Declines of freshwater mussel (order Unionida) populations worldwide are attributed to habitat degradation, pollution, and invasive species, among other factors. However, these purported causes do not fully explain the enigmatic decline and large-scale die-offs of mussels that have occurred in apparently healthy streams across a wide geographic region. The roles of the microbiota and pathogens in mussel health have been understudied, and, as a result, few data exist to compare the microbiota of healthy mussels to that of stressed or dying mussels. Captive propagation and stocking programs have expanded across the globe without standard diagnostic protocols to assess health or potential diseases in hatchery-reared or wild stocks. Nonindigenous species, contaminants of emerging concern, and anthropogenic climate change could alter adversely the underlying processes that support mussel health, such as nutritional status and microbial composition, and these factors could increase the risk for outbreaks of opportunistic and emergent mussel disease. We propose a coordinated, collaborative, and multidisciplinary effort to advance methods for assessing freshwater mussel health. We identify research and resources needed to answer central questions surrounding mussel health, including identifying potential agents of disease, defining clinical signs of declining condition, refining stress-specific biomarkers for health assessment, and developing protocols specific for mussels.

Freshwater mussels have disappeared from many U.S. streams since the 1960s. These declines are enigmatic: there are no clear causes and other components of aquatic communities appear unaffected. I review the characteristics, spatial occurrence, timing, and potential causes of enigmatic mussel declines. They share some or all of the following characteristics: (1) fauna-wide collapse, affecting all species; (2) recruitment failure, leading to a senescent fauna; (3) no well-documented impact sufficient to affect all species rapidly; (4) specific to mussels; (5) recent occurrence, since the 1960s; (6) rapid action, often leading to faunal collapse within 10 yr; and (7) upstream progression in some cases. Enigmatic declines are largely restricted to upland regions south of maximum Pleistocene glaciation and north or west of the Gulf and Atlantic coastal plains, and they appear restricted to small- to medium-sized streams. In contrast, mussel declines with different characteristics are reported nationwide. Their consistent characteristics, restricted spatial occurrence, and similar timing suggest that enigmatic declines represent a distinct, diagnosable phenomenon. Many commonly invoked factors are not plausible explanations for enigmatic declines, and others are vague or poorly supported. Other factors are plausible in some cases (e.g., agricultural effects) but cannot explain declines across the affected area. I identified only two factors that could broadly explain enigmatic declines: disease and introduction of Corbicula fluminea, but these factors are poorly understood. The occurrence of enigmatic declines overlies the region with the highest mussel species richness on Earth, but I believe their severity and importance are underappreciated. Streams affected by enigmatic declines are vital research and management opportunities, deserving of increased attention; I propose ways that research can be focused to rigorously evaluate the specific mechanisms for these declines. Until we understand the causes of enigmatic declines, mussel conservation in affected areas is substantially hamstrung.

The freshwater pearl mussel Margaritifera margaritifera is an endangered species in Sweden with more than 600 known populations distributed in 16 out of 21 counties. Only approximately one-third of these populations are considered viable and healthy with signs of recent juvenile recruitment. From 2011 to 2017, research documented an increased mortality in this species, of up to 100% in some populations, but no etiological cause of these mortalities has been identified. With this paper, we provide current knowledge of locations where mass mortality of freshwater pearl mussel has been found in Sweden and discuss possible causes. Postmortem sampling and histopathological findings from two counties in 2016–17 detected lesions in digestive glands indicating a reduced capacity for nutrient uptake. Results from these macroscopic and microscopic investigations also indicate a reduction in, or a lack of, reproductive output compared with reference populations.

The diagnosis of bacterial disease in freshwater unionid mussels has been hindered by a lack of baseline information regarding the microbial communities associated with these animals. In this study, we cultured and identified bacteria from the hemolymph of stable mussel populations from Wisconsin portions of the upper Mississippi River basin and compared the results to those from mussel populations experiencing a mortality event in the Clinch River in Virginia and Tennessee. Several bacterial genera were consistently identified across mussel species and locations, appearing to be part of the natural bacterial flora. One noteworthy bacterial species identified from the Clinch River was Yokenella regensburgei, which occurred in relatively high prevalence during the mortality event but was absent from samples acquired afterward. Its role in the mortality event, if any, is unknown but deserves further investigation. We suggest that future studies of freshwater mussel health incorporate hemolymph as a sample type due to its relative separation from the aquatic environment, its role in the circulatory system, and the fact that it can be collected nonlethally.

Unionid mussels are threatened by multiple environmental stressors and have experienced mass mortality events over the last several decades, but the role of infectious disease in unionid health and population declines remains poorly understood. Although several microbial agents have been found in unionids, to date only one virus has been documented—Lea plague virus (Arenaviridae) in propagated Triangle Shell mussels (Hyriopsis cumingii) in China. We used next-generation DNA sequencing to screen hemolymph of seven individuals of five unionid species from the Upper Mississippi River basin, USA for viruses. We identified the complete polyprotein gene of a novel picornalike virus in one individual of the Wabash Pigtoe (Fusconaia flava). The virus is a member of the Nora virus clade of picornalike viruses and is most closely related to viruses from arthropods in China. We did not detect viruses in another Wabash Pigtoe or in animals of the other four species. It is premature to make inferences about the role of this virus in the health of Wabash Pigtoes or other unionid species or the origin or transmission of this virus. Nevertheless, to our knowledge, our results represent the first report of a virus in wild North American unionids. Technologies based on next-generation DNA sequencing should prove useful for identifying new viruses and investigating their role in unionid health and disease.

Freshwater mussels (Mollusca: Bivalvia: Unionida) consist of 843 species in six families, but many are imperiled. Significant causes of mussel declines include contaminants and loss of substrate. Potentially, etiological agents are also contributing factors, but parasites and pathogens of freshwater mussels are understudied relative to those affecting marine bivalves. Published accounts of viral pathogens have been reported exclusively from Hyriopsis cumingii (Unionidae) in China. There are limited records of possible bacterial and fungal pathogens from unionids in the USA and Finland. Parasitic and commensal organisms generally include ciliates (Ciliophora), trematodes (Platyhelminthes: Aspidogastrea and Digenea), roundworms (Nematoda), moss animals (Ectoprocta, Entoprocta), oligochaetes and leeches (Annelida: Sedentaria: Clitellata), mites (Arthropoda: Acari), copepods (Arthropoda: Copepoda), insects (Arthropoda: Insecta), and fish eggs (Chordata: Actinopterygii). Parasites injure the host through attachment or feeding or when they invade host tissue to complete their life cycles (e.g., digeneans). Commensals are small organisms living in or on mussels that may use the mantle cavity or shell as a refuge or substrate, and commensals also may feed on particulates that have been gathered by their molluscan host. Typically, however, the relationship between the two parties is subject to speculation (e.g., leeches). We are in the midst of a biodiversity crisis, and this minireview highlights the relationships among these organisms and the need to understand the health of wild and captive mussels.

Wildlife disease concerns are global and broad in scope and involve a wide diversity of expertise from multiple disciplines. In the realm of freshwater mollusk conservation, there is a paucity of information on pathogens of freshwater mollusks or pathogens of other species they might harbor. Consequently, it is a daunting task to manage and mitigate disease in freshwater mollusks. Disease risk analysis (DRA) is a structured, evidence-based process that aids decision making in the face of uncertainty by characterizing the potential impact of infectious and noninfectious diseases on ecosystems, wildlife, domestic animals, and people. In March 2018, as part of the 11th biennial meeting of the Freshwater Mollusk Conservation Society, a team from the University of Minnesota College of Veterinary Medicine's Risk Analysis Unit and the Conservation Planning Specialist Group (CPSG, previously the Conservation Breeding Specialist Group: http://www.cpsg.org) of the International Union of the Conservation of Nature Species Survival Commission (IUCN SSC) offered a mini-workshop series; its aim was to help conservationists, animal resource managers, and industry professionals integrate science and policy to frame, characterize, and manage health risks using international standards in DRA. Participants worked through the initial stages of DRA to examine the risks of disease introduction into aquatic systems as a result of freshwater mollusk translocation. They formulated and prioritized problems in the larger effort to (1) train the community in DRA, (2) leverage funding for further work, and (3) begin communicating with policy makers in this area. Here we report the results of this working group activity and demonstrate the utility of the DRA process in addressing concerns, real and perceived, regarding the risk of diseases associated with freshwater mollusk conservation activities.

Energy storage is critical for gametogenesis and successful spawning in bivalve mollusks. However, it often is overlooked as an endpoint in toxicological studies of freshwater mussels. Energy storage can be assessed through direct measurement of energy substrates or the use of the condition index (CI) as an indicator of overall nutritional status. Our study focused on the CI of adult Lampsilis fasciola exposed to treatment conditions designed to mimic the Powell River (Virginia, USA), which historically supported an exceptionally diverse freshwater mussel community. Coal mining operations have impacted the upper Powell River, and low-flow specific conductance frequently exceeds 900 µS/cm. We used four treatments in a full-factorial design to evaluate mussel responses to diluted pond water (control), simulated Powell River water, control sediment, and Powell River sediment. We measured glycogen content of mantle tissue and CI and compared several CI metrics. Exposure to simulated Powell River water caused a significant decrease in several CI metrics compared to control water. There was no effect of sediment type, nor was there any effect of sex; both males and females lost body mass in simulated Powell River water. However, males had significantly lower glycogen content of mantle tissue, indicating females likely were using other sources of energy to compensate for salinity stress. Comparison of CI metrics demonstrated that dissection was necessary to discern the effect of major ions on energy storage and that the use of tissue weight (g)/shell cavity capacity (g) had lower variability than tissue weight (g)/shell cavity volume (mL). The observed decrease in CI of adult L. fasciola after exposure to elevated concentrations of major ions has implications for maintaining mussel populations in the Powell River and in other rivers with rapidly increasing salinity.