Animal Diversity Web

There are ap­prox­i­mately 940 species of echi­noids dis­trib­uted world­wide in ma­rine habi­tats from the in­ter­tidal to 5000 me­ters deep. Their fos­sil record is ex­ten­sive due to their test (an in­ter­nal skele­ton), and dates back to the mid­dle Or­dovi­cian pe­riod.

Echi­noids are com­monly grouped as reg­u­lar or ir­reg­u­lar, with the great­est dif­fer­ences per­tain­ing to the oral struc­ture, shape of the or­gan­ism, and lo­ca­tion of the anus. Reg­u­lar echi­noids are the sea urchins; they are gen­er­ally found on rocky sub­strates. Ir­reg­u­lar echi­noids are the sand dol­lars, which are gen­er­ally found on sandy or soft ground.

Like all echin­o­derms, echi­noids are pen­tara­di­ally sym­met­ri­cal, have a wa­ter-vas­cu­lar sys­tem, and have an in­ter­nal skele­ton made of cal­citic os­si­cles (plates). A dis­tin­guish­ing fea­ture of the echi­noids is that the os­si­cles im­bri­cate (over­lap) and are fused into a glob­u­lar or dis­coidal test; its flat­tened or con­cave oral side faces the sub­stra­tum and the ab­o­ral side is arched in most species. The mouth, in the peri­s­tomal mem­brane, con­tains a pow­er­ful chew­ing ap­pa­ra­tus called the Aris­to­tle's lantern. The lantern is com­posed of five jaws and is ca­pa­ble of ex­tend­ing through the mouth of some urchins. The mouth leads to the in­tes­tine and anus, which is lo­cated in the cen­ter of the ab­o­ral sur­face in reg­u­lar echi­noids. The anus is ei­ther pos­te­rior or on the oral sur­face of ir­reg­u­lar echi­noids.

Spines and tube feet sur­round­ing the peri­s­tome func­tion in lo­co­mo­tion, bur­row­ing, and food-gath­er­ing. Gen­er­ally, urchins have longer spines; sand dol­lars have shorter spines which give them a fuzzy ap­pear­ance. Tube feet are a part of the water vas­cu­lar sys­tem char­ac­ter­is­tic of all echin­o­derms. Pin­cers lo­cated be­tween spines are called pedi­cel­lar­iae. Some types of pedi­cel­lar­iae and spe­cial­ized spines of urchins con­tain venom used in self-de­fense.

In reg­u­lar urchins, the os­si­cles, or plates, of the test are ar­rayed in ten lon­gi­tu­di­nally ori­ented columns. Two ad­ja­cent columns each form one of five am­bu­lacral se­ries. These are the plates through which tube feet ex­tend. On the ab­o­ral side, the tube feet func­tion in res­pi­ra­tion and sen­sa­tion. The am­bu­lacral se­ries of plates are con­spic­u­ous in the cleaned test of a sand dol­lar: re­stricted to the ab­o­ral side, they are ar­rayed in a petaloid pat­tern. At the ab­o­ral end of the in­ter­am­bu­lacral se­ries of reg­u­lar urchins are the five (some­times four) gen­i­tal plates, through which the gono­pores open. One of the gen­i­tal plates serves as the sieve plate, or madreporite, for the water vas­cu­lar sys­tem. To­gether the madreporite, the anus, and the gono­pores make up the periproct.

Most echi­noids have five con­spic­u­ous go­nads ar­rayed in­ter­am­bu­lacrally. The sexes are sep­a­rate. In some species, ga­me­to­ge­n­e­sis is reg­u­lated by pho­tope­riod so that spawn­ing of most or all mem­bers of a pop­u­la­tion oc­curs dur­ing the same time. Some fe­male urchins brood their young ex­ter­nally, within the pro­tec­tion of their spines or tube feet. In species with in­di­rect de­vel­op­ment, an echino­plu­teus larva is pro­duced. Such a larva is bi­lat­er­ally sym­met­ri­cal, and un­der­goes meta­mor­pho­sis to at­tain the pen­tara­dial sym­me­try of the adult.

Echi­noids graze on just about any­thing they come across, plant or an­i­mal. This in­cludes algae, bry­ozoans, and dead an­i­mals.

Mem­bers of this class are food for crabs, sea stars, fish, birds, ot­ters, and other mam­mals. Prob­a­bly the sin­gle most im­por­tant con­tri­bu­tion of these an­i­mals to sci­en­tific knowl­edge is their em­bry­olog­i­cal de­vel­op­ment. Re­searchers in­ves­ti­gate the de­vel­op­ment of deuteros­tomes using sea urchin eggs, due the clear ra­dial cleav­age dur­ing a zy­gote's de­vel­op­ment. Echi­noids of eco­nomic im­por­tance for the U.S. are the red (Strongy­lo­cen­tro­tus fran­cis­canus), the pur­ple (S. pur­pu­ra­tus), and the green (S. droe­bachien­sis) sea urchins. These urchins are har­vested for their roe and are ex­ported to Japan; the roe, called uni, is used in sushi.

Ref­er­ences:

Br­usca, R.C. and G.J. Br­usca. 1990. Chap­ter 22: Phy­lum Echin­o­der­mata. In­ver­te­brates. Sin­auer As­so­ci­ates, Inc., Sun­der­land, Mass­a­chu­setts.

Hyman, L.H. 1955. The In­ver­te­brates: Echin­o­der­mata: The Coelo­mate Bi­la­te­ria. Vol­ume IV. Mc­Graw-Hill Book Com­pany, Inc. New York and other cities.

Ko­zloff, E.N. 1990. Chap­ter 21: Phy­lum Echin­o­der­mata. In­ver­te­brates. Saun­ders Col­lege Pub­lish­ing. Philadel­phia and other cities.

Price, R.J. and P.D. Tom 1995. Sea urchins. Sea Grant Ex­ten­sion Pro­gram Pub­li­ca­tion. http://​seaurchin.​org/​Sea-Grant-Urchins.​html

Con­trib­u­tors

Judy Follo (au­thor), Daphne G. Fautin (au­thor).