Anthropogenic introduction of steroidal estrogens and synthetic estrogenic compounds to aquatic environments has increased interest in assessment of the endocrine-disrupting effects of these compounds on aquatic organisms, including mollusks. For gonochoristic freshwater snails in the family Pleuroceridae, little is known about the spatial and temporal variation in population sex ratios or potential relationships between sex ratios and exposure to estrogenic compounds. Population sex ratios of Leptoxis (Rafinesque, 1819) spp., were evaluated within the Shenandoah River watershed (Virginia, U.S.A.), where agricultural operations are a source of estrogenic compounds to streams, and in four other river basins in Virginia. Proportions of females varied among streams within the Shenandoah River watershed, with overall mean proportions from four sampling periods ranging from 0.46–0.87. Sex ratios were consistently female-biased at nine of fifteen stream sampling sites. There was little within-site variation across generations of snails or when the same generation was examined in two different seasons. Proportions of females were not directly related to in-stream concentrations of estrogenic compounds or watershed densities of agricultural operations, but were negatively related to mean summer temperature at the sampling sites. Population sex ratios of Leptoxis spp. were female-biased at two of six sites in the Shenandoah River and one of five sites outside the basin. At the river sampling sites, proportions of females were only related to summer-specific conductivity. Overall, results suggest that site-specific factors can affect population sex ratios of Leptoxis spp. However, until more is known regarding mechanisms of sex determination and sexual differentiation in gastropods, population sex ratios should not be used as indicators of potential biological effects of estrogenic compounds.
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Vol. 30 • No. 2