Freshwater Mussel Anodonta cygnea Reduces Concentration, Germination Rate and Infectivity of Pathogenic Oomycete Saprolegnia spp. Spores

Saprolegniosis, also known as water mould, is a lethal disease caused by Saprolegnia oomycetes that threatens freshwater animals including fish, amphibians, and crustaceans, as well as aquaculture. It causes biodiversity loss and population declines, including those of endangered fishes. Controlling saprolegniosis in aquaculture is based on chemicals, which are often expensive, ineffective and hazardous to the environment. Filter‐feeding bivalves (Mollusca) remove pathogens, along with other particles, from the water. Therefore, testing of the capacity of freshwater mussels (Unionida) in biofiltering Saprolegnia spp. is important to find out if the mussels could provide an environmentally friendly and sustainable solution to mitigate the problems caused by Saprolegnia . We tested (1) the capacity of swan mussels ( Anodonta cygnea ) to filter particles (spores and hyphal fragments) of Saprolegnia ferax and S. diclina , and (2) the effect of mussel biofiltration on the concentration of infective spores and the germination rate of S. diclina spores, and (3) the effectiveness of mussel biofiltration to prevent saprolegniosis in fish eggs. Mussels effectively filtered Saprolegnia particles from the water: particle concentrations decreased in mussel aquaria compared to controls within 1 h of start and the difference remained until the end of the 24 h experiment. The number of colonies of S. diclina on oomycete‐optimized culture plates was lower in samples originating from the water and sediment of mussel aquaria than in samples from control aquaria. Furthermore, mussel biofiltration reduced the germination rate of S. diclina spores in both water and sediment compared to controls. Additionally, we found that the proportion of fish eggs infected with saprolegniosis was much lower when the eggs were exposed to mussel aquaria water compared to control aquaria water. Our results show that freshwater mussels can reduce the concentration of infective Saprolegnia particles and the infectivity of remaining particles, and protect fish eggs from contracting saprolegniosis, thus highlighting the effect of valuable ecosystem service ‘pathogen removal’ provided by the mussels. Our findings encourage deployment of freshwater mussel biofiltration as a nature‐based solution to mitigate the problems caused by Saprolegnia in freshwater ecosystems, aquaculture and fish conservation.