A “parasite-tag” approach reveals long-distance dispersal of the riverine mussel Margaritifera laevis by its host fish

Abstract

Long-distance dispersal of freshwater mussels (order Unionoida) has been assumed to occur mainly during a parasitic larval stage (glochidia) via movements of host fishes, but its empirical evidence is largely lacking. Here, we applied a “parasite-tag” approach to the riverine mussel Margaritifera laevis and its obligate host fish Oncorhynchus masou masou. This method examines the relationship between the prevalence of glochidia and distance from the nearest mussel population (i.e., the putative source of glochidia), a proxy that should quantify distance moved by host fish. We hypothesized that infected fish would be found in wider habitats at the end of the parasitic period (August) than at the beginning (July) if they were functioning as effective dispersal agents. In July, the prevalence of glochidia was highest in the vicinity of mussel beds but decreased rapidly with distance from the nearest mussel bed. In August, however, infected fish were distributed diffusively across the riverine network and dispersed over 4.8 km, demonstrating substantial dispersal of glochidia by the host fish. The results of our study build upon current knowledge of mussel’s dispersal ecology by providing highly needed evidence: host fish can be effective in mediating long-distance dispersal of a riverine mussel species.