Distribution of parasitic larvae of non-native Sinanodonta woodiana on fish hosts across an invasion gradient

The Chinese pond mussel ( Sinanodonta woodiana ) is a highly invasive freshwater bivalve, recognised for its broad host range and ecological plasticity. Its obligate parasitic larvae (glochidia) must attach to fish hosts to complete development, potentially disrupting native host-parasite interactions. We investigated patterns of natural infection by glochidia of S. woodiana and native unionid species across six sites in Poland that differ in habitat type and invasion history. Using whole-body dissections of 601 wild fish belonging to eight species, we quantified glochidia prevalence, intensity of infection and attachment site preferences. Glochidia of S. woodiana were widespread and present on all fish species, with the greatest loads on Carassius gibelio and Rhodeus amarus . In contrast, native mussels (primarily Unio spp.) infected only a subset of hosts and at lower prevalence. Habitat type strongly influenced parasite loads, with higher infection levels observed in lentic compared to lotic habitats. Glochidia of S. woodiana attached both to gills and fins, whereas native unionids predominantly targeted the gills. This spatial disparity may reflect differences in infection strategies or host immune responses. Although previous studies have shown that prior infection by S. woodiana can reduce the success of subsequent native mussel infections, our study found simultaneous infections and no significant association between native and invasive glochidia under natural conditions. Our findings underscore the generalist nature and potential ecological dominance of S. woodiana , while highlighting risks posed to native mussel reproduction. Understanding these interactions is crucial for developing effective conservation strategies for native unionid populations in freshwater ecosystems under pressure from biological invasions.