PHYSIOLOGICAL-RESPONSES AND THE UPTAKE OF CADMIUM AND ZINC BY THE AMPHIPOD CRUSTACEAN ORCHESTIA-GAMMARELLUS

The rates of uptake of cadmium and zinc by the amphipod crustacean Orc hestia gammarellus (Pallas) increase with decrease in salinity from 36 .5 to 25 parts per thousand NaCl, as expected from physicochemical cha nges in the availabilities of free metal ions. Between 15 and 25 parts per thousand NaCl cadmium and zinc uptake rates plateau, and the cadm ium uptake rate falls at 12 parts per thousand NaCl. This pattern of c hange of uptake rate with salinity change is not dependent on trace me tal exposure concentrations, and cannot be explained by uptake via any enzyme-driven uptake route. It is concluded that at low salinity the amphipods effect one or more physiological responses that offset any i ncreases in cadmium and zinc uptake rates expected from physico-chemic al increases in the availabilities of free metal ions at low salinity. Such physiological responses are induced by changes in total osmolali ty, as opposed to inorganic salinity, and are not maintained on transf er from low to high osmolality. The physiological response is not expl icable only in terms of change of the uptake rate of calcium, nor only in terms of change in apparent water permeability which may play a ro le at extremely low salinities. In low salinity the amphipods do excre te newly accumulated cadmium and zinc but this excretion does not expl ain the lack of increase in net uptake of cadmium and zinc at low sali nities. The identification of the physiological response of O. gammare llus reducing trace metal uptake at low salinity remains enigmatic, an d may turn out to be combination of several effects.