Ecological stoichiometry has been proposed as a mechanistic framework that can facilitate the integration of evolutionary biology with ecosystem science. By condensing organisms and their environment into elemental components (e.g., carbon, nitrogen, and phosphorus), ecological stoichiometry provides a powerful framework to make predictions about how changes at one level affects the other, such as how elemental availability in resources can shape consumer phenotypes or how organismal composition can alter nutrient recycling. Organismal stoichiometry can therefore act as a proxy for elemental demand of the organism and can help identify potential agents of selection through the characterization of elemental mismatches between consumers and their resources. Here we characterized interspecific and intraspecific variation in organismal body tissue stoichiometry within a family of freshwater bivalve mussels (Unionidae) across seven river systems and 21 sites. We found that interspecific variation in species life-history and body size traits were correlated to body tissue stoichiometry. Specifically, average growth rates were correlated with tissue percent phosphorus content. We also observed intraspecific variation in body stoichiometry among sites and in that genetic distances among sites (Fst) were correlated to stoichiometric distances. Our results shed light on some of the characteristic determinants of body stoichiometry. We propose that future research should better assess the mechanistic underpinnings of the complex relationships between life history characteristics, phenotypic variation, and stoichiometric requirements of organisms so can better predict responses to ongoing changes in nutrient supplies and further isolation and reduced gene flow due to changes in aquatic systems.