Freshwater mussels: Hidden biodiversity on the brink

Blog written by Cheryl L. Morrison1, Nathan A. Johnson2 and Jess W. Jones3 . Read the full article here.

1U.S. Geological Survey, Eastern Ecological Science Center, Leetown Research Laboratory, Kearneysville, WV 25430, USA.

2U.S. Geological Survey, Wetland and Aquatic Research Center, Gainesville, FL 32653, USA

3U.S. Fish and Wildlife Service, Department of Fish and Wildlife Conservation, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061-0321

For many of the authors of this paper, this study has been a long journey, spanning more than a decade. Though the work is rooted in providing critical information for imperiled freshwater mussel species, we have been motivated since the start by our sense of wonder and concern for one of the planet’s most imperiled groups of organisms.

Within North America’s rivers, streams, and lakes live an often overlooked yet fascinating and diverse group of animals- the freshwater mussels (Order Unionoida). With more than 350 species, North America is home to the highest diversity of freshwater mussels on the planet. These unassuming organisms play an important ecological role as they improve water quality by filter feeding, contribute to nutrient cycling and food webs, and create habitat for other invertebrates and fishes. Although adult mussels live their lives buried in riverbeds, they have evolved intricate life histories that include reliance on host fishes to disperse their mussel larvae to new habitats. This mutually beneficial relationship may help keep distant, patchy mussel beds ecologically connected, often creating broad habitat ranges.

Unfortunately, with threats to our freshwater ecosystems mounting, freshwater mussels also have the distinction of being among the most imperiled group of organisms globally, with at least 30 extinctions documented in the last century and many additional species considered vulnerable to extinction. Some of the unique characteristics mentioned above (intricate life histories with sessile adults, broad ranges) make them prone to human-induced threats, such as changes to river flows (dams, water withdraws, climate change), toxins, invasive species, and habitat destruction, loss, and fragmentation. However, considerable knowledge gaps regarding basic biology, ecology (distribution and abundance), and taxonomy for some species have diminished efforts to implement conservation actions. Also, the limited and subtle differences in shell shape and sculpture, which are typically used to identify species, may be altered by river environments, which contributes to taxonomic confusion. In some cases, a single species may display phenotypic plasticity; this means it may have multiple, distinctive shell shapes in different rivers or along the river’s length. For example, the morphology of a species may differ between headwater and mainstem or lake and riverine habitats. 

About one-third of the freshwater mussel diversity in North America exists within the Tribe Pleurobemini, a group that contains the clubshell species complex. The federally endangered Clubshell, Pluerobema clava, and its southern cousin, the Tennessee Clubshell, Pleurobema oviforme, had long been confused for each other due to their similar shell shapes. In 2010, a petition to list P. oviforme under the Endangered Species Act prioritized the importance of determining the relationship between the two species to alleviate taxonomic confusion. Though we initially focused on assessing connectivity among disjunct populations of P. clava in Pennsylvania, Michigan, and West Virginia as part of species recovery planning, we soon broadened our research, adding P. oviforme samples and the mitochondrial ND1 marker, as considerable sequence data were publicly available for use in our analyses.

The present study melded data from several field surveys in the most comprehensive examination of species status for clubshells to date. The results, however, are challenging to interpret as datasets were only partially congruent. While sequences from mitochondrial genes suggested P. clava and P. oviforme belong to a single species, both nuclear microsatellites and morphology suggested the presence of a distinctive population from the Upper Tennessee River Basin. How distinctive that population may be, and whether other unsampled populations from the Upper Tennessee River Basin may also be distinctive, remain unanswered questions and make it difficult to define precise boundaries between species.

Our results and the limitations of our findings point to a challenge with gathering the information needed to address species questions in freshwater mussels. First, due to the rarity of these animals, it is difficult to obtain high quality DNA samples from an adequate number of individuals and populations, which is required for most modern genetic approaches. Second, sampling over a wide geographic range requires coordination across different field teams and methods, and if not carefully planned, risks excluding key populations of wide-ranging species from analyses. Third, measuring additional structural features on shells may be necessary to tease apart cases of phenotypic plasticity within a species versus morphological differences between species. Lastly, there is still much to learn about the life histories of most unionid species, including the breadth of fish hosts used and their dispersal abilities, information that could help put bounds on population interactions that may be ecologically important.

As managers are faced with difficult decisions related to translocating individuals among habitats, supplementing habitats with hatchery-raised individuals, and other recovery options, being able to properly define the limits of gene pools in freshwater mussels becomes critical. Modern genetic approaches offer opportunities to further characterize relationships among species and could be used to refine species and population boundaries for conservation planning at appropriate scales of resolution. This approach, however, requires DNA of a sufficient quality, which can be difficult to obtain across the distribution of imperiled species. While our study expands the knowledge base and proverbial toolbox available to managers charged with designing effective, holistic conservation and recovery options for these two freshwater mussels, expanded geographic and genetic sampling are needed to precisely delineate species and conservation units for the imperiled clubshells.

One of the authors (Nathan Johnson) on the bank of the Green River, Kentucky, using a non-lethal method to collect tissue samples from Pleurobema clava specimens (image by Cheryl Morrison, U.S. Geological Survey).

Examples of shell morphologies and coloration patterns in clubshells including Pleurobema oviforme from the Cumberland River, Kentucky (top left and right; images by Wendell Haag, U.S. Forest Service), and Duck River, Tennessee (lower left, image by Don Hubbs, Tennessee Wildlife Resource Agency) and Pleurobema clava from the Allegheny River, Pennsylvania (lower right, image by Robert Anderson, U.S. Fish and Wildlife Service).

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