Invasive oysters as new hosts for native shell-boring polychaetes: Using historical shell collections and recent field data to investigate parasite spillback in native mussels in the Dutch Wadden Sea

Biological invasions can have manifold effects on native biota, including impacts on species interactions in invaded ecosystems. Among those are effects on parasite-host interactions, for example in cases where invaders serve as a new host for native parasites, leading to an amplification of the parasite population which may ultimately result in increased infection levels in the original native hosts (parasite spillback). In this study, we investigated the potential spillback of the native shell-boring polychaete Polydora ciliata from invasive Pacific oysters (Crassostrea (Magallana) gigas) to native mussels (Mytilus edulis) in the Dutch Wadden Sea. A field survey in the intertidal revealed that the majority of P. ciliata can nowadays be found in the shells of live but also dead Pacific oysters and much less in shells of other potential host species: mussels and periwinkles (Littorina littorea). Using a unique historical shell collection based on long-term sampling programmes in the intertidal and subtidal of the western Dutch Wadden Sea, we compared P. ciliata infections in mussels before and after the invasion of the Pacific oyster by means of x-ray scans. Both for the intertidal and subtidal, we did not detect differences in prevalence of P. ciliata in mussels between pre- and post-invasion periods. This suggests that the invasion of the Pacific oyster may not have caused a strong spillback to mussels regarding P. ciliata and thus the invasion probably had little indirect infection-mediated effects on the condition and fitness of native mussels. Instead, the acquisition of native P. ciliata by the invasive oysters suggests that they may themselves be affected by the new infections and this may warrant further research