THE ASSIMILATION OF ELEMENTS INGESTED BY MARINE PLANKTONIC BIVALVE LARVAE

The assimilation efficiencies of nine elements were measured in planktonic bivalve mollusc larvae (oysters, Crassostrea virginica, and hard clams, Mercenaria mercenaria) fed uniformly radiolabeled phytoplankton cells (Isochrysis galbana) in order to test whether the ''liquid'' digestion strategy observed in marine copepods operates in other planktonic herbivores with gut morphologies different from that of crustacean zooplankton. Of the elements studied (Ag, Am, C, Cd, Co, P, S, Se, and Zn), americium was assimilated the least by both the larval oysters (7.9%) and clams (2.6%), while selenium was assimilated with the highest efficiency by the larvae (oysters, 97%; clams, 100%). Assimilation efficiencies were directly related to the fraction of each element present in the cytoplasm of the ingested algae. Like copepods, bivalve larvae have short gut passage times and assimilate only the easily mobilized, cytoplasmic fraction of ingested phytoplankton cells. The cytoplasmic fraction of some elements (Se, Zn, and Cd) and of protein in I. galbana increased inversely with algal growth rate. Larvae feeding on senescent cells would therefore be expected to assimilate proportionately more of these elements and protein than when feeding on rapidly dividing cells.