Spatiotemporal variability in flow determines the physical structures of habitat. During low flows, aquatic organisms can be exposed to reduced dissolved oxygen concentrations, increased water temperature, and desiccation, whereas at high flows, increased velocity and hydraulic forces on the streambed can be equally detrimental. These constraints create a mosaic of habitat that influences the distribution and abundance of aquatic biota. This mosaic can change due to stochastic events or those mediated by humans. Understanding how low and high flow conditions affect aquatic organisms is critical not only for advancing ecological knowledge but also for protecting imperiled aquatic species such as unionid mussels. The overall goal of this project was to examine how substrate and hydrologic conditions affect mussel habitat and to then use the resulting information combined with life‐history traits and shell morphology (i.e., sculpturing) to better understand how flow shapes mussel assemblage structure. Using quantile regression, we found that low values of relative shear stress (RSS), a measure of substrate stability, were associated with high mussel species richness and density. Change point analysis using threshold indicator taxa analysis (TITAN) indicated species‐specific preferences for varying levels of bed stability. These preferences were best explained by life‐history strategy and shell morphology based on the results of a principal component analysis. Using these results, we then present a conceptual model from which to derive expectations concerning taxonomic composition, life‐history strategy, and shell sculpture type based on the degree of substrate mobility using RSS and variability in RSS.

Understanding the temperature tolerances of organisms is critical because the thermal regimes of freshwater ecosystems are changing globally. Native freshwater mussels are sensitive to increasing water temperatures because of their physiology and unique life history. Detailed knowledge on lethal temperatures for mussels has been limited to less than 5% of the species known to occur in North America, and little is known about the thermal tolerances of mussel species from rivers within the south‐western USA. To determine the effects of elevated water temperature on mussels, the upper thermal tolerances of larvae (glochidia) for the following species across four basins in Texas (Neches, Guadalupe, San Antonio, and Colorado) were tested: Amblema plicata, Cyclonaias necki, Fusconaia mitchelli, Lampsilis bracteata, Lampsilis hydiana, Lampsilis satura, Lampsilis teres, and Obovaria arkansasensis. Glochidia were acclimated to 27°C across a range of experimental temperatures (30–39°C) in 24‐h standard acute laboratory tests. The median lethal temperature (LT50) among glochidia averaged 32.4°C and ranged from 26.9 to 36.4°C. Thermal tolerances differed significantly among and within species, and by season. Comparing these results with current water temperatures in central and east Texas indicated that populations of the focal species studied are at risk from rising environmental temperatures and, as a consequence, their long‐term viability will be challenging in future years.

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1. Freshwater ecosystems are experiencing shifts in the natural range and variation of water temperatures due to anthropogenic activity, and these shifts can negatively affect survival, growth, and reproduction of aquatic species. Among the groups most affected are freshwater mussels of the family Unionidae.