The shells of Etheria elliptica as recorders of seasonality at Lake Victoria

Reconstruction of past environments is an essential part of evaluating the ecology of organisms including early man and assessing the pressures for evolutionary change. In this study we have demonstrated that Etheria elliptica, the freshwater oyster of equatorial Africa, accretes layers of shell having a lunar month periodicity. Separation and oxygen isotopic analyses of the individual monthly layers affords a qualitative picture of seasonal change. Fourteen monthly samples of E. elliptica, collected near Kisumu on the Winam Gulf of Lake Victoria, gave oxygen isotope ratios in the penultimate layer of their shells in excellent agreement with the individual layers of a shell whose growth covered the same period. The variation in isotope ratios correlates clearly with the rainfall pattern of that region of East Africa.