Rhodeus amarus (Bloch, 1782): A new potential threat for Margaritifera auricularia (Spengler, 1793) (Unionoida, Margaritiferidae)

Rhodeus amarus is a cyprinid fish that spawns in the mantle cavity of freshwater mussels, resulting in a fitness cost to the host mussel. Rhodeus amarus is expanding its geographic range in Europe, hence the exposure of different freshwater mussel species to R. amarus is also rising. This expansion of contact creates potential risk to affected mussels, and especially for mussel species already facing extinction risk. Here, we review the expansion of R. amarus in Europe and the mussel species that are known hosts for R. amarus. We discuss the potential danger of this expansion into new areas that are inhabited by spatially restricted mussel species, and we confirm that the critically endangered Margaritifera auricularia is now a host for R. amarus.