The reproductive costs of bitterling fish and zebra mussel parasitism to a unionid mussel

Unionid mussels are one of the most vulnerable groups of animals, with multitude of factors responsible for the decline of their populations in the freshwater ecosystems. Many of those causes stem from interspecific relationships. Direct data on the reproductive costs incurred to unionids through parasitism by fish and other mussel species are incomplete. We used field experiments to test whether reproduction of the swollen river mussel ( Unio tumidus ) is affected by parasitism by the European bitterling fish ( Rhodeus amarus ) and zebra mussel ( Dreissena polymorpha ). We documented that bitterling brood can develop in the gills of female, male and hermaphrodite mussels. Our research showed that male mussels carried more fish offspring than female U. tumidus and bitterling embryo survival appeared higher in the outer gill demibranchs. Parasitised female U. tumidus suffered reduced fecundity in terms of a reduced number of mature ovulated oocytes and increased oocyte degeneration. Our data lend clear support to the hypothesis that bitterling fish embryos parasitizing mussel gills and epibiont zebra mussel fouling unionid shells represent a major cost to the reproductive success of their host mussels. Extended gonadal activity (multiple spawning) of U. tumidus spreads their spawning time over a longer period, which may mitigate the costs of parasitism. Impact of bitterling fish parasitism and by zebra mussels fouling on Unio tumidus fecundity. • Reproductive output is a critical measure of the parasite impact on their host. • Oocyte development of Unio tumidus mussel was compromised by parasitism from bitterling fish as well as from zebra mussel. • Parasitised female U. tumidus suffered reduced number of mature ovulated oocytes and increased oocyte degeneration. • Multiple spawning of U. tumidus may mitigate the costs of parasitism.