Trait-Based Drivers of Host Specificity in Freshwater Mussel-Fish Parasitic Interactions.

Species interactions, particularly parasitism, are fundamental drivers of biodiversity and ecosystem function. Life history strategies, defined by differences in reproductive output, growth, and lifespan, play a key role in shaping host-parasite relationships. Freshwater mussels (Family: Unionidae) are a diverse and highly imperiled group that rely on an obligate ectoparasitic larval stage on host fishes. Despite this dependence, they are often excluded from general parasitology frameworks and differ from typical parasites due to their long and variable lifespans. In this study, we integrated publicly available data on mussel and fish phylogenies, life history traits, and host-parasite associations to evaluate patterns of host phylogenetic specificity in North American mussels. We tested whether mussel life history strategies predict differences in host phylogenetic breadth. Our results reveal that host phylogenetic specificity is phylogenetically conserved but variable among mussel species. Mussel life history traits were related to host use: equilibrium strategists (long-lived, low fecundity) disproportionately parasitize short-lived, opportunistic fishes; opportunistic mussels (short-lived, highly fecund) favor longer-lived, equilibrium hosts. Periodic strategists show intermediate traits and more variable patterns. These patterns suggest underlying ecological and evolutionary trade-offs in parasitic strategies, offering new insights into host-parasite dynamics and providing valuable information for conserving mussel-fish interactions and ecosystem function.