We present a revised list of freshwater mussels (order Unionida, families Margaritiferidae and Unionidae) of the United States and Canada, incorporating changes in nomenclature and systematic taxonomy since publication of the most recent checklist in 1998. We recognize a total of 298 species in 55 genera in the families Margaritiferidae (one genus, five species) and Unionidae (54 genera, 293 species). We propose one change in the Margaritiferidae: the placement of the formerly monotypic genus Cumberlandia in the synonymy of Margaritifera. In the Unionidae, we recognize three new genera, elevate four genera from synonymy, and place three previously recognized genera in synonymy. We recognize for the first time two species (one native and one nonindigenous) in the Asian genus Sinanodonta as occurring in North America. We recognize four new species and one subspecies and elevate 21 species from synonymy. We elevate 10 subspecies to species status and no longer recognize four subspecies. We change common names for five taxa, correct spelling for eight species, and correct the date of publication of original descriptions for four species.

We determined the number of samples necessary to accurately estimate species richness at three sites in the Choctawhatchee River watershed in Alabama and Florida. We sampled each site eight times using 5 person-hr timed searches with a combination of visual and tactile searching from June to October 2012. We estimated total species richness at each site using the Chao 2 estimator to construct rarefaction curves. We used these relationships to determine sampling effort necessary to detect 80%, 90%, 95%, and 99% of the estimated total species richness and the percentage of species detected with successive samples. We conducted the same analyses using a subset of the data including only federally threatened or endangered (TE) species. Species detection and effort requirements differed among streams and were primarily influenced by mussel abundance. We detected 62–88% of estimated total species richness with one sample, and detection of 90–99% of species required 2.1–8.0 samples. At two sites with high mussel abundance, detection of ≥90% of estimated total species richness required 1.3– 2.2 samples, but five samples were required to detect a similar percentage of species at a site with lower mussel abundance. A single sample was sufficient to detect all TE species present at two sites where these species were abundant, but a single sample in a stream with lower mussel abundance detected only 45% of TE species, and eight samples were required to detect 90% of TE species.

We evaluated the suitability of three cyprinid fishes previously proposed as hosts for the state threatened Texas Pigtoe (Fusconaia askewi). We collected naturally infested fishes from the wild, held them in captivity until glochidial development and juvenile excystment occurred, and identified a subsample of juveniles to species using the mitochondrial gene ND1. The Red Shiner (Cyprinella lutrensis), Blacktail Shiner (Cyprinella venusta), and Bullhead Minnow (Pimephales vigilax) all carried glochidial infestations from May to August. Red Shiners and Blacktail Shiners produced large numbers of juvenile mussels (metamorphosis success= 29.4% and 46.3%, respectively), and all sequenced individuals (N = 15) were identified as F. askewi, confirming that these species serve as hosts in the wild. Bullhead Minnows carried the highest glochidial infestation but produced only two juveniles (metamorphosis success = 0.3%), neither of which could be positively identified to species.

The recent declines in eastern North American species of freshwater mussels have been well documented, but the status of western species has been comparatively understudied. However, various local and regional studies and anecdotal observations indicate that western mussels are also declining, suggesting the need for range-wide assessments of extinction risk and changes in freshwater mussel distributions. Using historic (pre-1990) and recent (1990–2015) occurrence data from across western states and incorporating observations of recent population dynamics, we assessed the extinction risk of western freshwater mussels according to the categories and criteria of the International Union for Conservation of Nature (IUCN) Red List. Percent change in occupied watersheds (by area) between historic and recent time periods was evaluated against IUCN-established thresholds. Additionally, we considered whether evidence of declines was also supported by reported observations of changes in abundance or occurrence in studied water bodies, watersheds, or regions. We also assessed the proportion of watersheds that have reduced species richness as compared with historic levels. We evaluated four western freshwater mussel taxonomic entities: three currently recognized species and one clade consisting of two currently recognized species. Of the four entities assessed, two are Vulnerable (Anodonta nuttalliana and Gonidea angulata), one is Near Threatened (Margaritifera falcata), and one is Least Concern (Anodonta oregonensis/kennerlyi clade). Freshwater mussel richness declined 35% across western watersheds by area, and among the most historically diverse watersheds, nearly half now support fewer species/clades. Future research and conservation efforts should prioritize identifying the proximate causes for these declines and preserving existing habitat and populations.

Translocation of freshwater mussels is a conservation tool used to reintroduce extirpated populations or augment small populations. Few studies have evaluated the effectiveness of translocations, mainly because estimating survival is challenging and time-consuming. We used a mark-recapture approach to estimate survival of nearly 4,000 individually marked Clubshell (Pleurobema clava) and Northern Riffleshell (Epioblasma rangiana) translocated to eight sites over a five-year period into the Salt Fork and Middle Fork Vermilion rivers in central Illinois. Survival differed among sites and between species; Clubshell were approximately five times more likely to survive than Northern Riffleshell. Survival also increased in the fourth year following a release and decreased following high-flow events. Translocating numerous individuals into multiple sites over a period of years could spread the risk of catastrophic high-flow events and maximize the likelihood for establishing self-sustaining populations.

Historically, little thought was given to the value of freshwater mussels when making decisions that affected these animals and their habitats, even though these values may be considerable, and may be greatly changed by environmental alterations. Here, I review several kinds of values provided by freshwater mussels. Direct-use (market) values of mussels were substantial when the mussels were harvested to provide buttons and pearls, amounting to about $10 billion (2017 dollars) in the USA alone. Current harvests are much smaller but still valuable. Mussels also provide indirect-use value through the ecosystem functions that they provide (water clarification, nutrient cycling, pathogen suppression, etc.). The monetary value of these functions may be substantial, but has not yet been estimated. As interesting, rare creatures, freshwater mussels may also have existence value to society. This value probably is small at present, but could be increased greatly through outreach and education, as could their option and bequest values (the value of saving them for the future). The total value of a freshwater mussel community would be the sum of direct use, indirect use, existence, option, and bequest values, and has not yet been estimated for any real mussel community. Alternatively, one could calculate the replacement value of freshwater mussels (the cost of replacing a mussel community that was damaged or destroyed); procedures for estimating replacement costs have been published. Despite uncertainty about the precise value of freshwater mussels, it is clear that they have substantial value to humans, possibly many millions of dollars in individual ecosystems, which should be taken into account in environmental decision making. Mussel ecologists and biologists can play important roles in helping society better value freshwater mussels.

Despite the increasing use of passive integrated transponder (PIT) tags in freshwater mussel research and conservation, there has been no evaluation of the trade-offs in cost and effort between commonly used adhesive types. These factors could be important to consider if tag retention rates do not vary by adhesive, the effects of handling are large, or resources are limited. We modeled and evaluated how material costs and effort function over a range of sample sizes by using field data from the relocation of 3,749 PIT-tagged Clubshell (Pleurobema clava) and Northern Riffleshell (Epioblasma rangiana) in Illinois, 261 Eastern Elliptio (Elliptio complanata) in Maryland, and the release of 99 Cumberland Combshell (Epioblasma brevidens) in Virginia. Each study used externally affixed 12.5-mm, 134.2-kHz PIT tags, but used a different adhesive to encapsulate tags (Illinois, underwater epoxy resin; Maryland, surface-insensitive gel cyanoacrylate; and Virginia, dental cement). We determined the total cost-per-tag-effort (CPTE) after parameterizing cost, quantity required, application time, and time for each adhesive. After accounting for standardized costs of staff time and adhesive, cyanoacrylate was the least costly adhesive to affix, encapsulate, and cure PIT tags on a per mussel basis. Differences in CPTE were small when the number of mussels tagged was low, but they increased by US$2–6 mussel^-1. A primary goal in mussel projects is reduced stress from aerial exposure. Using underwater epoxy, which requires time above water to cure, can negate this goal and increase costs as it requires more handling effort than cyanoacrylate or dental cement. Nevertheless, more resourceintensive adhesives may still be an appropriate choice when the number of study animals is low. Further study is warranted to understand how our model may vary by adhesive brand, application rate, staffing level, and environmental factors.