Trematode prevalence−occupancy relationships on regional and continental spatial scales in marine gastropod hosts

The positive inter-specific relationship between local abundance and large-scale spatial occupancy is one of the most universal patterns in the distribution of species. However, evidence for the validity of this relationship in the marine realm is still scarce, especially for parasites. Using data from published studies, we investigated this relationship in trematode parasites infecting several marine gastropod species. On a regional spatial scale (<100 km between any pair of sites), we found a positive relationship between mean local prevalence (percentage of infected individuals in a population) and large-scale site occupancy among trematode species in all 4 gastropod host species investigated (Littorina obtusata, L. saxatilis, Hydrobia ventrosa, Ilyanassa obsoleta), although this was not significant in the case of L. saxatilis. Similar positive relationships were observed on a continental scale (>1000 km between the most distant sites) in 2 host species (L. littorea, H. ulvae). Further analyses pointed to the role of dispersal by the definitive hosts in shaping these prevalence−occupancy relationships as we found a significant interaction between definitive host type and mean local prevalence affecting the spatial occupancy of the trematodes infecting H. ulvae. While trematode species that use highly dispersive birds as definitive hosts exhibited a significant positive relationship, the ones that use less dispersive fish did not. Our results indicate that a positive relationship between local abundance and large-scale distribution also holds true for marine parasites, and they suggest a strong role of definitive host dispersal in linking local epidemiological infection patterns of parasites with their large-scale biogeographic distributions.