchin-eye.html}} In general, sea urchins are negatively attracted to light, and seek to hide themselves in crevices or under objects. Most species, apart from pencil urchins, have statocysts in globular organs called spheridia. These are stalked structures and are located within the ambulacral areas; their function is to help in gravitational orientation.

File:Male Flower Sea Urchin (toxopneustes roseus).theora.ogv) releasing milt, November 1, 2011, Lalo Cove, Sea of Cortez]]

Sea urchins are dioecious, having separate male and female sexes, although no distinguishing features are visible externally. In addition to their role in reproduction, the gonads are also nutrient storing organs, and are made up of two main type of cells: germ cells, and somatic cells called nutritive phagocytes.{{Cite journal |pmc = 5090362|year = 2016|last1 = Gaitán-Espitia|first1 = J. D.|title = Functional insights into the testis transcriptome of the edible sea urchin Loxechinus albus|journal = Scientific Reports|volume = 6|pages = 36516|last2 = Sánchez|first2 = R.|last3 = Bruning|first3 = P.|last4 = Cárdenas|first4 = L.|pmid = 27805042|doi = 10.1038/srep36516|bibcode = 2016NatSR...636516G}} Regular sea urchins have five gonads, lying underneath the interambulacral regions of the test, while the irregular forms mostly have four, with the hindmost gonad being absent; heart urchins have three or two. Each gonad has a single duct rising from the upper pole to open at a gonopore lying in one of the genital plates surrounding the anus. Some burrowing sand dollars have an elongated papilla that enables the liberation of gametes above the surface of the sediment. The gonads are lined with muscles underneath the peritoneum, and these allow the animal to squeeze its gametes through the duct and into the surrounding sea water, where fertilization takes place.

File:Sea Urchin Blastula.jpg

During early development, the sea urchin embryo undergoes 10 cycles of cell division,{{cite journal|title =Arsenic Exposure Affects Embryo Development of Sea Urchin, Paracentrotus lividus (Lamarck, 1816) | journal = Bulletin of Environmental Contamination and Toxicology| volume = 39| issue = 2| pages = 124–8|date =2013 | last= A. Gaion|first = A. Scuderi|author2= D. Pellegrini|author3= D. Sartori | doi = 10.3109/01480545.2015.1041602| pmid = 25945412| s2cid = 207437380}} resulting in a single epithelial layer enveloping the blastocoel. The embryo then begins gastrulation, a multipart process which dramatically rearranges its structure by invagination to produce the three germ layers, involving an epithelial-mesenchymal transition; primary mesenchyme cells move into the blastocoel{{cite journal |last1=Kominami |first1=Tetsuya |last2=Takata |first2=Hiromi |title=Gastrulation in the sea urchin embryo: a model system for analyzing the morphogenesis of a monolayered epithelium |journal=Development, Growth & Differentiation |volume=46 |issue=4 |pages=309–26 |year=2004 |doi=10.1111/j.1440-169x.2004.00755.x|pmid=15367199 |s2cid=23988213 }} and become mesoderm.{{cite journal |doi=10.1016/j.mod.2003.06.005 |last1=Shook |first1=D |last2=Keller |first2=R |title=Mechanisms, mechanics and function of epithelial-mesenchymal transitions in early development |journal=Mechanisms of Development |volume=120 |issue=11 |pages=1351–83 |year=2003 |pmid=14623443|s2cid=15509972 |doi-access=free }}; {{cite journal |last1=Katow |first1=Hideki |last2=Solursh |first2=Michael |title=Ultrastructure of primary mesenchyme cell ingression in the sea urchinLytechinus pictus |journal=Journal of Experimental Zoology |volume=213 |pages=231–246 |year=1980 |doi=10.1002/jez.1402130211 |issue=2|bibcode=1980JEZ...213..231K }}; {{cite journal |doi=10.1016/0014-4827(59)90275-7 |last1=Balinsky |first1=BI |title=An electro microscopic investigation of the mechanisms of adhesion of the cells in a sea urchin blastula and gastrula |journal=Experimental Cell Research |pmid=13653007 |volume=16 |issue=2 |pages=429–33 |year=1959}}; {{cite journal |last1=Hertzler |first1=PL |last2=McClay |first2=DR |title=alphaSU2, an epithelial integrin that binds laminin in the sea urchin embryo |journal=Developmental Biology |volume=207 |issue=1 |pages=1–13 |year=1999 |pmid=10049560 |doi=10.1006/dbio.1998.9165|doi-access=free }}; {{cite journal |doi=10.1016/0012-1606(85)90376-8 |last1=Fink |first1=RD |last2=McClay |first2=DR |title=Three cell recognition changes accompany the ingression of sea urchin primary mesenchyme cells |pmid=2578117 |journal=Developmental Biology |volume=107 |issue=1 |pages=66–74 |year=1985}}; {{cite journal |doi=10.1016/0012-1606(91)90425-3 |last1=Burdsal |first1=CA |last2=Alliegro |first2=MC |last3=McClay |first3=DR |title=Tissue-specific, temporal changes in cell adhesion to echinonectin in the sea urchin embryo |pmid=1707016 |journal=Developmental Biology |volume=144 |issue=2 |pages=327–34 |year=1991}}; {{cite journal |last1=Miller |first1=JR |last2=McClay |first2=DR |title=Characterization of the Role of Cadherin in Regulating Cell Adhesion during Sea Urchin Development |journal=Developmental Biology |volume=192 |issue=2 |pages=323–39 |year=1997 |pmid=9441671 |doi=10.1006/dbio.1997.8740|doi-access=free }}; {{cite journal |last1=Miller |first1=JR |last2=McClay |first2=DR |title=Changes in the pattern of adherens junction-associated beta-catenin accompany morphogenesis in the sea urchin embryo |journal=Developmental Biology |volume=192 |issue=2 |pages=310–22 |year=1997 |pmid=9441670 |doi=10.1006/dbio.1997.8739|doi-access=free }}; {{cite journal |doi=10.1016/S0012-1606(89)80058-2 |last1=Anstrom |first1=JA |title=Sea urchin primary mesenchyme cells: ingression occurs independent of microtubules |journal=Developmental Biology |pmid=2562830 |volume=131 |issue=1 |pages=269–75 |year=1989}}; {{cite journal |last1=Anstrom |first1=JA |title=Microfilaments, cell shape changes, and the formation of primary mesenchyme in sea urchin embryos |journal=The Journal of Experimental Zoology |volume=264 |issue=3 |pages=312–22 |year=1992 |pmid=1358997 |doi=10.1002/jez.1402640310}} It has been suggested that epithelial polarity together with planar cell polarity might be sufficient to drive gastrulation in sea urchins.{{cite journal | last1 = Nissen | first1 = Silas Boye | last2 = Rønhild | first2 = Steven | last3 = Trusina | first3 = Ala | last4 = Sneppen | first4 = Kim | title=Theoretical tool bridging cell polarities with development of robust morphologies | journal=eLife | date=November 27, 2018 | volume=7 |pages=e38407 |doi=10.7554/eLife.38407 | pmid = 30477635 | pmc = 6286147 | doi-access = free }}

File:Left-right asymmetry in the sea urchin - journal.pbio.1001404.g001.png

An unusual feature of sea urchin development is the replacement of the larva's bilateral symmetry by the adult's broadly fivefold symmetry. During cleavage, mesoderm and small micromeres are specified. At the end of gastrulation, cells of these two types form coelomic pouches. In the larval stages, the adult rudiment grows from the left coelomic pouch; after metamorphosis, that rudiment grows to become the adult. The animal-vegetal axis is established before the egg is fertilized. The oral-aboral axis is specified early in cleavage, and the left-right axis appears at the late gastrula stage.{{cite journal |last1=Warner |first1=Jacob F. |last2=Lyons |first2=Deirdre C. |last3=McClay |first3=David R. |title=Left-Right Asymmetry in the Sea Urchin Embryo: BMP and the Asymmetrical Origins of the Adult |journal=PLOS Biology |volume=10 |issue=10 |year=2012 |pages=e1001404 |doi=10.1371/journal.pbio.1001404|pmid=23055829 |pmc=3467244 |doi-access=free }}

File:Echinocardium cordatum (Pennant, 1777) early pluteus width ca.JPEG.]]

In most cases, the female's eggs float freely in the sea, but some species hold onto them with their spines, affording them a greater degree of protection. The unfertilized egg meets with the free-floating sperm released by males, and develops into a free-swimming blastula embryo in as few as 12 hours. Initially a simple ball of cells, the blastula soon transforms into a cone-shaped echinopluteus larva. In most species, this larva has 12 elongated arms lined with bands of cilia that capture food particles and transport them to the mouth. In a few species, the blastula contains supplies of nutrient yolk and lacks arms, since it has no need to feed.

Several months are needed for the larva to complete its development, the change into the adult form beginning with the formation of test plates in a juvenile rudiment which develops on the left side of the larva, its axis being perpendicular to that of the larva. Soon, the larva sinks to the bottom and metamorphoses into a juvenile urchin in as little as one hour. In some species, adults reach their maximum size in about five years. The purple urchin becomes sexually mature in two years and may live for twenty.{{Cite web|url=http://animaldiversity.org/accounts/Strongylocentrotus_purpuratus/|title=Strongylocentrotus purpuratus|website=Animal Diversity Web|year=2001|author=Worley, Alisa|access-date=2016-12-05|archive-date=2024-04-23|archive-url=https://web.archive.org/web/20240423125155/https://animaldiversity.org/accounts/strongylocentrotus_purpuratus/|url-status=live}}

Red sea urchins were originally thought to live 7 to 10 years but recent studies have shown that they can live for more than 100 years. Canadian red urchins have been found to be around 200 years old.{{cite web | url=https://www.aquariumofpacific.org/onlinelearningcenter/species/red_sea_urchin#:~:text=Southern%20California%20red%20sea%20urchins,probably%20about%20200%20years%20old! | title=Red Sea Urchin | access-date=2023-05-18 | archive-date=2023-05-18 | archive-url=https://web.archive.org/web/20230518000337/https://www.aquariumofpacific.org/onlinelearningcenter/species/red_sea_urchin#:~:text=Southern%20California%20red%20sea%20urchins,probably%20about%20200%20years%20old! | url-status=live }}{{cite journal |author1=Thomas A. Ebert |author2=John R. Southon |name-list-style=amp |year=2003 |title=Red sea urchins (Strongylocentrotus franciscanus) can live over 100 years: confirmation with A-bomb ¹⁴carbon |journal=Fishery Bulletin |volume=101 |issue=4 |pages=915–922 |url=http://fishbull.noaa.gov/1014/19ebertf.pdf |access-date=2023-05-18 |archive-date=2013-05-03 |archive-url=https://web.archive.org/web/20130503065837/http://fishbull.noaa.gov/1014/19ebertf.pdf |url-status=live }}

File:Sea Urchin.theora.ogv

Sea urchins feed mainly on algae, so they are primarily herbivores, but can feed on sea cucumbers and a wide range of invertebrates, such as mussels, polychaetes, sponges, brittle stars, and crinoids, making them omnivores, consumers at a range of trophic levels.{{cite journal |last1=Baumiller |first1=Tomasz K. |title=Crinoid Ecological Morphology |journal=Annual Review of Earth and Planetary Sciences |volume=36 |pages=221–49 |year=2008 |doi=10.1146/annurev.earth.36.031207.124116 |bibcode=2008AREPS..36..221B}}

Mass mortality of sea urchins was first reported in the 1970s, but diseases in sea urchins had been little studied before the advent of aquaculture. In 1981, bacterial "spotting disease" caused almost complete mortality in juvenile Pseudocentrotus depressus and Hemicentrotus pulcherrimus, both cultivated in Japan; the disease recurred in succeeding years. It was divided into a cool-water "spring" disease and a hot-water "summer" form.{{cite book |author=Lawrence, John M. |title=Edible Sea Urchins: Biology and Ecology |url=https://books.google.com/books?id=6T2JomruARoC |year=2006 |publisher=Elsevier |isbn=978-0-08-046558-6 |pages=167–168}} Another condition, bald sea urchin disease, causes loss of spines and skin lesions and is believed to be bacterial in origin.{{cite journal |title=Diseases of Echinodermata. I. Agents microorganisms and protistans |journal=Diseases of Aquatic Organisms |volume=2 |year=1987 |author=Jangoux, Michel |pages=147–162 |doi=10.3354/dao002147|doi-access=free }}

Adult sea urchins are usually well protected against most predators by their strong and sharp spines, which can be venomous in some species.{{cite web |url=http://www.nhm.ac.uk/research-curation/research/projects/echinoid-directory/intro/defence1.html |title=Defence – spines |website=Echinoid Directory |publisher=Natural History Museum }} The small urchin clingfish lives among the spines of urchins such as Diadema; juveniles feed on the pedicellariae and sphaeridia, adult males choose the tube feet and adult females move away to feed on shrimp eggs and molluscs.{{cite journal |author= Sakashita, Hiroko |year=1992 |title=Sexual dimorphism and food habits of the clingfish, Diademichthys lineatus, and its dependence on host sea urchin |journal=Environmental Biology of Fishes |volume=34 |issue=1 |pages=95–101 |doi=10.1007/BF00004787 |bibcode=1992EnvBF..34...95S |s2cid=32656986 }}

Sea urchins are one of the favourite foods of many lobsters, crabs, triggerfish, California sheephead, sea otter and wolf eels (which specialise in sea urchins). All these animals carry particular adaptations (teeth, pincers, claws) and a strength that allow them to overcome the excellent protective features of sea urchins. Left unchecked by predators, urchins devastate their environments, creating what biologists call an urchin barren, devoid of macroalgae and associated fauna.{{cite book|author1=Terborgh, John|author2=Estes, James A |title=Trophic Cascades: Predators, Prey, and the Changing Dynamics of Nature|url=https://books.google.com/books?id=tjOT8KJ6mF8C&pg=PA38 |year=2013|publisher=Island Press |isbn=978-1-59726-819-6 |pages=38}} Sea urchins graze on the lower stems of kelp, causing the kelp to drift away and die. Loss of the habitat and nutrients provided by kelp forests leads to profound cascade effects on the marine ecosystem. Sea otters have re-entered British Columbia, dramatically improving coastal ecosystem health.{{cite web |url=http://www.dfo-mpo.gc.ca/species-especes/species/species_seaOtter_e.asp |archive-url=https://web.archive.org/web/20080123224702/http://www.dfo-mpo.gc.ca/species-especes/species/species_seaOtter_e.asp |archive-date=2008-01-23|title=Aquatic Species at Risk – Species Profile – Sea Otter |publisher=Fisheries and Oceans Canada |access-date=2007-11-29}}

File:Wolf eel eating a sea urchin.jpg|Wolf eel, a highly specialized predator of sea urchins

File:Sea otter with sea urchin.jpg|A sea otter feeding on a purple sea urchin.

File:Carpilius convexus is consuming Heterocentrotus trigonarius in Hawaii.jpg|A marbled stone crab (Carpilius convexus) attacking a slate pencil sea urchin (Heterocentrotus mamillatus)

File:Saddle Wrasse are feeding on sea urchin in Kona.jpg|A wrasse finishing the remains of a damaged Tripneustes gratilla

File:Flower urchin by Vincent C Chen.jpg is a dangerous, potentially lethally venomous species.]]

The spines, long and sharp in some species, protect the urchin from predators. Some tropical sea urchins like Diadematidae, Echinothuriidae and Toxopneustidae have venomous spines. Other creatures also make use of these defences; crabs, shrimps and other organisms shelter among the spines, and often adopt the colouring of their host. Some crabs in the Dorippidae family carry sea urchins, starfish, sharp shells or other protective objects in their claws.{{cite book|author1=Thiel, Martin|author2=Watling, Les|title=Lifestyles and Feeding Biology |url=https://books.google.com/books?id=RZ-eBgAAQBAJ&pg=PA200 |year=2015 |publisher=Oxford University Press |isbn=978-0-19-979702-8 |pages=200–202}}

Pedicellariae{{cite web |url=http://www.nhm.ac.uk/research-curation/research/projects/echinoid-directory/intro/defence3.html |title=Defence – pedicellariae |author= |website=Echinoid Directory |publisher=Natural History Museum |access-date=2014-08-04 |archive-date=2014-10-06 |archive-url=https://web.archive.org/web/20141006072057/http://www.nhm.ac.uk/research-curation/research/projects/echinoid-directory/intro/defence3.html |url-status=live }} are a good means of defense against ectoparasites, but not a panacea as some of them actually feed on it.Hiroko Sakashita, " Sexual dimorphism and food habits of the clingfish, Diademichthys lineatus, and its dependence on host sea urchin ", Environmental Biology of Fishes, vol. 34, no 1, 1994, p. 95–101 The hemal system defends against endoparasites.{{cite journal |url=https://link.springer.com/content/pdf/10.1007%252FBF01989305.pdf |title=Diseases of echinoderms |journal=Helgoländer Meeresuntersuchungen |volume=37 |issue=1–4 |pages=207–216 |author=Jangoux, M. |year=1984 |access-date=23 March 2018 |bibcode=1984HM.....37..207J |doi=10.1007/BF01989305 |s2cid=21863649 |doi-access=free |archive-date=29 October 2020 |archive-url=https://web.archive.org/web/20201029002716/https://link.springer.com/content/pdf/10.1007%252FBF01989305.pdf |url-status=live }}

Sea urchins are established in most seabed habitats from the intertidal downwards, at an extremely wide range of depths.{{cite journal |last=Kroh |first=Andreas |date=2010 |title=The phylogeny and classification of post-Palaeozoic echinoids |journal=Journal of Systematic Palaeontology |volume=8 |issue=2 |pages=147–212 |doi= 10.1080/14772011003603556|doi-access=free |bibcode=2010JSPal...8..147K }}. Some species, such as Cidaris abyssicola, can live at depths of several kilometres. Many genera are found in only the abyssal zone, including many cidaroids, most of the genera in the Echinothuriidae family, and the "cactus urchins" Dermechinus. One of the deepest-living families is the Pourtalesiidae,{{cite web |url=http://echinoblog.blogspot.fr/search?q=pourtalesiid |title=Sizes and Species in the Strangest of the Strange : Deep-Sea Pourtalesiid Urchins |last1=Mah |first1=Christopher |date=April 12, 2011 |website=The Echinoblog }}. strange bottle-shaped irregular sea urchins that live in only the hadal zone and have been collected as deep as 6850 metres beneath the surface in the Sunda Trench.{{cite web |url=http://echinoblog.blogspot.fr/2014/04/what-are-deepest-known-echinoderms.html |title=What are the Deepest known echinoderms? |last=Mah |first=Christopher |date=8 April 2014 |website=The Echinoblog |access-date=22 March 2018 |archive-date=16 May 2018 |archive-url=https://web.archive.org/web/20180516111356/http://echinoblog.blogspot.fr/2014/04/what-are-deepest-known-echinoderms.html |url-status=live }}. Nevertheless, this makes sea urchin the class of echinoderms living the least deep, compared to brittle stars, starfish and crinoids that remain abundant below {{convert|8000|m|ft|-1|abbr=on}} and sea cucumbers which have been recorded from {{convert|10687|m|ft|-2|abbr=on}}.

Population densities vary by habitat, with more dense populations in barren areas as compared to kelp stands.{{cite journal |author1=Mattison, J.E. |author2=Trent, J.D. |author3=Shanks, AL |author4=Akin, T.B. |author5=Pearse, J.S. |title=Movement and feeding activity of red sea urchins (Strongylocentrotus franciscanus) adjacent to a kelp forest |journal=Marine Biology |volume=39 |pages=25–30 |year=1977 |doi=10.1007/BF00395589 |issue=1|bibcode=1977MarBi..39...25M |s2cid=84338735 }}{{Cite conference |last=Konar |first=Brenda |author-link=Brenda Konar |title=Habitat Influences on Sea Urchin Populations |editor1-last=Hallock |editor1-first=Pamela |editor2-last=French |editor2-first=Llyn |date=January 2000 |book-title=Diving for Science: Proceedings of the 20th Annual Scientific Diving Symposium, 11–15 October 2000 |url=http://archive.rubicon-foundation.org/8990 |archive-url=https://archive.today/20130415211125/http://archive.rubicon-foundation.org/8990 |url-status=dead |archive-date=April 15, 2013 |location=St. Pete Beach, Florida |publication-place=Nahant, MA |publisher=American Academy of Underwater Sciences |oclc=45371343 |access-date=7 January 2011}} Even in these barren areas, greatest densities are found in shallow water. Populations are generally found in deeper water if wave action is present. Densities decrease in winter when storms cause them to seek protection in cracks and around larger underwater structures. The shingle urchin (Colobocentrotus atratus), which lives on exposed shorelines, is particularly resistant to wave action. It is one of the few sea urchin that can survive many hours out of water.{{cite web |url=http://echinoblog.blogspot.com/2008/04/holding-on-in-rough-world.html |title=The Echinoblog |author=ChrisM |work=echinoblog.blogspot.com |date=2008-04-21 |access-date=2011-04-01 |archive-date=2024-08-25 |archive-url=https://web.archive.org/web/20240825043204/http://echinoblog.blogspot.com/2008/04/holding-on-in-rough-world.html |url-status=live }}

Sea urchins can be found in all climates, from warm seas to polar oceans. The larvae of the polar sea urchin Sterechinus neumayeri have been found to use energy in metabolic processes twenty-five times more efficiently than do most other organisms.[https://www.sciencedaily.com/releases/2001/03/010309080313.htm Antarctic Sea Urchin Shows Amazing Energy-Efficiency in Nature's Deep Freeze 15 March 2001] {{Webarchive|url=https://web.archive.org/web/20180322204623/https://www.sciencedaily.com/releases/2001/03/010309080313.htm |date=22 March 2018 }} University of Delaware. Retrieved 22 March 2018 Despite their presence in nearly all the marine ecosystems, most species are found on temperate and tropical coasts, between the surface and some tens of meters deep, close to photosynthetic food sources.

File:Sea urchins in california tide pools.jpg|Purple sea urchins at low tide in California. They dig a cavity in the rock to hide from predators during the day.

File:Expl1825 - Flickr - NOAA Photo Library.jpg|Dermechinus horridus, an abyssal species, at thousands of meters deep

File:Underwater mcmurdo sound.jpg|Antarctic sea urchin (Sterechinus neumayeri) inhabits frozen seas.

File:Colobocentrotus atratus Shingle urchin.jpg|The shape of the shingle urchin allows it to stay on wave-beaten cliffs.

File:Cidaridae - radiola (cropped).JPG were used for walking on the soft seabed.]]

The earliest echinoid fossils date to the Middle Ordovician period (circa 465 Mya).{{cite journal |last1=Botting |first1=Joseph P. |last2=Muir |first2=Lucy A. |date=March 2012 |title=Fauna and ecology of the holothurian bed, Llandrindod, Wales, UK (Darriwilian, Middle Ordovician), and the oldest articulated holothurian |journal=Palaeontologia Electronica |volume=15 |issue=1 |pages=1–28 |doi=10.26879/272 |s2cid=55716313 |doi-access=free }}{{cite journal |last1=Thompson |first1=Jeffrey R. |last2=Cotton |first2=Laura J. |last3=Candela |first3=Yves |last4=Kutscher |first4=Manfred |last5=Reich |first5=Mike |last6=Bottjer |first6=David J. |date=14 April 2022 |title=The Ordovician diversification of sea urchins: systematics of the Bothriocidaroida (Echinodermata: Echinoidea) |url=https://www.tandfonline.com/doi/abs/10.1080/14772019.2022.2042408 |journal=Journal of Systematic Palaeontology |volume=19 |issue=20 |pages=1395–1448 |doi=10.1080/14772019.2022.2042408 |s2cid=248192052 |access-date=29 October 2022}}{{cite web |title=Echinoids |url=http://www.bgs.ac.uk/discoveringGeology/time/Fossilfocus/echinoid.html |publisher=British Geological Survey |access-date=14 March 2018 |date=2017 |archive-date=15 March 2018 |archive-url=https://web.archive.org/web/20180315133241/http://www.bgs.ac.uk/discoveringGeology/time/Fossilfocus/echinoid.html |url-status=live }} There is a rich fossil record, their hard tests made of calcite plates surviving in rocks from every period since then.{{cite web |title=The Echinoid Directory {{!}} Introduction |url=http://www.nhm.ac.uk/our-science/data/echinoid-directory/intro/introduction.html |publisher=Natural History Museum |access-date=16 March 2018 |archive-date=25 August 2024 |archive-url=https://web.archive.org/web/20240825043212/https://www.nhm.ac.uk/our-science/data/echinoid-directory/intro/introduction.html |url-status=live }}

Spines are present in some well-preserved specimens, but usually only the test remains. Isolated spines are common as fossils. Some Jurassic and Cretaceous Cidaroida had very heavy, club-shaped spines.{{cite web |title=The Echinoid Directory {{!}} Spines |url=http://www.nhm.ac.uk/our-science/data/echinoid-directory/morphology/regulars/spine.html |publisher=Natural History Museum |access-date=16 March 2018}}

Most fossil echinoids from the Paleozoic era are incomplete, consisting of isolated spines and small clusters of scattered plates from crushed individuals, mostly in Devonian and Carboniferous rocks. The shallow-water limestones from the Ordovician and Silurian periods of Estonia are famous for echinoids.{{cite book|last1=Kirkaldy|first1=J. F. |title=Fossils in Colour|date=1967|publisher=Blandford Press|location=London|pages=161–163}} Paleozoic echinoids probably inhabited relatively quiet waters. Because of their thin tests, they would certainly not have survived in the wave-battered coastal waters inhabited by many modern echinoids. Echinoids declined to near extinction at the end of the Paleozoic era, with just six species known from the Permian period. Only two lineages survived this period's massive extinction and into the Triassic: the genus Miocidaris, which gave rise to modern cidaroida (pencil urchins), and the ancestor that gave rise to the euechinoids. By the upper Triassic, their numbers increased again. Cidaroids have changed very little since the Late Triassic, and are the only Paleozoic echinoid group to have survived.

The euechinoids diversified into new lineages in the Jurassic and Cretaceous periods, and from them emerged the first irregular echinoids (the Atelostomata) during the early Jurassic.{{cite book|last=Schultz|first=Heinke A.G.|title=Echinoidea: with pentameral symmetry|url=https://books.google.com/books?id=Ff4-CwAAQBAJ&pg=PT36|year=2015|publisher=Walter de Gruyter|isbn=978-3-11-038601-1|pages=36 ff, section 2.4|access-date=2018-03-16|archive-date=2024-08-25|archive-url=https://web.archive.org/web/20240825043211/https://books.google.com/books?id=Ff4-CwAAQBAJ&pg=PT36#v=onepage&q&f=false|url-status=live}}

Some echinoids, such as Micraster in the chalk of the Cretaceous period, serve as zone or index fossils. Because they are abundant and evolved rapidly, they enable geologists to date the surrounding rocks.{{cite book |title=The Science of life |first1=H. G. |last1=Wells |author1-link=H. G. Wells |first2=Julian |last2=Huxley |author2-link=Julian Huxley |first3=G. P. |last3=Wells |year=1931 |pages=346–348 |url=https://archive.org/details/in.ernet.dli.2015.221951/mode/2up}}

In the Paleogene and Neogene periods (circa 66 to 2.6 Mya), sand dollars (Clypeasteroida) arose. Their distinctive, flattened tests and tiny spines were adapted to life on or under loose sand in shallow water, and they are abundant as fossils in southern European limestones and sandstones.

File:Archaeocidaris brownwoodensis MHNT.jpg|Archaeocidaris brownwoodensis, Cidaroida, Carboniferous, c. 300 mya

File:Miocidaris coaeva MHNT.PAL.2006.94.jpg|Miocidaris coaeva, Cidaroida, Middle Triassic, c. 240 mya

File:Clypeus plotti, echinoid, Middle Jurassic, Gloucester, Gloucestershire, England - Houston Museum of Natural Science - DSC01800.JPG|Clypeus plotti, Irregularia, Middle Jurassic, c. 162 mya

File:Fossil Echinoid Echinocorys.jpg|Echinocorys, Holasteroida, Upper Cretaceous, c. 80 mya

File:Echinolampas ovalis M Eocene Civrac-en-Médoc France.JPG|Echinolampas ovalis, Cassiduloida, Middle Eocene, c. 40 mya

File:Clypeaster portentosus.jpg|Clypeaster portentosus, Clypeasteroida, Miocene, c. 10 mya

File:Clypeus plotii.JPG|Clypeus ploti gives its name to the Clypeus Grit of Western England, part of the Oolite.{{cite web |title=Clypeus Grit Member |url=https://webapps.bgs.ac.uk/lexicon/lexicon.cfm?pub=CG |publisher=British Geological Survey |access-date=21 February 2025 |ref=CG |date=2025 |quote=Parent Unit: Salperton Limestone Formation}}

Echinoids are deuterostome animals, like the chordates. A 2014 analysis of 219 genes from all classes of echinoderms gives the following phylogenetic tree.{{cite journal |last1=Telford |first1=M. J. |last2=Lowe|first2=C. J.|last3=Cameron |first3=C. B. |last4=Ortega-Martinez |first4=O. |last5=Aronowicz |first5=J. |last6=Oliveri|first6=P. |last7=Copley |first7=R. R. |title=Phylogenomic analysis of echinoderm class relationships supports Asterozoa |journal=Proceedings of the Royal Society B: Biological Sciences |volume=281 |issue=1786 |year=2014 |pages=20140479 |doi=10.1098/rspb.2014.0479|pmid=24850925 |pmc=4046411 }} Approximate dates of branching of major clades are shown in millions of years ago (mya).

{{Clade

|label1=Bilateria

|1={{Clade

|label1=Xenacoelomorpha

|1=80 px

|label2=Nephrozoa |sublabel2=650 mya

|2={{clade

|label1=Deuterostomia |sublabel1=>540 mya

|1={{clade

|label1=Chordata and allies

|1=80 px

|label2=Echinodermata |sublabel2=c. 500 mya

|2={{clade

|1={{clade

|label1=Echinozoa

|1={{clade

|label1=Holothuroidea |sublabel1= Sea cucumbers

|1=80 px

|label2=Echinoidea |sublabel2=c. 450 mya

|2=70 px

}}

|label2=Asterozoa

|2={{clade

|label1=Ophiuroidea |sublabel1=Brittle stars

|1=80 px

|label2=Asteroidea |sublabel2=Starfish

|2=80 px

}}

}}

|label2=Crinoidea |sublabel2=Crinoids

|2=50 px

}}

}}

|label2 =Protostomia |sublabel2=610 mya

|2={{Clade

|1=Ecdysozoa 80 px

|2=Spiralia 80 px

}}

}}

}}

}}

The phylogeny of the sea urchins is as follows:{{cite journal|last1=Planet|first1=Paul J. |last2=Ziegler |first2=Alexander |last3=Schröder |first3=Leif |last4=Ogurreck |first4=Malte |last5=Faber |first5=Cornelius |last6=Stach |first6=Thomas |title=Evolution of a Novel Muscle Design in Sea Urchins (Echinodermata: Echinoidea) |journal=PLOS ONE |volume=7 |issue=5 |year=2012 |pages=e37520 |doi=10.1371/journal.pone.0037520|pmid=22624043 |pmc=3356314 |bibcode=2012PLoSO...737520Z |doi-access=free }}{{cite journal |last1=Kroh |first1=Andreas |last2=Smith |first2=Andrew B. |title=The phylogeny and classification of post-Palaeozoic echinoids |journal=Journal of Systematic Palaeontology |volume=8 |issue=2 |year=2010|pages=147–212 |doi=10.1080/14772011003603556|doi-access=free |bibcode=2010JSPal...8..147K }}

{{clade

|label1=Echinoidea |sublabel1=450 mya

|1={{clade

|1=Cidaroida 50 px

|label2=Euechinoidea

|2={{clade

|1=Echinothurioida 60 px

|label2=Acroechinoidea

|2={{clade

|1=Diadematoida 60 px

|2={{clade

|1=Irregularia 45 px 60 px

|2={{clade

|1=Pedinoida 60 px

|2={{clade

|1=Salenioida 60 px

|label2=Echinacea

|2={{clade

|1=Stomopneustidae 60 px

|2={{clade

|1=Arbaciidae 60 px

|label2=Camarodonta

|2={{clade

|1=Parasaleniidae 60 px

|2={{clade

|1={{clade

|1=Temnopleuridae 60 px

|2=Trigonocidaridae

}}

|label2=Echinoida

|2={{clade

|1=Echinidae 60 px

|2={{clade

|1=Parechinidae 60 px

|2={{clade

|1=Toxopneustidae 60 px

|2={{clade

|1=Echinometridae 60 px

|2=Strongylocentrotidae 60 px

}}

}}

}}

}}

}}

}}

}}

}}

}}

}}

}}

}}

}}

}}

}}

The phylogenetic study from 2022 presents a different topology of the Euechinoidea phylogenetic tree. Irregularia are sister group of Echinacea (including Salenioida) forming a common clade Carinacea, basal groups Aspidodiadematoida, Diadematoida, Echinothurioida, Micropygoida, and Pedinoida are comprised in a common basal clade Aulodonta.{{cite journal |last1=Koch |first1=Nicolás Mongiardino |last2=Thompson |first2=Jeffrey R |last3=Hiley |first3=Avery S |last4=McCowin |first4=Marina F |last5=Armstrong |first5=A Frances |last6=Coppard |first6=Simon E |last7=Aguilera |first7=Felipe |last8=Bronstein |first8=Omri |last9=Kroh |first9=Andreas |last10=Mooi |first10=Rich |last11=Rouse |first11=Greg W |title=Phylogenomic analyses of echinoid diversification prompt a re-evaluation of their fossil record |journal=eLife |date=Mar 22, 2022 |volume=11 |pages=e72460 |doi=10.7554/eLife.72460 |pmid=35315317 |pmc=8940180 |doi-access=free}}

{{main|Sea urchin injury}}

File:Sea-urchin-injury.jpg

Sea urchin injuries are puncture wounds inflicted by the animal's brittle, fragile spines.{{cite book |author1=James, William D. |author2=Berger, Timothy G. |title=Andrews' Diseases of the Skin: clinical Dermatology |publisher=Saunders Elsevier |year=2006 |page=431 |isbn=0-7216-2921-0 |display-authors=etal}}

These are a common source of injury to ocean swimmers, especially along coastal surfaces where coral with stationary sea urchins are present. Their stings vary in severity depending on the species. Their spines can be venomous or cause infection. Granuloma and staining of the skin from the natural dye inside the sea urchin can also occur. Breathing problems may indicate a serious reaction to toxins in the sea urchin.{{cite web|last=Gallagher|first=Scott A.|title=Echinoderm Envenomation|url=http://emedicine.medscape.com/article/770053-overview|work=eMedicine|access-date=12 October 2010|archive-date=4 December 2010|archive-url=https://web.archive.org/web/20101204174610/http://emedicine.medscape.com/article/770053-overview|url-status=live}} They inflict a painful wound when they penetrate human skin, but are not themselves dangerous if fully removed promptly; if left in the skin, further problems may occur.{{cite journal |author1=Matthew D. Gargus |author2=David K. Morohashi |title=A sea-urchin spine chilling remedy |journal=New England Journal of Medicine |year=2012 |volume=30 |issue=19 |pages=1867–1868 |doi=10.1056/NEJMc1209382 |pmid=23134402 |doi-access=free }}

Sea urchins are traditional model organisms in developmental biology. This use originated in the 1800s, when their embryonic development became easily viewed by microscopy. The transparency of the urchin's eggs enabled them to be used to observe that sperm cells actually fertilize ova.{{cite web |url=http://www.exploratorium.edu/imaging-station/research/urchin/story_urchin1.php |title=Insight from the Sea Urchin |work=Microscope Imaging Station |publisher=Exploratorium |access-date=2018-03-07 |archive-url=https://web.archive.org/web/20170312223637/https://www.exploratorium.edu/imaging-station/research/urchin/story_urchin1.php |archive-date=2017-03-12}} They continue to be used for embryonic studies, as prenatal development continues to seek testing for fatal diseases. Sea urchins are being used in longevity studies for comparison between the young and old of the species, particularly for their ability to regenerate tissue as needed.{{Cite journal |last1=Bodnar |first1=Andrea G. |last2=Coffman |first2=James A. |date=2016-08-01 |title=Maintenance of somatic tissue regeneration with age in short- and long-lived species of sea urchins |journal=Aging Cell |volume=15 |issue=4 |pages=778–787 |doi=10.1111/acel.12487 |issn=1474-9726 |pmc=4933669 |pmid=27095483}} Scientists at the University of St Andrews have discovered a genetic sequence, the '2A' region, in sea urchins previously thought to have belonged only to viruses like foot-and-mouth disease virus.{{Cite journal | title='2A-Like' Signal Sequences Mediating Translational Recoding: A Novel Form of Dual Protein Targeting | pmc=4981915 | pmid=27161495 | doi=10.1111/tra.12411 | volume=17 | issue=8 | year=2016 | pages=923–39 | last1=Roulston | first1=C. | last2=Luke | first2=G.A. | last3=de Felipe | first3=P. | last4=Ruan | first4=L. | last5=Cope | first5=J. | last6=Nicholson | first6=J. | last7=Sukhodub | first7=A. | last8=Tilsner | first8=J. | last9=Ryan | first9=M.D. | journal=Traffic | url=https://research-repository.st-andrews.ac.uk/bitstream/10023/8924/1/Roulston_2016_Traffic_2A_Like_CCBY_FinalPublishedVersion.pdf | access-date=2018-10-24 | archive-date=2024-08-25 | archive-url=https://web.archive.org/web/20240825043716/https://research-repository.st-andrews.ac.uk/bitstream/10023/8924/1/Roulston_2016_Traffic_2A_Like_CCBY_FinalPublishedVersion.pdf | url-status=live }} More recently, Eric H. Davidson and Roy John Britten argued for the use of urchins as a model organism due to their easy availability, high fecundity, and long lifespan. Beyond embryology, urchins provide an opportunity to research cis-regulatory elements.{{Cite web |url=http://sugp.caltech.edu/SUGP/intro/index.php |title=Sea Urchin Genome Project |website=sugp.caltech.edu |access-date=2016-12-05 |archive-url=https://web.archive.org/web/20161220061841/http://sugp.caltech.edu/SUGP/intro/index.php |archive-date=2016-12-20 |url-status=dead }} Oceanography has taken an interest in monitoring the health of urchins and their populations as a way to assess overall ocean acidification,{{Cite web |url=http://news.stanford.edu/pr/2013/pr-urchins-ocean-acidity-040813.html |title=Stanford seeks sea urchin's secret to surviving ocean acidification {{!}} Stanford News Release |website=news.stanford.edu |access-date=2016-12-05 |date=2013-04-08 |archive-date=2016-12-20 |archive-url=https://web.archive.org/web/20161220065330/http://news.stanford.edu/pr/2013/pr-urchins-ocean-acidity-040813.html |url-status=dead }} temperatures, and ecological impacts.

The organism's evolutionary placement and unique embryology with five-fold symmetry were the major arguments in the proposal to seek the sequencing of its genome. Importantly, urchins act as the closest living relative to chordates and thus are of interest for the light they can shed on the evolution of vertebrates.{{Cite journal |date=2006-11-10 |title=The Genome of the Sea Urchin Strongylocentrotus purpuratus |journal=Science |volume=314 |issue=5801 |pages=941–952 |doi=10.1126/science.1133609 |pmc=3159423 |pmid=17095691 | last1 = Sodergren | first1 = E | last2 = Weinstock | first2 = GM | last3 = Davidson | first3 = EH |display-authors=etal |bibcode=2006Sci...314..941S }} The genome of Strongylocentrotus purpuratus was completed in 2006 and established homology between sea urchin and vertebrate immune system-related genes. Sea urchins code for at least 222 Toll-like receptor genes and over 200 genes related to the Nod-like-receptor family found in vertebrates.{{cite journal |last1=Rast |first1=JP |last2=Smith |first2=LC |last3=Loza-Coll |first3=M |last4=Hibino |first4=T |last5=Litman |first5=GW |title=Genomic Insights into the Immune System of the Sea Urchin |journal=Science |volume=314 |issue=5801 |pages=952–6 |year=2006 |pmid=17095692 |doi=10.1126/science.1134301|bibcode = 2006Sci...314..952R |pmc=3707132 }} This increases its usefulness as a valuable model organism for studying the evolution of innate immunity. The sequencing also revealed that while some genes were thought to be limited to vertebrates, there were also innovations that have previously never been seen outside the chordate classification, such as immune transcription factors PU.1 and SPIB.

File:Fresh Sea Urchin (2678940158).jpg

The gonads of both male and female sea urchins, sometimes euphemized as sea urchin "roe" or "corals",Laura Rogers-Bennett, "The Ecology of Strongylocentrotus franciscanus and Strongylocentrotus purpuratus{{-"}} in John M. Lawrence, Edible sea urchins: biology and ecology, p. 410 are culinary delicacies in many parts of the world, especially Japan.John M. Lawrence, "Sea Urchin Roe Cuisine" in John M. Lawrence, Edible sea urchins: biology and ecology"[http://www.smithsonianmag.com/science-nature/rise-sea-urchin-180951859/?all The Rise of the Sea Urchin] {{Webarchive|url=https://web.archive.org/web/20170304181028/http://www.smithsonianmag.com/science-nature/rise-sea-urchin-180951859/?all |date=2017-03-04 }}", Franz Lidz July 2014, Smithsonian In Japan, sea urchin is known as {{nihongo|uni|うに}}, and its gonads (the only meaty, edible parts of the animal) can retail for as much as ¥40,000 ($360) per kilogram;{{cite news |first=Richard |last=Macey |url=http://www.smh.com.au/articles/2004/11/08/1099781322260.html |work=The Sydney Morning Herald |title=The little urchins that can command a princely price |date=November 9, 2004 |access-date=May 6, 2009 |archive-date=September 17, 2011 |archive-url=https://web.archive.org/web/20110917103252/http://www.smh.com.au/articles/2004/11/08/1099781322260.html |url-status=live }} they are served raw as sashimi or in sushi, with soy sauce and wasabi. Japan imports large quantities from the United States, South Korea, and other producers. Japan consumes 50,000 tons annually, amounting to over 80% of global production.{{cite news |last1=Zatylny |first1=Jane |title=Searchin' for Urchin: A Culinary Quest |url=https://www.hakaimagazine.com/features/searchin-for-urchin-a-culinary-quest/ |access-date=10 September 2018 |work=Hakai Magazine |date=6 September 2018 |archive-date=11 September 2018 |archive-url=https://web.archive.org/web/20180911002422/https://www.hakaimagazine.com/features/searchin-for-urchin-a-culinary-quest/ |url-status=live }} Japanese demand for sea urchins has raised concerns about overfishing."Sea Urchin Fishery and Overfishing", TED Case Studies 296, American University [http://www.american.edu/TED/urchin.htm full text] {{Webarchive|url=https://web.archive.org/web/20090628151427/http://www1.american.edu/TED/urchin.HTM |date=2009-06-28 }}

Sea urchins are commonly eaten stuffed with rice in the traditional oko-oko dish among the Sama-Bajau people of the Philippines.{{cite web |title=Making Oko'-Oko', A Sama Sea Urchin Delicacy |url=https://sinama.org/2013/06/making-oko-oko-a-sama-sea-urchin-delicacy/ |website=Kauman Sama Online |date=27 June 2013 |access-date=3 June 2023 |archive-date=3 June 2023 |archive-url=https://web.archive.org/web/20230603025132/https://sinama.org/2013/06/making-oko-oko-a-sama-sea-urchin-delicacy/ |url-status=live }} They were once foraged by coastal Malay communities of Singapore who call them {{lang|zsm|jani}}.{{Cite magazine|url=https://biblioasia.nlb.gov.sg/vol-17/issue-3/oct-dec-2021/|title=The Role of Foraging in Malay Cuisine|last=Khir Johari|date=Oct–Dec 2021|magazine=BiblioAsia|volume=17|issue=3|pages=20–23|publisher=National Library Board, Singapore|access-date=2023-02-01|archive-date=2023-11-06|archive-url=https://web.archive.org/web/20231106234414/https://biblioasia.nlb.gov.sg/vol-17/issue-3/oct-dec-2021/|url-status=live}} In New Zealand, Evechinus chloroticus, known as {{lang|mi|kina}} in Māori, is a delicacy, traditionally eaten raw. Though New Zealand fishermen would like to export them to Japan, their quality is too variable.{{cite encyclopedia |first=Maggy |last=Wassilieff |date=March 2, 2009 |title=sea urchins |url=http://www.teara.govt.nz/en/starfish-sea-urchins-and-other-echinoderms/2 |encyclopedia=Te Ara: The Encyclopedia of New Zealand |access-date=October 8, 2010 |archive-date=October 25, 2010 |archive-url=https://web.archive.org/web/20101025163949/http://www.teara.govt.nz/en/starfish-sea-urchins-and-other-echinoderms/2 |url-status=live }}

In Mediterranean cuisines, Paracentrotus lividus is often eaten raw, or with lemon,for Puglia, Italy: Touring Club Italiano, Guida all'Italia gastronomica, 1984, p. 314; for Alexandria, Egypt: Claudia Roden, A Book of Middle Eastern Food, p. 183 and known as {{lang|it|ricci}} on Italian menus where it is sometimes used in pasta sauces. It can also flavour omelettes, scrambled eggs, fish soup,Alan Davidson, Mediterranean Seafood, p. 270 mayonnaise, béchamel sauce for tartlets,Larousse Gastronomique{{Page needed|date=November 2010}} the {{lang|fr|boullie}} for a soufflé,Curnonsky, Cuisine et vins de France, nouvelle édition, 1974, p. 248 or Hollandaise sauce to make a fish sauce.Davidson, Alan (2014) Oxford Companion to Food. Oxford University Press, 3rd edition. p. 280 In the region of Marseille, sea urchin are commonly eaten in dedicated food festival called oursinade.Every year, 'oursinades' (sea urchin festivals) are held in and around Marseille to promote this 'sea hedgehog https://www.marseille-tourisme.com/en/discover-marseille/gastronomy-in-marseille/culinary-specialities-of-marseille/the-oursinades-in-marseille/

On the Pacific Coast of North America, Strongylocentrotus franciscanus was praised by Euell Gibbons; Strongylocentrotus purpuratus is also eaten. Native Americans in California are also known to eat sea urchins.{{cite book|title=Marine and Freshwater Products Handbook|author1=Martin, R.E.|author2=Carter, E.P.|author3=Flick, G.J.|author4=Davis, L.M.|date=2000|publisher=Taylor & Francis|isbn=978-1-56676-889-4|url=https://books.google.com/books?id=OFKLk3S0fzgC|page=268|access-date=2014-12-03|archive-date=2024-08-25|archive-url=https://web.archive.org/web/20240825043715/https://books.google.com/books?id=OFKLk3S0fzgC|url-status=live}} The coast of Southern California is known as a source of high quality {{translit|ja|uni}}, with divers picking sea urchin from kelp beds in depths as deep as 24 m/80 ft.{{Cite news|url=http://www.bonappetit.com/people/article/california-sea-urchin-divers-interviewed-by-francis-lam|title=California Sea Urchin Divers, Interviewed by Francis Lam|last=Lam|first=Francis|date=2014-03-14|work=Bon Appetit|access-date=2017-03-26|language=en|archive-date=2017-03-26|archive-url=https://web.archive.org/web/20170326231148/http://www.bonappetit.com/people/article/california-sea-urchin-divers-interviewed-by-francis-lam|url-status=live}} As of 2013, the state was limiting the practice to 300 sea urchin diver licenses. Though the edible Strongylocentrotus droebachiensis is found in the North Atlantic, it is not widely eaten. However, sea urchins (called {{lang|ems|uutuk}} in Alutiiq) are commonly eaten by the Alaska Native population around Kodiak Island. It is commonly exported, mostly to Japan.{{cite news |first=Dena |last=Kleiman |title=Scorned at Home, Maine Sea Urchin Is a Star in Japan |work=The New York Times |date=October 3, 1990 |page=C1 |url=https://www.nytimes.com/1990/10/03/garden/scorned-at-home-maine-sea-urchin-is-a-star-in-japan.html |access-date=February 12, 2017 |archive-date=February 2, 2017 |archive-url=https://web.archive.org/web/20170202001653/http://www.nytimes.com/1990/10/03/garden/scorned-at-home-maine-sea-urchin-is-a-star-in-japan.html |url-status=live }} In the West Indies, slate pencil urchins are eaten.Davidson, Alan (2014) Oxford Companion to Food. Oxford University Press, 3rd edition. pp. 730–731.

In Chilean cuisine, it is served raw with lemon, onions, and olive oil.

Unidon Murakami.jpg|Japanese {{translit|ja|uni-don}}, or rice bowl with sea urchin roe

Sushi uni.jpg|Japanese {{translit|ja|nigirizushi}} with sea urchin roe

Sushi Saito IMG 1773 (23720371141).jpg|Sea urchin roe ({{translit|ja|uni}}) sashimi

Fried rice with sea urchin 1.jpg|Fried rice with sea urchin ({{lang|zh|海胆}}, {{translit|zh|hǎidǎn}}) served in China

File:Fossil sea urchin (FindID 551527).jpg, thought to have been used as an amulet{{cite web|title=Amulet {{!}} LIN-B37563|url=https://finds.org.uk/database/artefacts/record/id/551527|website=Portable Antiquities Scheme|access-date=14 March 2018|archive-date=15 March 2018|archive-url=https://web.archive.org/web/20180315070608/https://finds.org.uk/database/artefacts/record/id/551527|url-status=live}}]]

Some species of sea urchins, such as the slate pencil urchin (Eucidaris tribuloides), are commonly sold in aquarium stores. Some species are effective at controlling filamentous algae, and they make good additions to an invertebrate tank.{{cite book|last=Tullock|first=John H.|title=Your First Marine Aquarium: Everything about Setting Up a Marine Aquarium, Including Conditioning, Maintenance, Selecting Fish and Invertebrates, and More|url=https://archive.org/details/yourfirstmarinea0000tull |url-access=registration|year=2008 |publisher=Barron's Educational Series |isbn=978-0-7641-3675-7 |page=[https://archive.org/details/yourfirstmarinea0000tull/page/63 63]}}

A folk tradition in Denmark and southern England imagined sea urchin fossils to be thunderbolts, able to ward off harm by lightning or by witchcraft, as an apotropaic symbol.{{cite web |last1=McNamara |first1=Ken |title=Prehistoric fossil collectors |url=https://www.geolsoc.org.uk/Geoscientist/Archive/June-2012/Prehistoric-fossil-collectors |publisher=The Geological Society |access-date=14 March 2018 |date=2012 |archive-date=17 February 2019 |archive-url=https://web.archive.org/web/20190217090444/https://www.geolsoc.org.uk/Geoscientist/Archive/June-2012/Prehistoric-fossil-collectors |url-status=live }} Another version supposed they were petrified eggs of snakes, able to protect against heart and liver disease, poisons, and injury in battle, and accordingly they were carried as amulets. These were, according to the legend, created by magic from foam made by the snakes at midsummer.{{cite book |author1=Marren, Peter |author2=Mabey, Richard |author1-link=Peter Marren |author2-link=Richard Mabey |title=Bugs Britannica |url=https://books.google.com/books?id=Ah62bUZLDOwC |year=2010 |publisher=Chatto & Windus |isbn=978-0-7011-8180-2 |pages=469–470 |access-date=2018-03-22 |archive-date=2023-07-28 |archive-url=https://web.archive.org/web/20230728001019/https://books.google.com/books?id=Ah62bUZLDOwC |url-status=live }}

{{notelist}}

{{Reflist}}

{{Commons category|Echinoidea}}

• {{WRMS | 123082 | Echinoidea Leske, 1778}}
• [https://archive.today/20121228104735/http://spbase.org/ The sea urchin genome project]
• [https://web.archive.org/web/20070827120312/http://www.seaurchin.org/ Sea Urchin Harvesters Association – California] Also, (604) 524-0322.
• [http://www.nhm.ac.uk/research-curation/research/projects/echinoid-directory/ The Echinoid Directory] from the Natural History Museum.
• [https://web.archive.org/web/20100723152249/http://virtualurchin.stanford.edu/urchinanatomy.swf Virtual Urchin] at Stanford
• [http://www.calurchin.org/index.html California Sea Urchin commission]
• [http://www.ucmp.berkeley.edu/echinodermata/echinoidea.html Introduction to the Echinoidea] at UCMP Berkeley

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