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 know n about how food limitation during the larval stage influences length of larval life or postmetamorphic performance. This paper documents th e 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) o n postmetamorphic growth rate. To assess the magnitude of nutritional stress imposed by different food concentrations, we measured growth ra tes (as changes in shell length and ash-free dry weight) for larvae re ared in either 0.45-mu m filtered seawater or at phytoplankton concent rations (Isochrysis galbana, clone T-ISO) of 1x10(3), 1x10(4), or 1.8x 10(5) cells ml(-1). Larvae increased both shell length and biomass at 1x10(4) cells ml(-1), although significantly more slowly than at the h ighest food concentration. Larvae did not significantly increase(p >0. 10) mean shell length in filtered seawater or at a phytoplankton conce ntration 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 a bility 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 lar vae were competent to metamorphose, as assessed by the numbers of indi viduals metamorphosing in response to elevated K+ concentration in sea water, 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 w ere maintained at 1.8 x 10(5) cells ml(-1). All larvae were monitored daily for metamorphosis. Individuals that metamorphosed in each food t reatment were transferred to high ration conditions (1.8 x 10(5) cells ml(-1)) for four additional days to monitor postmetamorphic growth. C ompetent larvae responded to all food-limiting conditions by metamorph osing precociously, typically 1 wk or more before larvae metamorphosed when maintained at the highest food ration. Surprisingly, juveniles r eared 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 ra p:d decline in phytoplankton concentration during the larval developme nt 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.