Evolutionary distinctiveness and parasite specificity determine infection of non-native fish.

While parasites frequently cross the host species barrier and increase their host range, the mechanisms underlying successful novel cross-species transmission remain poorly understood. Host range expansion of complex life-cycle parasites is especially challenging to assess because they use different host species for development and reproduction. Addressing this knowledge gap, we use a large, published dataset on fish-helminth interactions, which includes introduced, non-native fish species, providing an opportunity to examine parasite infection of novel species in the context of a parasite's known phylogenetic host range and the evolutionary distance between non-native fish and native hosts. We found that larval and adult-stage parasites are influenced by distinct factors in determining their host range expansion. Larval parasites with broad phylogenetic host ranges exhibit higher infection success in novel hosts than specialists. In contrast, both specialist and generalist adults can successfully infect new host species. Notably, specialists achieve the highest infection success when novel hosts are closely related to their typical hosts. Our findings provide evidence of the trade-offs associated with specialism and generalism in parasites and enhance our understanding of the ecological and evolutionary constraints shaping host range expansion. These insights are critical for predicting future host-parasite assemblies in the context of global change.