METAL ACCUMULATION IN THE CLAM TRIDACNA-CROCEA UNDER NATURAL AND EXPERIMENTAL CONDITIONS

Clams (Tridacna crocea) were collected from three sites in the Torres Strait along a transect from the mouth of the heavily polluted Fly River towards the south. The concentrations of Cu, Cd and Zn in different organs of the clams were determined and, with the exception of Cd levels in the gills and mantle, were found to decrease with increasing distance from the river outfall. The levels of metals in the clam tissues, however, fall within a similar range of values found in control clams collected from Orpheus Island. In addition, the rates of metal bioaccumulation in clams were studied under laboratory conditions. Experiments carried out with either Cd (0.20 mg/l) or Cu (0.06 mg/l) in seawater revealed a linear uptake of metal throughout the experimental period in different clam organs. The highest accumulation rates were found in the kidney and the lowest in the muscle. The accumulation rate of Cd was higher in the mantle than in the gills, whereas the inverse was found for Cu. Cu treatment resulted in significant expulsion of the symbiotic zooxanthellae from the mantle, a recognized stress response. In clams collected from the wild, the concentration of renal metal-binding proteins (metallothionein-like proteins) increased with the levels of total metal in the kidneys, and indirectly reflected the environmental levels.