Large-scale determinants of trematode infections in intertidal gastropods

Abstract

The influence of latitude, substrate type, tidal level, and host shell size on trematode species-richness and prevalence were assessed using published data on 255 samples from 54 species of marine intertidal gastropods. The analyses were performed at different levels of statistical inde- pendence: among all samples, across snail species, and using phylogenetically independent con- trasts. Repeatability analysis showed that both trematode species richness and prevalence could be considered as snail species traits. Although there was much within-species variation among values obtained from different samples, the among-species variation was much higher. After removing the confounding effect of sample size, positive relationships between trematode prevalence and species richness were observed at all levels of analysis, highlighting the additive nature of trematode infec- tions in snails. The latitude at which samples were collected had a significant effect on trematode species richness across all samples, but this influence disappeared at higher levels of analyses. There was no evidence that host shell size (as well as life-span and population density) was associated with either trematode species richness or prevalence. Tidal level and substrate type (rocky versus soft- sediment) where samples were collected had significant effects on trematode species richness across all samples. These effects could not be verified at higher levels of analyses. Overall, the results point toward an important role of local, small-scale factors in determining the number of trematode species infecting snails and the proportion of infected snails in a population.