Controls of Spatial Variation in the Prevalence of Trematode Parasites Infecting a Marine Snail

Abstract

Geographic variability in abundance can be driven by multiple physical and biological factors operating at multiple scales. To understand the determinants of larval trematode prevalence within populations of the marine snail host Littorina littorea, we quantified many physical and biological variables at 28 New England intertidal sites. A hierarchical, mixed-effects model identified the abundance of gulls (the final hosts and dispersive agents of infective trematode stages) and snail size (a proxy for time of exposure) as the primary factors associated with trematode prevalence. The predominant influence of these variables coupled with routinely low infection rates (21 of the 28 populations exhibited prevalence <12%) suggest broad-scale recruitment limitation of trematodes. Although infection rates were spatially variable, formal analyses detected no regional spatial gradients in either trematode prevalence or independent environmental variables. Trematode prevalence appears to be predominantly determined by local site characteristics favoring high gull abundance.

Can the studies at a spatial scale of 100s meters detect the spatiotemporal fluctuations of a parasite assemblage?

“…In summary, the results obtained during this study revealed that the fluctuations in the community of larval digeneans of the snail H. australis can be detected at small spatial scale, using its natural distribution of 500 m. Given that local factors play an important role in determining the richness and abundance of larval digeneans in their snail host (Poulin and Mouritsen 2003;Byers et al 2008), therefore, differences in the spatio-temporal distribution of the community of larval digeneans in H. australis should not be attributed to the effect of only a limited number of factors. This study also highlights the importance of seasonality as a factor that must be considered in studies focused on the search for patterns structuring the communities of larval digeneans, at medium and large scales.…”

“…In the latest years, parasitological studies regarding different aspects of the host-parasite relationships have placed special emphasis on the spatial scale at which those relationships are examined (Poulin and Mouritsen 2003;Byers et al 2008). In general, infection processes are proximately driven by local factors (environmental conditions, resources or biotic interactions) which ultimately may be correlated over large regional scale (Byers et al 2008).…”

“…In general, infection processes are proximately driven by local factors (environmental conditions, resources or biotic interactions) which ultimately may be correlated over large regional scale (Byers et al 2008). For this reason, studies conducted at large, nested spatial scales are considered as particularly informative (Byers et al 2008).…”

“…In general, infection processes are proximately driven by local factors (environmental conditions, resources or biotic interactions) which ultimately may be correlated over large regional scale (Byers et al 2008). For this reason, studies conducted at large, nested spatial scales are considered as particularly informative (Byers et al 2008). Nevertheless, in the case of the larval digeneans parasitizing snail hosts, Poulin and Mouritsen (2003) stressed the importance of small-scale heterogeneity in local factors which overrides that of larger-scales factors, suggesting that local factors play a major role in determining the richness and prevalence of larval digeneans in intertidal gastropods.…”

Distribution of parasites in their second ­intermediate host, the cockle Cerastoderma edule: community heterogeneity and spatial scale

“…The density of cockles and/or their shell length could contribute to spatial infection heterogeneity (Thieltges & Reise 2006, 2007, but in the present study dissected cockles displayed similar lengths and abundance everywhere. Among possible drivers of trematode distribution, the presence of birds (= definitive hosts) should be explored (Hechinger & Lafferty 2005, Byers et al 2008. Characteristics of the water body could also interfere with parasite transmission by miracidium or cercariae larvae and may explain heterogeneity at this scale.…”