Histological description of the gonad, reproductive cycle, and fertilization of Pisidium amnicum (Muller, 1774) (Bivalvia : Sphaeriidae)

A detailed study of the reproductive cycle of a Spanish population of Pisid ium amnicum (Muller, 1774) based on monthly histological gonadal samples is presented. Results of the study of the gonadal cycle perfectly match previ ously reported data on the dynamics of this population, with mature gametes of both sexes present between July and October. Specimens surviving one re productive cycle undergo a second gametogenesis resulting in a new brood. N evertheless, most of them die before birth because of the adults' limited l ife span. We suggest that cross-fertilization in these freshwater bivalves occurs in summer in the gills or in the suprabranchial chamber instead of i n the gonoduct as has been proposed by other authors. Illustrations of all stages of the gametogenic processes, both male and female, as well as of th e first stages of embryonic development, are given. Differences between the reproductive strategies of P. amnicum and other sphaeriid species are also discussed.