Seasonal Dynamics of Halipegus occidualis (Trematoda: Hemiuridae) in Helisoma anceps and Its Impact on Fecundity of the Snail Host

Helisoma anceps, a pulmonate snail, was infected with 8 species of larval trematodes in Charlie's Pond, located in the Piedmont area of North Carolina. Each species except Halipegus occidualis occurred in less than 4% of the snails; prevalence of H. occidualis was 54.6%. The peak in H. occidualis occurred in late spring, declined in summer, and increased in fall. A field study designed to monitor egg production by H. anceps revealed that nearly all snails shedding cercariae of H. occidualis were castrated by the parasite. Castration was mechanical, by direct ingestion of host tissue by trematode rediae. One of the most pervasive effects of parasitism is the impact of larval trematodes on the reproductive biology of molluscan intermediate hosts. While the primary site of infection in snails is usually the hepatopancreas, they frequently spread into the gonad and cause host castration (Pratt and Barton, 1941; Etges and Gresso, 1965; Robson and Williams, 1971; Cheng et al., 1983). During the summer of 1983, preliminary observations on the pulmonate snail, Helisoma anceps, in a small North Carolina pond revealed several species of larval trematodes, including the hemiurid Halipegus occidualis. Because of the extent to which host tissue was replaced by parasite biomass, it seemed likely that such infections, especially those by H. occidualis, might have a serious effect on the population and reproductive biology of the snail hosts. Accordingly, a study was undertaken to examine the seasonal dynamics of H. occidualis in H. anceps and to determine the effect of H. occidualis on the fecundity of H. anceps. MATERIALS AND METHODS