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

Abstract

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.

Latitudinal differences in early growth of largehead hairtail (Trichiurus japonicus) in relation to environmental variables

“…The model that allowed for region‐specific growth patterns was vastly better in terms of AIC than the model assuming a common growth pattern. This indicated that the environmental influences on daily increment widths over 41–110 days were more marked compared to the first 40 days, suggesting more habitat‐specific influences rather than inherent growth influences over 41–110 days and more inherent growth rather than strong environmental effects in the first 40 days (Table 2) (e.g., Scharsack et al., 2007; Wullschleger & Jokela, 1999). Therefore, we focus on the growth at ages 41–110 days and the associated environmental variables on both spatial and temporal scales.…”

“…Because variation in early growth rate is closely related to early life stage mortality, early growth plays an important role in determining recruitment and subsequent adult populations (Cowen & Sponaugle, 2009; Houde, 1987; Lett, Ayata, Huret, & Irisson, 2010; Munday et al., 2009; Takahashi, McCormick, Munday, & Jones, 2012; Watanabe, Ochiai, & Fukamichi, 2014). While early growth variation can be inherent, it can also be influenced by environmental variability, that is, growth plasticity, which can have an important influence on recruitment and eventually population dynamics (e.g., Scharsack, Kalbe, Harrod, & Rauch, 2007; Wullschleger & Jokela, 1999). Previous studies based on long‐term catch data of largehead hairtail in the East China Sea have shown that both fishing and climate change have influenced the largehead hairtail population dynamics (Chen, Wang, Bai, Bai, & Ji, 2004; Wang et al., 2011).…”

Increase in multiple paternity across the reproductive lifespan in a sperm‐storing, hermaphroditic freshwater snail

“…The absence of differences in embryonic performance between singly and multiply sired clutches indicates that genetic benefits associated with more diverse, compatible, complementary, competitive or intrinsically better sperm cells (Bernasconi et al., ; Jennions & Petrie, ; Simmons, ; Zeh & Zeh, ) may only appear after embryos have hatched or not at all. In its natural habitat, R. balthica is frequently exposed to both parasites (Wiehn, Kopp, Rezzonico, Karttunen, & Jokela, ; Wullschleger & Jokela, ) and predators (Ahlgren & Brönmark, ; Brönmark et al., ), and thus selective pressures may be rather heterogeneous in space and time. Testing for genetic benefits of polyandry at later stages of life, ideally by following the fate of singly and multiply sired progenies in the field, would thus be very interesting, although logistically difficult for obvious reasons.…”

Range limits and parasite prevalence in a freshwater snail

“…Prevalence was significantly higher close to the range limit than in more central parts of its UK geographical range. Prevalence was not related to habitat type, suggesting that a habitat-specific risk of parasitism did not contribute to the observed pattern (see also Wullschleger & Jokela 1999). Prevalence data were based on determination of mature cercarial infections and hence do not include snails that are infected, but not yet shedding cercariae.…”