Associations between substrate and hydraulic variables and the distributions of a sculptured and an unsculptured unionid mussel

Abstract

Freshwater mussels in the family Unionidae are a diverse faunal group in North America. Anthropogenic disturbances have caused many unionid species to be listed as threatened, endangered, or of concern across the USA. Previous studies have shown that substrate stability, which is related to flow and sediment size, is an important predictor of the richness and abundance of species within mussel beds. The ability of unionids to withstand scouring, move into and out of flow refugia, or burrow after dislodgement may influence species habitat selection. The shell morphology of some species, specifically external ornamentation or sculpturing, may be an adaptation to deal with variation in sediment stability and flows in stream habitats. We examined how shell morphology is related to hydraulic variables by surveying 2 common unionid species that have different shell morphologies in 4 geomorphically diverse stream sections of the Sabine River of East Texas. We measured the presence and absence of each mussel species in a variety of quadrats in each stream section and compared the presence and absence of each with quadrat-scale variables associated with stream hydraulics and substrates. Quadrat-scale variables differed among stream sections and were correlated with the presence of the sculptured species. Tritogonia verrucosa was associated with sites with larger substrates and more turbulent flow conditions including higher shear stresses. The unsculptured species, Lampsilis teres, was not associated with any combination of environmental variables, which suggests it is a habitat generalist that can thrive in a variety of habitat types. The differences in habitat preferences of L. teres and T. verrucosa indicate the importance of species-specific models and studies, especially as they relate to the conservation of rare and endangered species.