2024

Deng, Binglin

The invasion of exotic bivalves can cause large changes in freshwater ecosystems and threaten native freshwater mussels (Unionida). The Enemy Release Hypothesis (ERH) proposes that (i) invasive species may lose enemies during the invasion process, and (ii) the invaders enjoy a competitive advantage caused by a lower enemy pressure than in the native species, contributing to the success of biological invasions. Freshwater mussels are infected by diverse parasite fauna, i.e. protozoans, trematodes, nematodes, watermites, associated with varying degrees of harm to the host. Thus, ERH was tested by using the invasive freshwater bivalves Chinese pond mussel Sinanodonta woodiana, Asian clam Corbicula fluminea and zebra mussel Dreissena polymorpha. First, parasite pressure, measured as parasite taxon richness and sum of prevalences of infection by different parasite taxa, was compared between native and invasive freshwater bivalves living in sympatry in two separate field studies covering a total of 11 European waterbodies inhabiting a total of 6 native Unionidae species. 16 parasite taxa were found, and in the native bivalves the mean site-specific parasite taxon richness was 2.3–3.4 times and the mean sum of prevalences of infection of different parasites was 2.4–2.6 times that of those in the invasive bivalves. Second, the parasite pressure in the invasive bivalves S. woodiana and C. fluminea was compared between populations in the original range (China, 5 waterbodies) and the invaded range (Europe, 11 waterbodies). For S. woodiana, the average site-specific parasite taxon richness in China was 2.1 times and the sum of prevalences of infection was 3.0 times of those in Europe. For C. fluminea, the average site-specific parasite taxon richness was 1.3 and the sum of prevalences of infection was 13.8 in China, while all the studied European C. fluminea populations were free of parasites. These results indicate a reduction in the parasite pressure among invasive species, and thus support ERH.