THE ASSIMILATION OF ELEMENTS INGESTED BY MARINE COPEPODS

The efficiency with which a variety of ingested elements (Ag, Am, C, Cd, P, S, Se, and Zn) were assimilated in marine calanoid copepods fed uniformly radiolabeled diatoms ranged from 0.9% for Am to 97.1% for Se. Assimilation efficiencies were directly related to the cytoplasmic content of the diatoms. This relation indicates that the animals obtained nearly all their nutrition from this source. The results suggest that these zooplankton, which have short gut residence times, have developed a gut lining and digestive strategy that provides for assimilation of only soluble material. Because the fraction of total cellular protein in the cytoplasm of the diatoms increased markedly with culture age, copepods feeding on senescent cells should obtain more protein than those feeding on rapidly dividing cells. Elements that are appreciably incorporated into algal cytoplasm and assimilated in zooplankton should be recycled in surface waters and have longer oceanic residence times than elements bound to cell surfaces.