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 pressu res for evolutionary change. In this study we have demonstrated that E theria elliptica, the freshwater oyster of equatorial Africa, accretes layers of shell having a lunar month periodicity. Separation and oxyg en isotopic analyses of the individual monthly layers affords a qualit ative picture of seasonal change. Fourteen monthly samples of E. ellip tica, collected near Kisumu on the Winam Gulf of Lake Victoria, gave o xygen isotope ratios in the penultimate layer of their shells in excel lent agreement with the individual layers of a shell whose growth cove red the same period. The variation in isotope ratios correlates clearl y with the rainfall pattern of that region of East Africa.