The probability of a marine gastropod being infected by a trematode.

Sentinel snails were employed to determine the likelihood of an estuarine snail becoming infected with a trematode under natural conditions. In summers of 1991 and 1993, Ilyanassa obsoleta (Mollusca: Gastropoda) (n = 1,400) were collected from a saltmarsh in Lewes, Delaware, where parasite prevalence was low, and screened for infections. Putatively uninfected snails were individually marked and released onto the nearby Cape Henlopen sandflat where prevalence of trematodes among native snails is very high (approximately 80%). Most sentinels were free in the field for 1-5 mo and 186 were ultimately recovered. The overall estimate of probability of infection is 1.6%. The snail lives for many years and this low probability of becoming infected indicates that high prevalence of trematodes in this host is brought about not by rapid colonization, but by slow accumulation over time.