Freshwater mussels of the family Unionidae, also known as naiads, pearly mussels, freshwater clams, or unionids, are a diverse group of bivalve mollusks that are distributed on every continent except Antarctica. Approximately 300 species are known from the United States, with most of this diversity residing in rivers of the Southeast where many endemic taxa have evolved.
The Science Issue and Relevance: Accurate and precise species identification and classification provide the foundation for understanding evolution relationships, are key to identifying biogeographic processes, and facilitate the targeting of conservation programs. The ability to identify distinct evolutionary lineages objectively in some groups remains an important challenge for modern systematics research. Urgency increases for assessments involving imperiled species, particularly those fraught with taxonomic instability, because conservation efforts are typically based on species-level designations.
Freshwater mussels of the family Unionidae, also known as naiads, pearly mussels, freshwater clams, or unionids, are a diverse group of bivalve mollusks that are distributed on every continent except Antarctica. Approximately 300 species are known from the United States, with most of this diversity residing in rivers of the Southeast where many endemic taxa have evolved. This fauna is highly imperiled, with about 78% of currently recognized species considered either extinct, endangered, threatened, or of special concern. At least 10% of North American mussel taxa became extinct in the past 100 years, which is comparable to extinction rates observed in the rainforest and for other freshwater organisms. Despite the realization that native unionid populations are in peril, taxonomic uncertainties remain for many species and limit the development of effective conservation management strategies. Furthermore, the identification of many unionid species is complicated by the lack of discrete morphological characters that are useful for diagnosing species accurately, delimiting species boundaries, or determining evolutionary relationships. The conservation of remaining North American mussel populations would benefit greatly from improved methods for identification and delineation of species boundaries, which would also enable a better understanding of the biogeographic processes responsible for creating this biodiversity.
Methodology for Addressing the Issue: A variety of molecular-based tools are used to test current taxonomic hypotheses at both the generic and species level. We rely primarily on mitochondrial genes to test species-level relationships and more conserved, nuclear genes for supraspecific relationships. Most recently, we’ve began employing next generation sequencing technologies. Specifically, we are using restriction site-associated DNA sequencing (RADseq) and exon capture via anchored hybrid enrichment for massively high-throughput phylogenomics. These molecular data are then analyzed within an integrative taxonomic framework, in which we consider information from multiple independent lines of evidence to resolve taxonomy and better inform conservation decisions.
Future Steps: Information produced by this research is being used to guide U.S. Fish and Wildlife Service (FWS) listing decisions and actions. In addition, the findings will help FWS understand how to adaptively manage for freshwater mussels, along with other aquatic organisms, including potential changes that may occur as a result of climate change.
Below are publications associated with this project.
Integrative taxonomy resolves taxonomic uncertainty for freshwater mussels being considered for protection under the U.S. Endangered Species Act
Integrative taxonomy resolves taxonomic uncertainty for freshwater mussels being considered for protection under the U.S. Endangered Species Act
Molecular and morphometric analyses reveal cryptic diversity within freshwater mussels (Bivalvia: Unionidae) of the western Gulf coastal drainages of the USA
Molecular and morphological data reveal non-monophyly and speciation in imperiled freshwater mussels (Anodontoides and Strophitus)
Molecular phylogeny of the Nearctic and Mesoamerican freshwater mussel genus Megalonaias
Deciphering the link between doubly uniparental inheritance of mtDNA and sex determination in bivalves: Clues from comparative transcriptomics
The Pleurobemini (Bivalvia: Unionida) revisited: Molecular species delineation using a mitochondrial DNA gene reveals multiple conspecifics and undescribed species
A revised list of the freshwater mussels (Mollusca: Bivalvia: Unionida) of the United States and Canada
The extremely divergent maternally- and paternally-transmitted mitochondrial genomes are co-expressed in somatic tissues of two freshwater mussel species with doubly uniparental inheritance of mtDNA
- Overview
Freshwater mussels of the family Unionidae, also known as naiads, pearly mussels, freshwater clams, or unionids, are a diverse group of bivalve mollusks that are distributed on every continent except Antarctica. Approximately 300 species are known from the United States, with most of this diversity residing in rivers of the Southeast where many endemic taxa have evolved.
Figure 1. Phylogenetic tree of select freshwater mussels from North America The Science Issue and Relevance: Accurate and precise species identification and classification provide the foundation for understanding evolution relationships, are key to identifying biogeographic processes, and facilitate the targeting of conservation programs. The ability to identify distinct evolutionary lineages objectively in some groups remains an important challenge for modern systematics research. Urgency increases for assessments involving imperiled species, particularly those fraught with taxonomic instability, because conservation efforts are typically based on species-level designations.
Freshwater mussels of the family Unionidae, also known as naiads, pearly mussels, freshwater clams, or unionids, are a diverse group of bivalve mollusks that are distributed on every continent except Antarctica. Approximately 300 species are known from the United States, with most of this diversity residing in rivers of the Southeast where many endemic taxa have evolved. This fauna is highly imperiled, with about 78% of currently recognized species considered either extinct, endangered, threatened, or of special concern. At least 10% of North American mussel taxa became extinct in the past 100 years, which is comparable to extinction rates observed in the rainforest and for other freshwater organisms. Despite the realization that native unionid populations are in peril, taxonomic uncertainties remain for many species and limit the development of effective conservation management strategies. Furthermore, the identification of many unionid species is complicated by the lack of discrete morphological characters that are useful for diagnosing species accurately, delimiting species boundaries, or determining evolutionary relationships. The conservation of remaining North American mussel populations would benefit greatly from improved methods for identification and delineation of species boundaries, which would also enable a better understanding of the biogeographic processes responsible for creating this biodiversity.
Figure 2. Photo illustrating difficulties distinguishing between federally endangered and common freshwater mussel species using only external shell morphology. Methodology for Addressing the Issue: A variety of molecular-based tools are used to test current taxonomic hypotheses at both the generic and species level. We rely primarily on mitochondrial genes to test species-level relationships and more conserved, nuclear genes for supraspecific relationships. Most recently, we’ve began employing next generation sequencing technologies. Specifically, we are using restriction site-associated DNA sequencing (RADseq) and exon capture via anchored hybrid enrichment for massively high-throughput phylogenomics. These molecular data are then analyzed within an integrative taxonomic framework, in which we consider information from multiple independent lines of evidence to resolve taxonomy and better inform conservation decisions.
Future Steps: Information produced by this research is being used to guide U.S. Fish and Wildlife Service (FWS) listing decisions and actions. In addition, the findings will help FWS understand how to adaptively manage for freshwater mussels, along with other aquatic organisms, including potential changes that may occur as a result of climate change.
- Publications
Below are publications associated with this project.
Integrative taxonomy resolves taxonomic uncertainty for freshwater mussels being considered for protection under the U.S. Endangered Species Act
Objectively delimiting species boundaries remains an important challenge in systematics and becomes urgent when unresolved taxonomy complicates conservation and recovery efforts. We examined species boundaries in the imperiled freshwater mussel genus Cyclonaias(Bivalvia: Unionidae) using morphometrics, molecular phylogenetics, and multispecies coalescent models to help guide pending conservation aAuthorsNathan A. Johnson, Chase H. Smith, John M. Pfeiffer, Chalres R. Randklev, James D. Williams, James D. AustinIntegrative taxonomy resolves taxonomic uncertainty for freshwater mussels being considered for protection under the U.S. Endangered Species Act
Objectively delimiting species boundaries remains an important challenge in systematics and becomes urgent when unresolved taxonomy complicates conservation and recovery efforts. We examined species boundaries in the imperiled freshwater mussel genus Cyclonaias(Bivalvia: Unionidae) using morphometrics, molecular phylogenetics, and multispecies coalescent models to help guide pending conservation aAuthorsNathan A. Johnson, Chase H. Smith, John M. Pfeiffer, Chalres R. Randklev, James D. Williams, James D. AustinMolecular and morphometric analyses reveal cryptic diversity within freshwater mussels (Bivalvia: Unionidae) of the western Gulf coastal drainages of the USA
Past geological processes and climate change affected current species distributions and the genetic structure of riverine fauna. Western Gulf of Mexico coastal rivers harbour four mussel species within the genus Fusconaia(Bivalvia: Unionida). The distributions of these species are unclear owing to their indistinguishable shell morphologies. Using molecular phylogenetic and Fourier morphometric anaAuthorsAnna M. Pieri, Kentaro Inoue, Nathan A. Johnson, Chase H. Smith, John L. Harris, Clint Robertson, Charles R. RandklevMolecular and morphological data reveal non-monophyly and speciation in imperiled freshwater mussels (Anodontoides and Strophitus)
Accurate taxonomic placement is vital to conservation efforts considering many intrinsic biological characteristics of understudied species are inferred from closely related taxa. The rayed creekshell, Anodontoides radiatus (Conrad, 1834), exists in the Gulf of Mexico drainages from western Florida to Louisiana and has been petitioned for listing under the Endangered Species Act. We set out to resAuthorsChase H. Smith, Nathan A. Johnson, John M. Pfeiffer, Michael M. GangloffMolecular phylogeny of the Nearctic and Mesoamerican freshwater mussel genus Megalonaias
Megalonaias is the most geographically widespread genus of the subfamily Ambleminae and is distributed across much of the eastern half of North America, from Minnesota to Nicaragua. Despite the large geographic distribution, the species-level diversity of Megalonaias is quite depauperate (2 spp.), suggesting the genus may not be constrained by the same physical, ecological, or physiological barrieAuthorsJohn M. Pfeiffer, Ashley Sharpe, Nathan A. Johnson, Kitty F. Emery, Lawrence M. PageDeciphering the link between doubly uniparental inheritance of mtDNA and sex determination in bivalves: Clues from comparative transcriptomics
Bivalves exhibit an astonishing diversity of sexual systems and sex-determining mechanisms. They can be gonochoric, hermaphroditic or androgenetic, with both genetic and environmental factors known to determine or influence sex. One unique sex-determining system involving the mitochondrial genome has also been hypothesized to exist in bivalves with doubly uniparental inheritance (DUI) of mtDNA. HoAuthorsCharlotte Capt, Sébastien Renaut, Fabrizio Ghiselli, Liliana Milani, Nathan A. Johnson, Bernard E. Sietman, Donald Stewart, Sophie BretonThe Pleurobemini (Bivalvia: Unionida) revisited: Molecular species delineation using a mitochondrial DNA gene reveals multiple conspecifics and undescribed species
The Pleurobemini (Bivalvia: Unionida) represent approximately one-third of freshwater mussel diversity in North America. Species identification within this group is challenging due to morphological convergence and phenotypic plasticity. Accurate species identification, including characterisation of currently unrecognised taxa, is required to develop effective conservation strategies because many sAuthorsKentaro Inoue, David M. Hayes, John L. Harris, Nathan A. Johnson, Cheryl L. Morrison, Michael S. Eackles, Tim King, Jess W. Jones, Eric M. Hallerman, Alan D. Christian, Charles R. RandklevA revised list of the freshwater mussels (Mollusca: Bivalvia: Unionida) of the United States and Canada
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 speciAuthorsJames D. Williams, Arthur E. Bogan, Robert S. Butler, Kevin S. Cummings, Jeffrey T. Garner, John L. Harris, Nathan A. Johnson, G. Thomas WattersThe extremely divergent maternally- and paternally-transmitted mitochondrial genomes are co-expressed in somatic tissues of two freshwater mussel species with doubly uniparental inheritance of mtDNA
Freshwater mussel species with doubly uniparental inheritance (DUI) of mtDNA are unique because they are naturally heteroplasmic for two extremely divergent mtDNAs with ~50% amino acid differences for protein-coding genes. The paternally-transmitted mtDNA (or M mtDNA) clearly functions in sperm in these species, but it is still unknown whether it is transcribed when present in male or female soma.AuthorsSophie Breton, Karim Bouvet, Gabrielle Auclair, Stephanie Ghazal, Bernard E. Sietman, Nathan A. Johnson, Stefano Bettinazzi, Donald T. Dtewart, Davide Guerra