Investigating co-occurrences between freshwater mussels and host fish species in Southeastern Oklahoma

Freshwater mussels (Order: Unionida) and fishes are two imperiled taxa that make up an important part of global freshwater biodiversity. Mussels exhibit a complex life cycle that can contribute to their imperilment. Most mussels parasitize a host fish during their larval stage to reach maturity, making host fishes and the mussels that they support coevolved taxa thus mussel diversity and distribution can correspond to that of fishes. The Host-Habitat Continuum Concept (HHCC) seeks to explain mussel distribution by combining life history theory, host fish distributions, and stream ecological gradients. The HHCC predicts increases in fish and mussel species richness, shifts in mussel life history strategy and fish family composition as habitat area, complexity, and stability increase with stream size. Here, we combined observational data on fish and mussel assemblages from seven mussel bed reaches with published mussel and host fish relationships and mussel life history traits to evaluate if HHCC explains mussel and fish distribution. We found weak support for HHCC as fish and mussel species richness, increased with watershed area, but only mussel densities were positively related to watershed. The relationship between fish densities and watershed area was seasonally dependent, showing no relationship in summer when stream discharge was greater and a negative relationship in autumn when stream discharge was lower. Centrarchids (sunfish) and leuciscids (minnows) were the most abundant fishes among sites, equilibrium life history strategies and luring host infestation strategies dominated mussels assemblages, but there was no longitudinal change in proportional abundances of fish families, mussel life history or infestation strategies as expected. There was a high degree of host partitioning among coexisting mussels, but we found no relationship between known host fish occurrences and mussel occurrences. The complex nature of freshwater systems and biological interactions between species likely influenced our ability to detect strong associations. Strong host fish partitioning among mussels at sites distributed along a stream size gradient indicates that management aimed at protecting interactions between fishes and mussels may be valuable for conserving these codependent taxa.