Allopatric distribution of generalist parasites: interplay between postglacial dispersal and host species

Abstract

Dispersal of freshwater mussels occurs when the larvae parasitize fishes. In northeastern North America, distributions of floater mussels ( Pyganodon  spp.) are allopatric and arranged in an east–west pattern that corresponds poorly with the biogeographic patterns of the regional fish fauna. This study aims at determining whether associations with distinct fish species can explain the distribution of floaters. Here, we devised a framework based on a series of novel distributional null models to elucidate dispersal of species that are dependent on host species. The distributional patterns of floaters were tested by controlling for associations with fishes, while host–parasite associations were assessed with null models based on co-occurrence patterns. The disjoint distribution of the giant floater ( Pyganodon grandis ) and the eastern floater ( Pyganodon cataracta ) could not be explained by their association with distinct host fish and is likely the result of associations with distinct host populations rather than distinct species. In contrast, the distribution of the Newfoundland floater ( Pyganodon fragilis ) could be explained by its association with euryhaline fishes. Such associations may have also promoted differentiation with its sister taxa, the eastern floater. This study demonstrated the effects of positive biological associations on distributional patterns and the utility of a null model framework to uncover them.