Host and parasite counteradaptations: an example from a freshwater snail.

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Trematode infection of pulmonate snails is often associated with increased growth and/or survivorship of snail hosts. The freshwater pulmonate Lymnaea elodes and its trematode parasites are used to test whether this increase is a parasite adaptation, a host adaptation, or a side effect that serves no adaptive function for either participant. Field experiments indicate that trematode parasitism significantly reduces host fecundity and causes a temporary elevation and subsequent reduction in host growth. A 2- yr field survey of the prevalence of trematode infection in 3 snail populations revealed a significant positive relationship between shell size and prevalence. L. elodes does not outlive its trematode infections. Overall, results suggest that increased survivorship in trematode-infected L. elodes is a parasite strategy for providing a stable, long-term resource for the parasite. -from Authors

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American Naturalist

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