Trematode Prevalence and the Population Dynamics of Freshwater Pond Snails

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Abstract
We performed a 4-yr study of prevalence of larval trematodes in three populations of two snail species in three Indiana [USA] ponds and also recorded the relative abundance of cercarial types in these species (Lymnaea elodes and Physa gyrina), as well as a third species, Helisoma trivolvis. Yearly variation in L. elodes was considerable, but prevalence was greatest in the least productive pond (24% of snails for both L. elodes and P. gyrina) because longer generation times increased the frequency of older snails in samples, and prevalence increased with age. In dry seasons, prevalence averaged only 7.9% in vernal ponds, but increased to 25.6% in wet years. Prevalence actually increased in dry years in a permanent pond. These changes were due perhaps to alteration of final host visitation of ponds, indicating the importance of habitat predictability (e.g., time pond remains full). Over all ponds and years, 17% of Lymnaea elodes collected were infected, and prevalence increased with shell size (and age). The dominant cercarial type in L. elodes was an echinostoma, in P. gyrina, a notocotylid, and in H. trivolvis, a spirorchid. Life tables constructed here for the first time for Lymnaea elodes using these age-specific prevalences, along with field survivorship and fecundity data, indicate that parasites can lower replacement rates of snail populations by approximately 10 to 20%. Manipulatory experiments in field situations are necessary to determine if this loss of reproduction can regulate host populations.

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