Biogeographical patterns of marine larval trematode parasites in two intermediate snail hosts in Europe

Aim We used published inventories of trematodes in Littorina littorea (L.) and Hydrobia ulvae (Pennant) in European seas to search for two basic biogeographical patterns in the spatial occurrence of various trematode species: (1) do parasite distribution and richness patterns in the two host snails overlap with known ecoregions of free‐living organisms; and (2) does trematode species richness in the snails follow latitudinal or longitudinal gradients? Location North East Atlantic. Methods We used multidimensional scaling (MDS), analysis of similarity (ANOSIM) and analysis of variance (ANOVA) to test whether there were overlaps of parasite distribution and richness with known ecoregions of free‐living organisms. In addition, we used linear regression analyses to test whether trematode richness in snails (corrected for sampling effort) was correlated with the latitude or longitude of the sampling sites. Results When corrected for sampling effort, mean trematode species richness per site did not differ among the different ecoregions in L. littorea. In contrast, in H. ulvae, mean species richness was much lower for sites from the Celtic Sea compared with sites from the Baltic Sea and the North Sea. Based on the results of MDS analyses, trematode species composition was distinct among ecoregions; in particular, communities from the Baltic Sea differed markedly from communities in the Celtic Sea, for both snail species. Latitude and longitude were not significantly correlated with parasite species richness in either snail species. Most trematode species had restricted distributions, and only three species in L. littorea and five species in H. ulvae occurred at more than 50% of the sites. Main conclusions There is more structure in the large‐scale distribution of trematodes in gastropods than one would expect from the large‐scale dispersal capabilities of their bird and fish final hosts. We propose mechanisms based both on limited dispersal via fish and bird final hosts and on gradients in environmental factors to explain the observed patterns.