Larval trematode assemblages in the snail Lymnaea stagnalis from southeastern Wisconsin

From May through November 1993 and 1994, Lymnaea stagnalis appressa were collected from a stream adjacent to Cedarburg Bog, Ozaukee County, Wisconsin. Observations of cercarial shedding and subsequent dissection of snails revealed a relatively depauperate digenean component community consisting primarily of 3 common species. The most frequently occurring form was the metacercaria of Cotylurus flabelliformis. Sporocysts and cercariae of this species were rarely observed. Snails also harbored sporocysts and shed cercariae of Schistosomatium douthitti and a plagiorchid. Seasonal and yearly dynamics of the component trematode community reflect the apparent inability of sporocysts to overwinter in snails at this location. They also seem to be associated closely with snail life histories in terms of the availability of large snails, most often infected by the sporocysts of S. douthitti and the plagiorchid. Analyses of double infections revealed the occurrence of a negative interspecific interaction between the tetracotyles of C. flabelliformis and sporocysts of S. douthitti. Other combinations of trematode species did not reveal negative interactions. Competition does not seem to play a major role in the structure of the component community at this location.