Invasive Chinese pond mussel Sinanodonta woodiana threatens native mussel reproduction by inducing cross-resistance of host fish

The effects of invasive alien species (IAS) on host-affiliate relationships are often subtle and remain unnoticed or insufficiently quantified. The global decline of freshwater unionid mussel species has been attributed to many causes, but little is known about the interactions of IAS, with their complex life cycle, which includes an obligatory parasitic stage (the glochidium) that develops on fishes. The capacity of a European freshwater mussel, Anodonta anatina, to develop on its widespread fish host, Squalius cephalus was tested experimentally, after previous infestations by the IAS, Sinanodonta (Anodonta) woodiana. The initial attachment of glochidia, the length of the parasitic period, and the metamorphosis success rate of A. anatina glochidia were compared among treatments of different priming infestation intensities. The metamorphosis success rate of the native A. anatina glochidia was strongly reduced (Wilcoxon Signed-Rank Test, P<0.001) and declined by 42.1 and 45.4% on fish hosts that were previously exposed to S. woodiana by single and multiple priming infestations, respectively, in comparison with the control group. Such cross-resistance is expected to decrease significantly the quality of the host resources available to native mussels. This study provides the first evidence of the host-mediated adverse impact of invasive S. woodiana on native mussel species. These results also highlight the importance of potential competition for hosts between threatened groups of affiliate species and their invasive counterparts, which should be reflected in conservation strategies.