Waterborne cadmium and zinc uptake in a euryhaline teleost Acanthopagrus schlegeli acclimated to different salinities

Metal uptake and toxicity in marine fish are usually much lower than those in freshwater fish, but the underlying mechanisms remain unclear. In this study, we investigated Cd and Zn uptake by the euryhaline black sea bream (Acanthopagrus schlegeli) over a salinity range from 0 to 35 psu. Cd and Zn uptake increased as salinity decreased. The gills were the most sensitive organs in response to salinity change, and played a more important role in Cd and Zn uptake at a lower salinity. Cd and Zn uptake in the viscera contributed to 34-36% of the overall accumulation at full salinity (35 psu), but decreased to 13-16% in freshwater despite the increase of uptake rate. Water permeability, drinking, and major ion uptake (Ca) in the fish at different salinities were also concurrently examined. The overall water uptake was comparable, whereas the drinking rate decreased at lowered salinities. In contrast, the Ca uptake increased significantly with decreasing salinity. The responses of Cd and Zn uptake to salinity challenge were correlated with the Ca uptake, suggesting that they may be taken up through the Ca uptake pathway. At a constant salinity, Cd and Zn uptake increased with reducing Ca concentration, indicating the competitive effect of Ca on metal uptake. Ca channel blockers (verapamil and lanthanum) significantly reduced the uptake of Cd, Zn, and Ca when the fish were acclimated in freshwater, but had no impact on their uptake in marine water. Furthermore, the chloride cell number in the gills could not explain the lower Cd and Zn uptake in seawater. Our results indicated that both ambient physicochemical factors and the physiological responses of fish resulted in difference of metal uptake in marine and freshwater environments.