THE FRESH-WATER MUSSELS (BIVALVIA, UNIONOIDEA) OF THE UPPER DELAWARE RIVER DRAINAGE

We surveyed the unionoidean fauna of 44 sites in the upper Delaware River drainage of New York during 1990. Seven species of unionoideans were found living in the basin, including the endangered Alasmidonta heterodon (Lea, 1829). Four other species are known historically from the upper Delaware basin, but now are either absent from the area or restricted to a few local sites. Neither calcium concentration nor stream size is a good predictor of unionoidean species richness in the study area. We hypothesize that sediment stability could regulate the occurrence of unionoideans in the streams of the upper Delaware drainage.