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Conservation Genetics
Mollusks

Samples of genetics and genomics research from the USGS Ecosystems Mission Area about the conservation genetics of mollusks.

Clubshell mussel. Photo credit: Craig Stihler, courtesy of USFWS National Digital Library Example of Pleurobema oviforme (UAUC 2733) Green floater (Lasmigona subviridis). Photo credit: Art Bogan, courtesy of USGS Nonindigenous Aquatic Species The endangered dwarf wedgemussel (Alasmidonta heterodon). Photo credit: Kira Hawk Island applesnail shells. Photo credit: Cassie Thibodeaux, USGS National Wetlands Research Center
Clubshells (King) Clubshells and Pigtoes (Morrison) Freshwater Mollusks: Range (King) Freshwater Mollusks: Recovery (King)

Island Applesnails (Carter)



Captive Breeding Management and Assessment of Augmentation Efforts with Multilocus Microsatellite DNA Genotypes in the Endangered Clubshell
Clubshell mussel. Photo credit: Craig Stihler, courtesy of USFWS National Digital Library
Clubshell mussel. Photo credit: Craig Stihler, courtesy of USFWS National Digital Library

The ultimate success of captive propagation programs relies, in part, on implementing a biologically sound genetic management program to determine the level of genetic diversity within and among wild populations and to ensure that this diversity is conserved within the captive population.  Practically speaking, managers of threatened and endangered species must be aware of the potential hazards of inbreeding and outbreeding depression within the captive breeding program, as well as, the potential threats posed by introducing genetically divergent congeners into vulnerable extant populations.  Measurements of success for mussel (i.e., unionids) reintroduction and augmentation efforts are necessary yet difficult given the small size of the released individuals, the disruptive nature of traditional techniques (i.e. repeated sampling), the challenge of using in-stream cages in small and flashy streams, and the need to discern between wild and captive-bred individuals.  Unique multi-loci genotypes provide managers with a robust tool for assessing and managing genetic biodiversity (individual identification, kinship, fine-scale population structure), and therefore can be utilized to follow the success of augmentation efforts.  As part of a previously funded project designed to assess population genetic structure from the largest remaining populations of clubshell (Pleurobema clava), from the Allegheny River in Pennsylvania, as well as other populations throughout its range, Leetown Science Center geneticists have developed a suite of polymorphic microsatellite markers.  This research constitutes the first use of multi-locus genotypes to manage the levels of genetic diversity among a unionid broodstock program and to provide direct evidence via gene marking of the effectiveness of an augmentation program. It follows that this project will serve as a model for future mussel captive breeding efforts throughout the nation.  Gene marking not only allows managers to measure the overall resource benefit of an augmentation effort, but also provides a research tool to measure the effectiveness of various breeding and introduction methodologies.  Moreover, a mechanism will be created to monitor the temporal effects (i.e., changes in allele and genotypic frequencies) on wild populations as supplementation proceeds.  Although this study is specific to Region 3 USFWS, the broodstock may also be appropriate for use outside the Region where clubshell recovery is also necessary per the recovery plan, such as in the Green River (KY).

For more information contact Timothy L. King at the Leetown Science Center.

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Clubshells and Pigtoes of the Pleurobemini:  Molecular Systematics to Clarify a Conchological Conundrum

Example of Pleurobema oviforme (UAUC 2733)
Example of Pleurobema oviforme (UAUC 2733)
Example of Pleurobema clava (Allegheny River, PA)
Example of Pleurobema clava (Allegheny River, PA)

Native freshwater mussels belonging to the Family Unionidae reach maximum diversity in North America, with nearly 300 species known from the U.S. Unionids are susceptible to human activities such as habitat modification, pollution, over-harvesting, and invasive species introductions, and thus are the most imperiled group of animals in North America. Knowledge of basic biology, ecology, and taxonomy is lacking, and severely limits the ability of conservationists to take action to protect this declining fauna. 

One group of unionids, members of the tribe Pleurobemini, are notoriously difficult to identify. Pleurobema species exhibit considerable shell morphology, often varying among river drainages, which has lead to taxonomic confusion, and is especially unfortunate since many species are considered threatened or endangered at either the state or national level.  USGS scientists have been utilizing DNA sequence data generated from samples obtained throughout the eastern U.S. as additional characters to assess evolutionary relationships within the Pleurobemini. At the molecular level, the story is equally complicated! On one hand, some species may be difficult to identify in the field and may be incorrectly labeled.  For others, what appear to be distinguishable species can not be separated based on their DNA. The clubshell, Pleurobema clava (endangered), and the Tennessee clubshell, P. oviforme (special concern), are two species that cannot be distinguished by sequence data, and species status is being investigated with fine-scale microsatellite markers.

For more information contact Cheryl Morrison, Leetown Science Center.

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Conservation Implications of Range Discontinuity in a Unionid Bivalve
Green floater (Lasmigona subviridis). Photo credit: Art Bogan, courtesy of USGS Nonindigenous Aquatic Species
Green floater (Lasmigona subviridis). Photo credit: Art Bogan, courtesy of USGS Nonindigenous Aquatic Species

A nucleotide sequence analysis of the first internal transcribed spacer region (ITS-1) between the 5.8S and 18S ribosomal DNA genes (640 base pairs) and cytochrome c oxidase subunit I (COI) of mitochondrial (mt) DNA (576 base pairs) was conducted for the freshwater bivalve the green floater (Lasmigona subviridis) and three congeners to determine the utility of these regions in identifying phylogeographic and phylogenetic structure.  Sequence analysis of the ITS-1 region indicated a zone of discontinuity in the genetic population structure between a group of green floater populations inhabiting the Susquehanna and Potomac rivers and more southern populations.  Moreover, haplotype patterns resulting from variation in the COI region suggested an absence of gene exchange between tributaries within two different river drainages, as well as between adjacent rivers systems.  Leetown Science Center geneticists recommended that the northern and southern populations, which are reproductively isolated and constitute evolutionarily significant lineages, be managed as separate conservation units.  Results from the COI region suggest that, in some cases, unionid relocations should be avoided between tributaries of the same drainage because these populations may have been reproductively isolated for thousands of generations.  Therefore, unionid bivalves distributed among discontinuous habitats (e.g., Atlantic slope drainages) potentially should be considered evolutionarily distinct.  The phylogenetic portion of this study suggested that the genus Lasmigona, as represented by the four species surveyed in this study, may not be monophyletic.

For more information, contact Timothy L. King at the Leetown Science Center.

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Applying Conservation Genetics to the Recovery of Freshwater Mollusks
The endangered dwarf wedgemussel (Alasmidonta heterodon). Photo credit: Kira Hawk
The endangered dwarf wedgemussel (Alasmidonta heterodon). Photo credit: Kira Hawk

Delineation of the appropriate unit of management is especially critical when the composition of a population is manipulated, whether by reintroduction from external stocks or by reestablishment of gene flow and migration patterns by the exchange of individuals from different populations.  The intended use of cultured unionids as a conservation tool underscores the need to recognize the genetic composition of natural and managed populations.  If a goal of unionid conservation efforts is to permit the continued evolution of a species (or any unit of management), then it is important to establish the genetic and taxonomic relationships among managed individuals or populations. To address these research needs and to allow more informed management decisions that may preserve ecological and evolutionary integrity of unionid species, Leetown Science Center geneticists have undertaken studies to recognize the hierarchical structure of genetic variation in multiple at-risk species.  This group of researchers has conducted research to: 1) assess the population structure within watersheds; 2) determine the phylogeographic structure among watersheds representing each species’ range; and 3) position the observed genetic variation in the appropriate taxonomic context.  Although unionid populations that constitute an important component in the evolutionary legacy of the species can only be protected under the U.S. Endangered Species Act if the entire species or subspecies is listed, recognition and protection of significant intraspecific differentiation should be an integral component of legislatively mandated recovery plans. 

For more information contact Timothy L. King at the Leetown Science Center.

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Island Applesnails (Pomacea insularum)
Island applesnail shells. Photo credit: Cassie Thibodeaux, USGS National Wetlands Research Center
Island applesnail shells. Photo credit: Cassie Thibodeaux, USGS National Wetlands Research Center
Island applesnail egg masses along Hwy. 20 between Chacahoula, Louisiana and Schriever, Louisiana. April 19, 2008. Photo credit: Eric Broussard
Island applesnail egg masses along Hwy. 20 between Chacahoula, Louisiana and Schriever, Louisiana. April 19, 2008. Photo credit: Eric Broussard

The exotic applesnails significantly impact wetland plant communities and rice agriculture due to their voracious grazing. They are also a potential vector for disease transmission to humans and animals. The snail tolerates a range of salinities and temperatures and can forage both in and out of water through the use of a gill and lung. Egg masses are laid on solid surfaces (e.g. dock pilings, plants) just above the water line. Once established, they are very difficult to remove. However, applesnails are species complex of similar looking but distinct species. Each species can have very different environmental tolerances and life history. For example some species are algae eaters while others will eat vascular plants. Because it can be very difficult to accurately identify applesnails species through their external morphology genetic tools are the most reliable way to correctly identify which species of applesnails is invading a system.

For photographs of Island applesnails, go to http://www.nwrc.usgs.gov/invasive_species/applesnail_images1.htm and http://www.nwrc.usgs.gov/invasive_species/applesnail_images2.htm

For more information view Island Applesnails (Pomacea insularum) in Louisiana: A Rapid Assessment of Status and Risk (PDF; 1.72 MB) and contact Jacoby Carter, National Wetlands Research Center.

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