The importance of microhabitat factors and habitat stability to the threatened Louisiana pearl shell, Margaritifera hembeli (Conrad)

Margaritifera hembeli, the Louisiana pearl shell, is a threatened mussel with a distribution limited to the headwaters of three tributaries of the Red River in central Louisiana, U.S.A. We assessed the role that several habitat characters played in determining its abundance and distribution. Pearl shell mussels were more common in second-order streams with elevated conductivity (approximately 0.04 mS/cm) and water hardness (8 mg/L). A discriminant analysis indicated that mussel density was related to water depth, substrate size, substrate compaction, and water velocity. Mussels were rare in deep, stagnant pools with silt-covered bottoms, and were more common in shallow, wide areas of streams with higher current velocities and in sediments with larger particle sizes. Mussel beds were also more likely to occur in sections of the stream where the substrate was more stable through time. These habitat associations may occur because individuals that recruit into, or later select, more stable microhabitats, have an advantage owing to the relatively long life cycle of this mussel. We suggest that the measurement of microhabitat characteristics can be important when evaluating habitat preferences and management plans for endangered mussel species in headwater streams.