Animal Diversity Web

Two thou­sand species have been species iden­ti­fied in Sub­class Ophi­uroidea. All are ma­rine, as are all echin­o­derms. They are found in all seas, at all lat­i­tudes, and from the in­ter­tidal to the abyssal zone (to a depth of 6,000 me­ters).

Ophi­uroids ex­hibit the three dis­tinc­tive char­ac­ter­is­tics of the phy­lum Echin­o­der­mata: a body plan with five part sym­me­try (pen­tara­dial), an in­ter­nal cal­cium car­bon­ate skele­ton in the min­eral form of cal­cite, and a water vas­cu­lar sys­tem of fluid-filled ves­sels that end in tube feet. Nearly all ophi­uroids have five equal rays or arms that are clearly set off from the cen­tral disk. The rays dif­fer from those of as­ter­oids and crinoids in being long and slen­der, and nearly solid. They are very flex­i­ble and en­able the an­i­mals to make snake-like move­ments (which is the source of the class name: G. ophis = ser­pent) -- an ophi­uroid lo­co­motes by using two rays to pro­duce a row­ing mo­tion. The rays of bas­ket stars branch. Ophi­uroids pos­sess con­sid­er­able pow­ers of re­gen­er­a­tion: an arm can be re­gen­er­ated at any point, but if the disc is to­tally sep­a­rated from all arms, the an­i­mal will die. An ophi­uroid can eas­ily cast off por­tions of an arm if at­tacked by a preda­tor. This abil­ity to au­to­tomize is the source of the com­mon name brit­tle star.

The water vas­cu­lar sys­tem, which plays a role in lo­co­mo­tion, gen­er­ally has one madreporite, but some species lack a madreporite. A ra­dial canal ex­tends into each arm from the ring canal, and lat­eral canals from each ra­dial canal sup­ply the tube feet. The tube feet of ophi­uroids lack suck­ers and am­pul­lae.

Ophi­uroids are car­ni­vores, fil­ter feed­ers, and scav­engers; those of some species use more then one method to ob­tain food. Mucus on the tube feet can trap phy­to­plank­ton, bac­te­ria, and even medusae from the water or par­ti­cles from sed­i­ment that are moved by the tube feet to the mouth; some ophi­uroids cap­ture small crus­taceans or worms by loop­ing the or­gan­isms with their rays. The mouth is sur­rounded by five jaws and leads to an esoph­a­gus that con­nects to the sac-like stom­ach. The stom­ach fills much of the disc, but does not ex­tend into the arms, and ends blindly (that is, it has no anus). Di­ges­tion oc­curs within 10 pouches or in­folds of the stom­ach.

Gas ex­change and ex­cre­tion occur through cilia-lined sacs called bur­sae; each opens onto the in­ter­am­bu­lacral area of the oral sur­face of the disc, and typ­i­cally there are 10 per an­i­mal. Go­nads are lo­cated in the disc; sexes are sep­a­rate in most species. Ga­metes are shed into the water by way of the bur­sal sacs. Fer­til­iza­tion may re­sult in a free-swim­ming larva called an ophio­plu­teus, which un­der­goes meta­mor­pho­sis with­out an at­tach­ment stage. How­ever, many ophi­uroids brood their young, com­monly in the bur­sae.

The ophi­uroid coelom is much re­duced com­pared to that of other echin­o­derms. The ner­vous sys­tem con­sists of a nerve ring in the disc that sends out a ra­dial nerve to each arm. Ophi­uroids lack eyes but the epi­der­mis is sen­si­tive to light and other stim­uli.

Ophi­uroids have lit­tle im­por­tance in human com­merce but they are very im­por­tant in the diets of many crus­taceans and fishes.

Ref­er­ences:

Hendler, G. L., M. P. Kier, J. E. Miller, and D. L. Paw­son. Class Ophi­uroidea, pages 89-195 in Sea Stars, Sea Urchins, and Al­lies. Wash­ing­ton: Smith­son­ian In­sti­tu­tion Press. 1995.

Hyman, L. H. Class Ophi­uroidea, pages 589-689 in The In­ver­te­brates vol­ume IV: Echin­o­der­mata. New York: Mc­Graw-Hill Book Com­pany Inc. 1955.

Ko­zloff, E. N. Class Ophi­uroidea, pages 745-750 in In­ver­te­brates. Philadel­phia and other cities: Saun­ders Col­lege Pub­lish­ing. 1990.

Con­trib­u­tors

Michelle Mor­ris (au­thor), Daphne G. Fautin (au­thor).