Microhabitat Use by an Assemblage of Stream-Dwelling Unionaceans (Bivalvia), including Two Rare Species of Alasmidonta

We studied the microhabitats of six species of freshwater mussels, including two rare species of Alasmidonta, in the Neversink River, New York. In each of 270, 1-m2 quadrats, we measured water depth, current speed, bottom roughness, spatial variation in current speed, distance to shore, presence or absence of macrophytes, presence or absence of an overhead canopy, the extent of patches of fine sediment, and sediment granulometry, as well as recording the mussels present. Mussel populations in the Neversink are dense (mean=3.2/m2) and highly clumped. Stepwise discriminant analyses showed that current speed and spatial variation in current speed were the most useful predictors of the occurrence of mussels in quadrats. Alasmidonta heterodon was found most frequently at moderate current speeds and in quadrats that contained many patches of fine sediments. Alasmidonta varicosa occurred most frequently at moderate current speeds and in sediments with a high proportion of medium sands (0.25-1 mm). Nevertheless, the predictive power of discriminant models based on microhabitat variables is so low that we question the adequacy of a traditional microhabitat approach to unionacean ecology. We suggest that including geomorphological descriptors of the streambed or working at spatial scales of hundreds of metres might be more useful than a traditional microhabitat approach for predicting the distribution of freshwater mussels in streams.