THE ASSIMILATION OF ELEMENTS INGESTED BY MARINE PLANKTONIC BIVALVE LARVAE

The assimilation efficiencies of nine elements were measured in plankt onic 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'' di gestion strategy observed in marine copepods operates in other plankto nic herbivores with gut morphologies different from that of crustacean zooplankton. Of the elements studied (Ag, Am, C, Cd, Co, P, S, Se, an d Zn), americium was assimilated the least by both the larval oysters (7.9%) and clams (2.6%), while selenium was assimilated with the highe st efficiency by the larvae (oysters, 97%; clams, 100%). Assimilation efficiencies were directly related to the fraction of each element pre sent in the cytoplasm of the ingested algae. Like copepods, bivalve la rvae have short gut passage times and assimilate only the easily mobil ized, cytoplasmic fraction of ingested phytoplankton cells. The cytopl asmic 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 proportionat ely more of these elements and protein than when feeding on rapidly di viding cells.