SCIENTISTS at the School of Marine Sciences in Cochin have discovered that shells of some molluscs, such as two-shelled clams, which store trace metals in their shells and tissues, are good biological indicators of water pollution levels.

Detailed investigations on accumulation of trace metals in the clam Villorita cyprinoides, found in the Cochin Estuarine System, indicate manganese, cobalt and lead tend to accumulate in shells, while copper, cadmium and zinc build-up in soft tissues.

Metal enrichment of shells usually occurs in two ways. Triggered by metabolic functions and a need for minerals, trace elements may be actively integrated into the shell. Alternately, metals may be passively absorbed onto surfaces exposed to polluted seawater.