Endosymbionts: An overlooked threat in the conservation of freshwater mussels?

Endosymbionts can often have profound impacts on the growth, reproduction and survivorship of their hosts. Freshwater unionid mussels (Unionida) are important ecosystem engineers, and one of the most globally imperilled taxa, yet evidence concerning their endosymbiotic fauna remains patchy. Further, endosymbionts are not considered in an IUCN assessment for any unionid mussel. Here, we conduct the first literature review of all endosymbionts of the 16 extant European and 279 extant North American unionids, in addition to the four most significant invasive bivalves in Europe. There were 1476 host-endosymbiont records from 239 different studies over a 168-year period, documenting at least 188 unique endosymbiont taxa. However, study effort was uneven in its distribution, with 53% of unionid species (n = 157) having no endosymbiont records. Eighty-eight percent of all hosts are considered under-sampled, including 99% of Endangered or Critically Endangered mussels. This is of significant concern given that when the effects of endosymbionts were examined, 72% showed potentially negative effects on their host, including complete castration in the case of digenean trematodes. However, only a small number of endosymbionts have had their effects quantified. Bipartite network analyses revealed invasive mussels may be competent for native parasites. This leads to the potential for parasite spillback, with conservation implications for vulnerable native species. Recommendations for future work include greater sampling of sympatric native and invasive populations (including non-destructive sampling of endangered species) and experimental manipulation of host-endosymbiont communities. This will facilitate better conservation outcomes for this crucial group of ecosystem engineers.