Metal concentrations in the annual shell layers of the Bivalve Elliptio complanata

Levels of four metals were measured in the shells of Elliptio complanata, collected from three localities in the Great Lakes, having widely different levels of contamination. The levels of ten metals were measured in the outermost shell and overlying periostracum, cut from each of the annual rings of two shells. Zn (unlike Pb) displayed a much greater affinity to the soft tissues than the shell. Levels of Al, Cu, Fe, Ni, Pb and Zn were significantly higher in the outer periostracum of the shell than in the inorganic prismatic shell layer. In spite of high Pb levels in industrial effluent at one of the locations, inorganic Pb was not deposited in the prismatic shell layer. Metal concentrations in the periostracum did not follow the chronological trends in the prismatic layer. Levels of Al, Fe and Mn decreased with clam age, whereas concentrations of Cu and Zn were greatest during years of fastest growth.