Rapid Clearance of Tetracapsuloides bryosalmonae Spores by Freshwater Sponge Ephydatia muelleri: Potential Implications for Controlling Proliferative Kidney Disease in Salmonids.

Host-parasite interactions are influenced by both abiotic and biotic factors. While abiotic drivers, particularly temperature, have received considerable attention in recent years due to global climate change, the role of biotic factors remains comparatively underexplored. The malacosporean parasite Tetracapsuloides bryosalmonae (Tb), which causes proliferative kidney disease in salmonids, is a well-documented example, where elevated temperature and eutrophication have been shown to intensify disease prevalence and severity. In this study, we investigated whether freshwater sponges, known for their particle-filtering capabilities, can graze on malacosporean spores in the aquatic environment, potentially acting as biotic filters that may influence parasite transmission dynamics. Using an experimental setup, we employed environmental DNA sampling to (1) characterise the short-term release of Tb spores from infected 1+ brown trout (Salmo trutta) and (2) assess the ability of freshwater sponge (Ephydatia muelleri) to remove Tb spores from water and function as a bio-sampler. Our results show that sponges significantly reduced Tb DNA concentrations in water. However, their effectiveness as bio-samplers and concentrators of Tb was limited, likely due to the rapid degradation of ingested Tb DNA. These results suggest that freshwater sponges may function as natural biological controllers of Tb by filter feeding on its infectious planktonic stages in natural environments.