Physicochemistry or physiology: cadmium uptake and effects of salinity and osmolality in three crabs of different ecologies

Abstract

The effects of salinity and osmolality differences on the uptake rates of dissolved cadmium were investigated in 3 crabs of different ecologies — the euryhaline common shore crab Carcinus maenas, the extremely euryhaline Chinese mitten crab Eriocheir sinensis, and a more stenohaline crab, the velvet swimming crab Necora puber. This study tests predictions derived from the interpretations of the authors from a study of Zn uptake by the same crabs, in terms of the balance between physicochemical and counteractive physiological effects controlling trace metal uptake rates from solution. As predicted, reduced salinities caused increases in the cadmium uptake rate of E. sinensis as expected from the free metal ion model, with increased free cadmium ion availabilities in conditions of reduced chloride complexation. In the cases of C. maenas and N. puber, however, decreased salinity was associated with reduced cadmium uptake, a result interpreted in terms of a physiological response by these crabs to low salinity offsetting the physicochemical effect of increased free cadmium ion availability. This physiological response may include reductions in apparent water permeability with reduced salinities. Osmolality experiments showed that the physiological responses made by the crabs to decreases in salinity could not be explained simply by responses to changes in osmotic pressure of the medium. The interaction of physiology and physico- chemistry controls rates of trace metal uptake from solution by these ecophysiologically distinct crab species.