ASSIMILATION EFFICIENCIES AND TURNOVER RATES OF TRACE-ELEMENTS IN MARINE BIVALVES - A COMPARISON OF OYSTERS, CLAMS AND MUSSELS

Assimilation efficiencies (AEs) and physiological turnover-rate consta nts (k) of six trace elements (Ag, Am, Cd, Co, Se, Zn)in four marine b ivalves (Crassostrea virginica Gmelin, Macoma balthica Linnaeus, Merce naria mercenaria Linnaeus, and Mytilus edulis Linnaeus) were measured in radiotracer-depuration experiments. Egestion rates of unassimilated elements were highest during the first 24 h of depuration and decline d thereafter. Significant egestion of unassimilated Co, however, conti nued for up to 5d in Macoma balthica, Mercenaria mercenaria and Mytilu s edulis. With the exception of the extremely low values for Ag-110m, Cd-109, and Zn-65 in C. virginica, physiological turnover-rate constan ts (ii) showed no general pattern of variation among elements, bivalve species or food types, and were relatively invariant. Values from les s than or equal to 0.001 to 0.1 d(-1) were observed, but excluding tho se for Co, most values were less than or equal to 0.04 d(-1). In all f our species, the AEs of Ag, Am, and Co were generally lower than those of Cd, Se, and Zn. The AEs of Ag, Cd, Se, and Zn in these bivalves ar e directly related to the proportion of each element in the cytoplasmi c fraction of ingested phytoplankton, indicating that >80% of elements in a prey alga's cytoplasm was assimilated. C. virginica, Macoma balt hica, and Mercenaria mercenaria assimilated similar to 36% of the Ag a nd Cd associated with the noncytoplasmic (membrane/organelle) fraction of ingested cells in addition to the cytoplasmic fraction. The ratio of AE:k, which is proportional to the consumer-prey trace-element bioa ccumulation factor (concentration in consumer:concentration in prey) w as generally greater for Cd, Se, and Zn than for Ag, Am, and Co. This ratio was lowest in Mytilus edulis, suggesting that this bivalve, the most widely employed organism in global biomonitoring, is relatively i nefficient at accumulating important elements such as Ag, Cd, and Zn f rom ingested phytoplankton.