ADAPTIVE ASPECTS OF PHENOTYPIC PLASTICITY IN ECHINODERMS

Echinoderms show phenotypic plasticity in which variation in environme ntal conditions elicit graded reaction norms of morphological change. With the sea urchin Strongylocentrotus purpuratus under laboratory con ditions, decreases in available food caused decreases in dry weights o f the gonad and gut coupled with increases in the mass of Aristotle's lantern and length of the demi-pyramids. Spine clipping caused increas ed spine growth as well as decreases in the gut and gonad masses. Surv ival rate remained unchanged over a range of feeding and clipping trea tments. In the field, at Sunset Bay, Oregon, well-fed sea urchins, wit h relatively small demi-pyramids, showed survival rates similar to poo rly fed sea urchins that had large demi-pyramids. The wide range of mo rphological change, coupled with the narrow range of-change in surviva l, suggests that morphological plasticity buffers survival and hence w ould increase fitness. Phenotypic plasticity shown by other sea urchin s includes changes in the relative sizes of Aristotle's lantern in Dia dema antillarum and Echinometra mathaei and seasonal variation in the density of pedicellariae in Echinus esculentus. Variation in body size of echinoids and asteroids in different habitats and variation in num ber of arms in crinoids also may be examples of adaptive plasticity. S easonal atrophy of internal organs in the sea cucumber Parastichopus c alifornicus has been interpreted as an adaptive response to food limit ation but, given the nature of responses in other echinoderms, such an interpretation seems unlikely and the significance of seasonal atroph y of organs in holothurians remains unknown.