Allocreadiid flukes often parasitize and castrate sphaeriid freshwater clams. The effects of parasitic castrators on host population life history can lead to earlier reproductive onset, changes in body size, or changes in the number of reproductive events per year. However, little is known about whether species- or clade-level traits are associated with parasite prevalence. In this study, we examine if phenotypic effects, e.g., size change, seen at the population level, sort to the species or clade level. To answer this question, we determined if different-sized host species in the ecologically important, widespread, and common sphaeriid subgenus Cyclocalyx have different prevalences of allocreadiid infection. After showing that large species are more than 12 times more likely to have patent infections than small species, we examined the evolution of size within Cyclocalyx lineages. Transitions from large size to small size or vice versa have occurred at least twice and probably many times during lineage diversification, perhaps relating to trade-offs between clutch size and parasite risk. Finally, based on existing data, we show that it is unlikely that age and body size are correlated; therefore, a simple age-exposure model is not a sufficient explanation for parasite prevalence in this clade.

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