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.