PHYSICOCHEMICAL AND PHYSIOLOGICAL-EFFECTS ON THE UPTAKE OF DISSOLVED ZINC AND CADMIUM BY THE AMPHIPOD CRUSTACEAN ORCHESTIA-GAMMARELLUS

An artificial medium of NaCl in distilled water, of predictable dissolved metal speciation equilibria, has been used to define the effects of physicochemical changes on the uptake of zinc (Zn) and cadmium (Cd) by the euryhaline littoral amphipod crustacean Orchestia gammarellus (Pallas). At salinities between 36.5 and 25 parts per thousand, changes in uptake rate of either metal correlated with predicted changes in free metal ion concentrations upon change in salinity, indicating the dominant role of medium physicochemistry in defining metal uptake rates. At 15 parts per thousand salinity, however, the change in uptake rate of each metal was no longer proportional to the change in free metal ion concentration, indicating a physiological response on the part of the amphipod to low salinity. It is proposed that such a response might be a reduction in apparent water permeability, characteristic of some euryhaline crustaceans. It is concluded that the use of a simple medium of defined physicochemistry has permitted the resolution of physicochemical and physiological effects on trace metal uptake rates, and that physiological adaptations of euryhaline organisms are significant in determining rates of trace metal uptake.