Growth and energy balance during the larval lives of three prosobranch gastropods

Abstract

Larvae of most marine invertebrates postpone metamorphosis in the absence of a suitable environment for the adult stage. Larvae of three marine gastropod species, Ilyanassa obsoleta (Say), Crepidula fornicata (L.), and Bittium alternatum (Say), differed in the potential durations of their delay periods and in their ultimate fates in the absence of preferred substratum for metamorphosis in the laboratory. Experiments were conducted to test the hypothesis that differences in the delaying capabilities of the three species result from differences in the magnitude of metabolic adjustments made during the delay period. Carbon and caloric balances were calculated for larvae of Ilyanassa obsoleta before and after their attainment of competence, using data on feeding rates, respiration rates, and retention efficiencies. An excess of carbon and energy intake over expenditure was calculated in all experiments. As predicted from those calculations, but not from measurements of shell length increase, tissue biomass of I. obsoleta increases at a constant rate throughout larval life. Similarly, growth rate of larvae of Crepidula fornicata does not decline with the onset of metamorphic competence. Larvae of Bittium alternatum also show substantial growth during delay of metamorphosis. Morphological differentiation continues during the delay periods of Ilyanassa obsoleta and Crepidula fornicata. Therefore, there is no indication that changes in energy balance directly limit the potential duration of the delay period in these species. However, these data suggest a relationship between growth rate prior to the onset of competence and the potential duration of the delay period. The hypothesis is presented that there may be a program for larval development with a genetically fixed endpoint and that potential duration of the delay period may be determined by the rate at which larvae progress through this developmental program. Ecological factors potentially operating to select for differences in delay capabilities among species are discussed.