A DECADE-LONG PERSPECTIVE ON A BIOINDICATOR OF POLLUTION - IMPOSEX IN ILYANASSA-OBSOLETA ON CAPE-HENLOPEN, DELAWARE BAY

Imposex refers to the imposition of male characteristics on female gastropods and is a bioindicator of organotin (TBT) pollution. In summer 1992 a survey, involving dissection of 1686 snails and covering 1400 m of shoreline, was made of an estuarine neogastropod, Ilyanassa obsoleta, on the Cape Henlopen sandflat at the mouth of Delaware Bay. The purpose was to determine the frequency, intensity of expression, and spatial distribution of imposex. Further, historical data on imposex frequency and intensity of expression in this population, gathered since 1981, have been assembled to put the 1992 data into a longer term perspective. In 1992 imposex occurred throughout the habitat, but at variable frequency among 44 locations (range 0-64%). Intensity of expression was everywhere slight, as it had been throughout the 1980s. Imposex frequency increased from about 1% in the early 1980s to about 30% in 1992. Weak correlations could be mane between percentage imposer in samples, position on the sandflat and trematode parasitism. Samples collected farther from the beach and with larger proportions of females parasitized by certain trematodes tended to have fewer imposexed females. It is suggested that low inter tidal microhabitats, such as those where parasitized snails often gather, may be exposed to less of the imposer-inducing agent (TBT) than higher parts of rite sandflat.