Temporal Dynamics of Biological Invasions: Perception of Host Quality Differs Between Native and Alien Host Species.

The spread of non-native species into new regions is a dynamic process driven by behavioural adaptations to local environments and species interactions. Interactions between coexisting populations can lead to localised coevolutionary patterns, shaped by the duration of their co-occurrence. We investigated the relationship between the European bitterling fish (Rhodeus amarus), a parasitic fish that lays eggs in the gills of unionid mussels, and an invasive mussel host, Sinanodonta woodiana, which has spread across Europe over the past 50 years. The bitterling, a host generalist, can parasitise any European unionid mussel species, but its reproductive success with S. woodiana is limited due to the mussel's ability to reject bitterling eggs. We tested three hypotheses on the role of rapid local coevolution in host choice using S. woodiana populations with recent (5 years), intermediate (17 years), and long-term (40+ years) associations with European bitterling. We experimentally evaluated the spawning preferences of four bitterling populations: three coexisting with their respective S. woodiana populations and one naïve to S. woodiana. All bitterling populations avoided S. woodiana from the most recently established population. Neither local coexistence nor individual mussel quality influenced bitterling responses to S. woodiana. In contrast, bitterling selected native Anodonta anatina mussels based on individual quality rather than population identity. These findings suggest that native species can recognise and avoid an invasive host at the invasion front, where co-occurrence is recent.