Greater diversification of freshwater than marine parasites of fish

[Display omitted] •Isolated freshwater habitats should promote greater speciation rates than open oceans.•The relative number of species per genus is compared among helminth faunas of fish.•Parasite faunas of freshwater fish show higher species-to-genus ratios than in marine fish.•Isolated populations of freshwater hosts promote higher parasite diversification. The species richness of freshwater environments is disproportionately high compared with that of the oceans, given their respective sizes. If diversification rates are higher in freshwaters because they are isolated and heterogeneous, this should apply to parasites as well. Using 14 large datasets comprising 677 species of freshwater and marine fish, the hypothesis that freshwater parasites experience higher rates of diversification than marine ones is tested by contrasting the relative numbers of species per parasite genus between the regional endohelminth faunas of fish in both environments. The relationship between the number of parasite genera and the number of parasite species per host was well described by a power function, in both environments; although the exponent of this function was slightly lower for freshwater parasite faunas than marine ones, the difference was not significant. However, the ratio between the number of parasite species and the number of parasite genera per host species was significantly higher in freshwater fish than in marine ones. These findings suggest fundamental differences between the way parasite faunas diversify in freshwater versus marine habitats, with the independent evolution of conspecific parasite populations in isolated host populations being a more common phenomenon in freshwater environments.