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 wi th 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 habit at characters played in determining its abundance and distribution. Pearl s hell mussels were more common in second-order streams with elevated conduct ivity (approximate to 0.04 mS/cm) and water hardness (8 mg/L). A discrimina nt analysis indicated that mussel density was related to water depth, subst rate size, substrate compaction, and water velocity. Mussels were rare in d eep, stagnant pools with silt-covered bottoms, and were more common in shal low, wide areas of streams with higher current velocities and in sediments with larger particle sizes. Mussel beds were also more likely to occur in s ections of the stream where the substrate was more stable through time. The se habitat associations may occur because individuals that recruit into, or later select, more stable microhabitats, have an advantage owing to the re latively long life cycle of this mussel. We suggest that the measurement of microhabitat characteristics can be important when evaluating habitat pref erences and management plans for endangered mussel species in headwater str eams.