The effect of food availability on phenotypic plasticity in larvae of the temperate sea cucumber Australostichopus mollis

Phenotypic plasticity in response to food availability results in either the achievement of larval metamorphic competence or exposing larvae to a reduced probability for survival. Larvae of the temperate aspidochirote sea cucumber, Australostichopus mollis fed the diatom Chaetoceros muelleri grew best in 3000 cells ml- 1 day- 1, reaching a maximum length of 896 ± 22.8 μm by day 12 (mean ± SE). When shape was quantified independent of growth and size larvae were consistently shorter and wider in 300 but became longer and narrower in 3000 cells ml- 1 day- 1. They were also consistently shorter and narrower in 6000 than those in 600 cells ml- 1 day- 1, which became longer and wider. In both 300 and 6000 gut shape did not change whereas in 600 and 3000 cells ml- 1 day- 1 the gut became more oval shaped. By day 12 most in 3000 cells ml- 1 day- 1 were late auricularia whereas development was delayed in other feeding regimes. Although by day 18 in all feeding regimes larvae had 2 hyaline spheres in the posterior folds of the ciliated band, only those fed 600 and 3000 cells ml- 1 day- 1 had more. Of the surviving larvae in these feeding regimes, most were competent, undergoing, or likely to complete metamorphosis. Phenotypic plasticity or a change in shape allows larvae to moderate growth and form in response to food availability but it may also act as a selection mechanism in determining the contribution of larvae to subsequent generations by delaying development.