Dam removal is a common conservation tool that has many potential benefits for freshwater mussels. We conducted qualitative and quantitative mussel surveys in the Mill River system, Massachusetts, where four dams have been removed or modified to benefit aquatic organisms. These data represent a baseline for future monitoring of the effects of dam removal or modification. Mussel assemblages were composed of six species and were dominated by Elliptio complanata; Lampsilis radiata was the second most abundant species. Two species of Special Concern in Massachusetts, Ligumia nasuta and Leptodea ochracea, were rare, as were Pyganodon cataracta and Utterbackiana implicata. We conducted catch-per-unit-effort (CPUE) surveys at 77 sites; mussels occurred throughout much of the watershed except for the lower portion of the Mill River. The highest CPUE values were found immediately downstream of the two lakes in the system. We conducted quadrat-based surveys at nine sites, including one site in each of the lakes. Precision of estimates of total mussel density was ≥80% at most sites, which will allow detection of moderate to large changes over time. Monitoring of changes for rarer species may require a watershed-based approach based on CPUE because quantitative estimates had wide confidence intervals.

Most freshwater mussels (Unionidae) require a specific host fish to advance their life cycle. Currently, hosts are known for only one-third of the mussel species endemic to the United States and Canada. Texas boasts the greatest diversity of freshwater mussels in the southwestern United States. However, information on mussel-host relationships for ∼52 species known to occur within the state is either lacking or incomplete, including two species, Cyclonaias necki (Guadalupe Orb) and Fusconaia mitchelli (False Spike), currently under review for listing under the U.S. Endangered Species Act. To address this deficiency, we conducted laboratory trials that tested 12 fish species (four families and 11 genera) for C. necki and eight species (four families and seven genera) for F. mitchelli. For C. necki, we identified four host species, Ictalurus punctatus (Channel Catfish), Pylodictis olivarus (Flathead Catfish), Noturus gyrinus (Tadpole Madtom), and Ameiurus natalis (Yellow Bullhead). The transformation period was 11 to 22 d for I. punctatus (peak metamorphosis at 15 d), 16 d for P. olivaris and A. natalis, and 10 d for N. gyrinus. For F. mitchelli, we identified two host species, Cyprinella lutrensis (Red Shiner) and Cyprinella venusta (Blacktail Shiner); for both, the transformation period was 18 d. Current information on the status of these six host species within the Guadalupe River suggests that imperilment of C. necki and F. mitchelli may be partly related to the status of their host fishes. Our results also provide critical information for informing recovery activities, such as translocation and captive propagation, if deemed necessary for one or both mussel species.

Only recently have substantial efforts been made to understand phylogenetic relationships among freshwater mussels of the western United States and Canada. Genetic studies show the existence of two divergent clades in western Anodonta, one containing Anodonta californiensis and Anodonta nuttalliana, and another containing Anodonta oregonensis and Anodonta kennerlyi, but relationships within these two clades remain unclear. For example, some authors have placed A. californiensis in the synonymy of A. nuttalliana, but additional taxonomic information is needed to resolve these issues. We examined glochidial shell size and fine structure of these four species to assess the taxonomic utility of these characters. Glochidia of A. oregonensis and A. kennerlyi were similar in size and fine structure, which supports their proposed close relationship. Glochidia of A. californiensis and A. nuttalliana were smaller in all dimensions than A. oregonensis and A. kennerlyi, which supports the existence of two divergent clades. However, shell size and fine structure also differed between A. californiensis and A. nuttalliana, which supports the distinctiveness of these two taxa. Glochidial characters may help to clarify evolutionary relationships among western U.S. Anodonta and other problematic groups.