Distribution of unionid freshwater mussels and host fishes in Texas. A study of broad-scale spatial patterns across basins and a strong climate gradient

Unionid freshwater mussels are a threatened fauna, and understanding their distribution is essential to aid and promote conservation efforts. Therefore, we (1) compared patterns of species richness and endemism of Texas mussel and fish species, as mussels depend on fish for their reproduction and dispersal; (2) examined how distribution and community composition of mussels and fishes varies across river basins; and (3) how much variation in mussel community composition could be explained by the distribution of potential host fish, river basin (as a spatial component), ecoregion (as proxy for large-scale environmental differences), and flow variability. Mussel and fish community compositions in rivers differed significantly between river basins with an east-to-west gradient of decreasing species richness following the transition from sub-humid to arid climate. River basin explained 25% of the variation in mussel community composition, and potential host fish presence explained 20%. The total variation explained by both variables was 34%, as part of the variation in host fish presence (11.5%) was spatially structured by differences in river basins. Flow variability explained an additional 14% of the variation in mussel community composition, and ecoregion an additional 9% compared with river basin alone. Locations of significantly higher mussel species richness and/or endemism were present in rivers from all regions of Texas. These locations should be protected, especially as human population continues to expand and urbanize in these regions. A better understanding of mussel–host fish relationships and the impact of flow variation on the distribution of mussels will be needed to inform conservation efforts.