Abundance of five sympatric stream dwelling mussels varies with physical habitat
Freshwater mussel species regularly co-occur in streams forming assemblages, but the extent of shared versus unique instream habitat features that contribute to their distribution and abundance is poorly understood. In Massachusetts, a rare species, Alasmidonta varicosa , is often found with four other species: Alasmidonta undulata , Strophitus undulatus , Margaritifera margaritifera , and Elliptio complanata , yet variation in species composition within assemblages raises questions of potential species-specific habitat associations. Identifying species-level habitat information is critical at a spatial scale that malacologists can use to identify translocation or restoration areas. This study investigated whether species abundance varied by mesohabitat type (riffle, run, dam pool, scour pool), instream habitat characteristics, and within-reach location (centre versus edge). From 2016 to 2019, freshwater mussel surveys were conducted in nine streams across Massachusetts and associated habitat information was collected. Species abundances were similar across mesohabitat types. Elliptio complanata was the exception, whereby higher abundances occurred in runs and dammed pools than in riffles. Unique species relationships with habitat existed for M. margaritifera with macroalgae and emergent vegetation, and A. varicosa with heterogeneous substrate. Flow transitions, such as depositional areas that create heterogeneous substrates, may provide habitats for A. varicosa . Most mussel species were distributed with higher abundance in the river centre than the edge; E. complanata was the only species with a higher abundance at the river edge. Locations with high abundance varied based on unique relationships with pebble heterogeneity ( A. varicosa ), depth ( A. undulata ), large wood ( A. undulata ), and canopy closure ( E. complanata ). Including physical characteristics in a holistic assessment of habitat that incorporates fish and landscape attributes may further an understanding of river reaches that best support translocated and propagated freshwater mussels.