Effects of combined waterborne Cd and Cu exposures on ionic composition and plasma cortisol in tilapia, Oreochromis mossambicus

Abstract

Plasma ions and cortisol levels were studied in immature tilapia for 6 days to a range of sublethal concentrations of Cu (50, 100 and 200 μg Cu l −1), Cd (20, 35 and 70 μg Cd l −1) and to combinations of these metals (50 μg Cu l −1 + 20 μg Cd l −1, 100 μg Cu l −1 + 35 μg Cd l −1and 200 μg Cu l −1 + 70 μg Cd l −1). Our data show that Na and Ca were markedly, although not exclusively, affected by Cu and Cd, respectively. Plasma Na concentrations were most prominently decreased in Cu-exposed fish, with less pronounced effects in Cd-exposed fish. In fish exposed to 70 μg Cd l −1, the plasma Ca concentration was half of the control value. Cu-induced changes of plasma Ca concentrations were less strongly marked. In combined Cu Cd exposed fish, Na, Ca and Cl concentrations were significantly changed. Important in the present study was the notion that, in combined Cu Cd exposed fish, the changes in Na and Ca levels could not be explained by synergism of addition of the effects observed in single metal exposed fish. Plasma cortisol levels were increased in Cu-exposed fish, but an increase was not observed in the Cu Cd co-exposed fish. It is argued that the absence of this cortisol response contributes to an inadequate recovery of ionic disturbances in the Cu Cd co-exposed fish.