Imposex in Ilyanassa obsoleta still common in a Delaware estuary

Populations of Ilyanassa obsoleta (Gastropoda) in Delaware estuaries were assessed for imposex. This phenomenon involves the abnormal imposition of male features on females and is associated with marinas and TBT (tributyltin) pollution from antifouling paints. Imposex was assessed in the northern section of the Lewes and Rehoboth Canal in a 1983–1984 study. In summer 1995, snails were taken from this section, the south Canal, and Rehoboth Bay. In the Canal, imposex was least frequent (52% of females) at the north terminus, reached a peak in Lewes Harbor (94%), and was 78–90% to the south. At the Rehoboth Bay site it was 23%. Imposex frequency has not changed in the northern section since 1983–1984. However, the intensity of imposex expression has declined. No female had a full-size penis in this study, while previously many did. Intense boating activity is confined to the Canalʹs northern section. Frequent imposex in the south Canal suggests that, over the years, TBT pollution has spread south.