Trematode prevalence and the population dynamics of freshwater pond snails

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Examined prevalence of larval trematodes in 3 populations of 2 snail species in 3 Indiana ponds and recorded the relative abundance of cercarial types in these species (Lymnaea elodes, Physa gyrina), as well as 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. Changes were due perhaps to alteration of final host visitation of ponds, indicating the importance of habitat predictability. Over all ponds and years, 17% of L. elodes collected were infected; prevalence increased with shell size (and age). The dominant cercarial type in L. elodes was an echinostome, in P. gyrina, a notocotylid, and in H. trivolvis, a spirorchid. Parasites can lower replacement rates of snail populations by 10-20%. -from Authors

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American Midland Naturalist

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