Food limitation stimulates metamorphosis of competent larvae and alters postmetamorphic growth rate in the marine prosobranch gastropod Crepidula fornicata

The effects of food limitation on growth rates and survival of marine invertebrate larvae have been studied for many years. Far less is known about how food limitation during the larval stage influences length of larval life or postmetamorphic performance. This paper documents the effects of food limitation during larval development (1) on how long the larvae of Crepidula fornicata (L.) can delay metamorphosis in the laboratory after they have become competent to metamorphose and (2) on postmetamorphic growth rate. To assess the magnitude of nutritional stress imposed by different food concentrations, we measured growth rates (as changes in shell length and ash-free dry weight) for larvae reared in either 0.45-mu m filtered seawater or at phytoplankton concentrations (Isochrysis galbana, clone T-ISO) of 1x10(3), 1x10(4), or 1.8x10(5) cells ml(-1). Larvae increased both shell length and biomass at 1x10(4) cells ml(-1), although significantly more slowly than at the highest food concentration. Larvae did not significantly increase(p >0.10) mean shell length in filtered seawater or at a phytoplankton concentration of only 1 x 10(3) cells ml(-1), and in fact lost weight under these conditions. To assess the influence of food limitation on the ability of competent individuals to postpone metamorphosis, larvae were first reared to metamorphic competence on a food concentration of I. galbana (1.8x10(5) cells ml(-1)). When at least 80%, of subsampled larvae were competent to metamorphose, as assessed by the numbers of individuals metamorphosing in response to elevated K+ concentration in seawater, remaining larvae were transferred either to 0.45-mu m filtered seawater or to (1 x 10(3), 1 x 10(4), or 5 x 10(4) cells ml(-1)), or were maintained at 1.8 x 10(5) cells ml(-1). All larvae were monitored daily for metamorphosis. Individuals that metamorphosed in each food treatment were transferred to high ration conditions (1.8 x 10(5) cells ml(-1)) for four additional days to monitor postmetamorphic growth. Competent larvae responded to all food-limiting conditions by metamorphosing precociously, typically 1 wk or more before larvae metamorphosed when maintained at the highest food ration. Surprisingly, juveniles reared at full ration grew more slowly if they had spent 2 or 3 d under food-limiting conditions as competent larvae. The data show that a rap:d decline in phytoplankton concentration during the larval development of C. fornicata stimulates metamorphosis, foreshortening the larval dispersal period, and may also reduce the ability of postmetamorphic individuals to grow rapidly even when food concentrations increase.