RESPONSE OF METALLOTHIONEIN CONCENTRATIONS IN A FRESH-WATER BIVALVE (ANODONTA-GRANDIS) ALONG AN ENVIRONMENTAL CADMIUM GRADIENT

Surficial (oxic) sediments, overlying water, and specimens of the freshwater bivalve Anodonta grandis were collected by divers at 11 littoral lacustrine sites located along a geochemical gradient of pH, [Cd], [Cu], and [Zn] in an area influenced by mining and smelting activities. The molluscs were dissected (gills; hepatopancreas; remaining tissues) and analyzed for Cd, Cu, Zn, and metallothioneins (MT). Tissue concentrations of MT in A. grandis varied between 100 and 440 nmol metal binding sites (g dry wt)-1. Metallothionein levels in the gills. the remaining tissues, and the whole organism were significantly correlated (P < 0.01) with tissue Cd concentrations. In contrast, correlations between [MT] and tissue levels of Cu or Zn were weak or nonexistent. The intersite variations in MT concentrations were best related, not to total dissolved metal concentrations at the time of sampling, nor to extractable metal concentrations in the sediments, but to the free Cd2+ concentration at the sediment-water interface, as estimated from sediment-water sorptive equilibria. These observations suggest that Cd2+ activity is the key environmental factor to which metallothionein levels in A. grandis are responding in the studied lakes. The mollusc condition indices (C.I. = 1,000 x total dry wt of flesh of the animals/total intervalval volume) deteriorated as Cd concentrations in the tissues increased.