Does habitat-specific variation in trematode infection risks influence habitat distribution of two closely related freshwater snails?

Parasitism may be an important factor determining the geographic distributi on of closely related species. A habitat-specific risk of parasitism may le ad to exclusion of susceptible host types from parasite-rich environments, and promote speciation if it leads to reproductive isolation between suscep tible and resistant types. We surveyed populations of the freshwater snail Lymnaea peregra for differences in habitat distribution and trematode paras itism between its two distinct shell morphs, L. ovata and L. peregra. We su rveyed 58 populations (43 L. ovata, 15 L. peregra). At each location we rec orded an array of habitat characteristics that were summarized using a nonl inear principal components analysis. This yielded two orthogonal habitat sc ore variables. Discriminant analysis with these habitat dimensions indicate d that the snail morphs differed in their habitat distribution. L. ovata pr eferred larger, more permanent natural habitats surrounded by forests, whil e L. peregra was found more often at a higher altitude, in nonpermanent hab itats, often surrounded by meadows. The snails were parasitized by four cer carial types of castrating trematodes. The morphs had a similar prevalence of infection by each of the parasite types, with one exception: monostomid cercariae were found at a higher prevalence in L. ovata than in L. peregra. However, monostomes were rare parasites, and the difference in prevalence of infection was not significant when only populations with monostomes were compared. Our results indicate that variation in the overall prevalence of infection seems to be independent of snail morph, and do not support the i dea that a difference in the rate of parasitism might explain differences i n the habitat distribution of these snail morphs.