Does habitat-specific variation in trematode infection risks influence habitat distribution of two closely related...

Parasitism may be an important factor determining the geographic distribution of closely related species. A habitat-specific risk of parasitism may lead to exclusion of susceptible host types from parasite-rich environments, and promote speciation if it leads to reproductive isolation between susceptible and resistant types. We surveyed populations of the freshwater snail Lymnaea peregra for differences in habitat distribution and trematode parasitism between its two distinct shell morphs, L. ovata and L. peregra. We surveyed 58 populations (43 L. ovata, 15 L. peregra). At each location we recorded an array of habitat characteristics that were summarized using a nonlinear principal components analysis. This yielded two orthogonal habitat score variables. Discriminant analysis with these habitat dimensions indicated that the snail morphs differed in their habitat distribution. L. ovata preferred larger, more permanent natural habitats surrounded by forests, while L. peregra was found more often at a higher altitude, in nonpermanent habitats, often surrounded by meadows. The snails were parasitized by four cercarial 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 idea that a difference in the rate of parasitism might explain differences in the habitat distribution of these snail morphs.