Variation in resistance and survival of the European bitterling fish across a gradient of Sinanodonta woodiana expansion.

Biological invasions impose new selective pressures on native species, prompting rapid evolutionary responses. We examined how the parasitic larvae (glochidia) of the invasive East Asian mussel Sinanodonta woodiana affect European bitterling fish (Rhodeus amarus) populations with different histories of exposure to the parasite (none, 15 years, or 40 + years). Glochidia parasitise fish fins and gills, and bitterling are especially vulnerable to parasitism due to their spawning association with mussels. In a common garden experiment, we found the lowest glochidia load in bitterling from populations with the longest association with S. woodiana, indicating evolved resistance. While infection reduced swimming capacity proportionally to parasite load, this effect was not affected by exposure history. However, post-infection mortality was highest in naïve and lowest in long-associated populations. No negative long-term effects on survival, growth, condition, or reproduction were observed, suggesting fitness costs are concentrated shortly after infection. These results show that novel host-parasite interactions during invasions can drive rapid adaptive change, emphasising the need to consider evolutionary dynamics in invasion management.