ASSIMILATION OF TRACE-ELEMENTS INGESTED BY THE MUSSEL MYTILUS-EDULIS - EFFECTS OF ALGAL FOOD ABUNDANCE

Pulse-chase feeding and multi-labeled radiotracer techniques were empl oyed to measure the assimilation of 6 trace elements (Ag-110, Am-241, Cd-109, Co-57, Se-75 and Zn-65) from ingested diatoms in the mussel My tilus edulis feeding at different rates (0.1, 0.49 and 1.5 mg dry wt h (-1)). Uniformly radio-labeled diatoms Thalassiosira pseudonana were f ed to mussels for 0.5 h, and the behavior of the radiotracers in indiv idual mussels was followed for 96 h in a depuration seawater system. A ssimilation efficiency (AE) of each element declined with increasing i ngestion rate and increased with gut passage time. The importance of e xtracellular digestion relative to intracellular digestion increased w ith inges tion activity, which, when coupled with a decline in AE, sug gested that extracellular digestion is less efficient in metal absorpt ion. Zn assimilation was most affected by ingestion rate, suggesting t hat AE may play a role in the physiological regulation of this metal i n M. edulis. In an experiment to simulate the effects of an acidic gut , lowered pH (5.5) enhanced the release of elements from intact diatom cells, especially at low particle concentration. These results indica te that both feeding components of the mussel (i.e. gut passage time, digestive partitioning) and metal chemistry (i.e. metal release at low ered pH within the bivalve gut) are responsible for the difference in the assimilation of trace metals at different food quantities observed in mussels.