View source for Sea urchin

{{Taxobox
| name = Sea urchin
| image = SeaUrchin.jpg
| image_width = 230px
| image_caption = 
| regnum = [[Animal]]ia
| phylum = [[Echinodermata]]
| classis = '''Echinoidea'''
<!--
| subdivision_ranks = Subclasses
| subdivision = 
*[[Euechinoidea]]
**Superorder [[Atelostomata]]
***Order [[Cassiduloida]]
***Order [[Spatangoida]] (heart urchins)
**Superorder [[Diadematacea]]
***Order [[Diadematoida]]
***Order [[Echinothurioida]]
***Order [[Pedinoida]]
**Superorder [[Echinacea (animal)|Echinacea]]
***Order [[Arbacioida]]
***Order [[Echinoida]]
***Order [[Phymosomatoida]]
***Order [[Salenioida]]
***Order [[Temnopleuroida]]
**Superorder [[Gnathostomata (echinoid)|Gnathostomata]]
***Order [[Clypeasteroida]] (sand dollars)
***Order [[Holectypoida]]
*[[Perischoechinoidea]]
**Order [[Cidaroida]] (pencil urchins)
-->| subphylum = [[Echinozoa]]
}}
'''Sea urchins''' are spiky members of the [[Class (biology)|class]] '''Echinoidea''' of the [[phylum]] [[Echinodermata]]. Like the rest of the echinoderms, they are entirely [[Ocean|marine]]. They are globe-shaped, and protected by [[calcareous]] plates and spines. 

''Urchin'' is an old word for [[hedgehog]], and in many foreign languages these animals are called sea hedgehogs.

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.

Sea urchins mostly feed on [[algae]] and small animals. They have a special chewing apparatus called ''[[Aristotle's lantern]]'', after the [[Ancient Greece|Greek]] [[philosopher]] [[Aristotle]] who was fascinated by sea urchins. With this apparatus they can scrape organisms stuck to the surface over which the urchin is moving.

== Spines==
The spines are long and sharp in some species,<ref>Rubber slippers will protect feet</ref> and protect the urchin from [[predator]]s. The spines inflict a painful wound when they penetrate human skin, but are usually not dangerous. Echinoids also have pincer-like pedicellaria all over their body between the spines. The job of these is to deal with anything (like larvae) settling on the test (outer shell).<ref name="Nichols">Nichols D. 1962. ''Echinoderms''. Hutchinson, London. {{ISBN|0-09-065994-5}}</ref><sup>p101</sup> It is not known if the spines are venomous unlike the pedicellariae between the spines, which are VENOMOUS. <ref>
{{cite journal |author=Slaughter RJ, Beasley DM, Lambie BS, Schep LJ |title=New Zealand's venomous creatures |journal=N.Z. Med. J. |volume=122 |issue=1290 |pages=83–97 |year=2009 |pmid=19319171}}</ref>

Typical sea urchins have spines that are {{convert|1|to|3|cm}} in length, {{convert|1|to|2|mm}} thick, and not terribly sharp. ''Diadema antillarum'', familiar in the Caribbean, has thin, potentially dangerous spines that can reach {{convert|10|to|30|cm}} long.

==Teeth==

Sea urchins have five [[jaws]] with one [[tooth]] each.  The teeth grow throughout the sea urchin's life.  The teeth of the pink sea urchin chip against each other to stay sharp.<ref name="PinkTeeth">{{cite news|publisher=Phys.org|title=These pink sea urchins have teeth that sharpen themselves|date=September 19, 2019|accessdate=May 27, 2020|url=https://phys.org/news/2019-09-pink-sea-urchins-teeth-sharpen.html|author=Cell Press}}</ref><ref name="Espinosa">{{cite journal|doi=10.1016/j.matt.2019.08.015|journal=Matter|title=In situ Wear Study Reveals Role of Microstructure on Self-Sharpening Mechanism in Sea Urchin Teeth|url=https://www.cell.com/matter/fulltext/S2590-2385(19)30173-0|author1=Horatio D. Espinosa|date=September 18, 2019|author2=Alireza Zaheri|author3=Hoang Nguyen|author4=David Restrepo|author5=Matthew Daly|author6=Michael Frank|author7=Joanna McKittrick|volume=1|issue=5|pages=1246–1261|s2cid=203651036|accessdate=May 27, 2020}}</ref>

==Ecology==
[[Image:SeaDSC01286.JPG|thumb|right|230px|''Echinothrix calamaris'', a sea urchin with huge banded spines. The sphere, top, middle, is its anus.]]
Sea urchins feed mainly on [[algae]], but can also feed on sea cucumbers, and a wide range of [[invertebrate]]s such as [[mussel]]s, [[polychaete]]s, [[sponge]]s, [[brittle star]]s and [[crinoid]]s.<ref>Baumiller T.K. 2008. Crinoid ecological morphology. ''Annual Review of Earth and Planetary Sciences'' '''36''': 221–249. doi:10.1146/annurev.earth.36.031207.124116.</ref> Sea urchins are one of the favorite foods of [[sea otter]]s and wolf [[eel]]s. Without predators to eat them, urchins breed in large numbers and eat almost all the nearby kelp, creating an ''[[urchin barren]]'', devoid of macroalgae and associated [[fauna]].<ref name="NHMag">{{cite magazine|title=Underwater Barrens: Monitoring the fate of Southern Californian kelp requires a long view.|date=February 2017|author=Doug Simpson|url=https://naturalhistorymag.com/features/153361/underwater-barrens|accessdate=May 22, 2020|publisher=Natural History Magazine}}</ref><ref name="Grant">{{cite web|title=With otters away, what keeps urchins at bay?|url=https://caseagrant.ucsd.edu/news/with-otters-away-what-keeps-urchins-at-bay|date=March 14, 2019|accessdate=May 22, 2020|publisher=California Sea Grant|archive-date=September 29, 2020|archive-url=https://web.archive.org/web/20200929135054/https://caseagrant.ucsd.edu/news/with-otters-away-what-keeps-urchins-at-bay|url-status=dead}}</ref> Sea otters have re-entered [[British Columbia]], dramatically improving coastal ecosystem health by eating sea urchins.<ref name=dfo>{{cite web|url=http://www.dfo-mpo.gc.ca/species-especes/species/species_seaOtter_e.asp|archiveurl=https://web.archive.org/web/20080123224702/http://www.dfo-mpo.gc.ca/species-especes/species/species_seaOtter_e.asp|archivedate=January 23, 2008|title=Aquatic Species at Risk - Species Profile - Sea Otter|publisher=Fisheries and Oceans Canada|accessdate=November 29, 2007}}</ref>

==  Fossil record and evolution ==
The first echinoid [[fossil]]s are from the Lower [[Ordovician]] period. The earliest forms had flexible tests, with plates that could slide over each other. Echinoids were a relatively unimportant part of the [[biota]] in the [[Palaeozoic]]. Only one group survived the [[P/Tr extinction event]] to form the basis of all later echinoids.<ref name="Clarkson">Clarkson E.N.K. 1998. ''Invertebrate palaeontology and evolution''. 4th ed, Blackwell, Oxford. {{ISBN|0-412-47990-7}}. (Chapter 9 covers Echinoderms).</ref> The group which survived into the Triassic, the Cidaroids, radiated into all other modern groups, which are known as the Euechinoids.

The break with perfect symmetry, which happened in the [[Jurassic]], gave them definite front and back ends. This opened up new habitats, in particular, the [[burrow]]ing habitat. [[Sand dollar]]s and [[heart urchin]]s have been highly successful.<ref name="Nichols"/> The living echinoids are now important members of the biota, especially in shallow and in-shore waters.

== Sea urchins as food ==
Some humans eat the reproductive organs of sea urchins (which they call [[roe]]). This is popular in [[Korea]]. In [[Japan]], this kind of [[sushi]] is called ''uni''.

== References ==
[[File:Seeigel-Saugfuesse(Galicien2005).jpg|thumb|left|200px|Tube feet in action: how a sea urchin moves]]
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[[Category:Echinoderms]]