Oyster mussel  (Epioblasma capsaeformis)

The oyster mussel is classified as Critically Endangered (CR) on the IUCN Red List (1).

The oyster mussel is a small freshwater mollusc with a relatively dull to slightly shiny outer shell surface (periostracum), which is yellowish to green in colour, and crossed with numerous dark green, narrow rays. The inner shell surface, or nacreous layer, is white to bluish-white in colour. The species shows pronounced sexual dimorphism, the female’s shell being expanded along the outer margin, towards the rear, where it is quite thin and fragile. This expanded portion of shell is dark in colour, helping to distinguish the female from other, similarly-shaped mussels. The male oyster mussel, in comparison, is much more elliptical in shape (3) (4) (5).

Recent studies have suggested that the population of oyster mussels from the Duck River in Tennessee may represent a separate species (4) (6). Specimens from this population grow to nearly twice the size of those from other areas, and show a spongy, greyish to blackish rather than smooth, bluish-white mantle pad, a region of the female’s mantle that is exposed during display (2) (4) (6).

The oyster mussel occurs in the Cumberland and Tennessee River systems in the United States. Once one of the most widely distributed Cumberlandian mussel species, occurring across six states, it is now found only in a small number of streams and rivers in Alabama, Kentucky, Tennessee and Virginia (1) (2) (3) (4) (5) (7). However, plans are underway to reintroduce the species into parts of its historical range (7) (8).

The oyster mussel inhabits small- to medium-sized rivers, and sometimes larger rivers, in areas with sand or gravel substrate and moderate to swift currents. It appears to prefer shallow sandbanks and riffles (shallow stretches where the water flows fast over stones or rocks), and is often found in pockets of gravel between bedrock ledges, or associated with beds of water willow (Justicia americana) (2) (3) (4) (5). The oyster mussel can bury itself beneath the substrate, although females have often been observed lying on top of the substrate when displaying or releasing larvae (3) (4) (5).

The adult oyster mussel feeds by filtering small food particles from the water, and orients the body into the best position for obtaining food and oxygen from the water column. The specific diet of this species is unknown, but, like other freshwater mussels, it is likely to feed on a mixture of plankton, detritus, bacteria, diatoms (microscopic, single-celled algae), and other microorganisms (2) (4). Juveniles may also use the muscular ‘foot’ to pick up food particles (4). The oyster mussel spawns from late spring to early autumn (2) (4) (5), the males releasing sperm into the water column, which is then taken in by the female through the siphon. The fertilised eggs are retained by the female in a specialised region of the gills known as the marsupium, the enlargement of which accounts for the female’s bulging shell shape. In this species, the female retains the larvae, known as glochidia, for some months, usually releasing them the following spring (2) (3) (4) (9).

Human modifications to streams and rivers have had a serious impact on this once common and widespread mussel. By 2000, the species had undergone an estimated 80 percent population decline, and is also now much reduced in range (1) (2) (4). Channel modifications, including dams, dredging and mining, together with pollution, sedimentation and water withdrawal, all directly threaten the oyster mussel’s habitat (1) (2) (3) (4), while impoundments such as dams can greatly alter factors such as water flow, temperature and oxygen levels, as well as presenting barriers to dispersal. Adverse effects on host fish can also indirectly impact the mussel’s reproductive cycle. Large portions of the oyster mussel’s historical range, including the entire length of the main Tennessee and Cumberland River channels, are now impounded or greatly modified, leading to the extirpation of the oyster mussel from many areas (2) (3) (4). The small and fragmented populations that remain are under increased risk from isolated events such as chemical spills, or from the potential negative effects of genetic isolation (4).

In 1997, the oyster mussel was listed as Endangered by the U.S. Fish and Wildlife Service (7), and a recovery plan was drawn up for the species (4). The conservation measures recommended by this plan include the protection of remaining mussel populations and habitat, identifying or re-establishing additional populations, undertaking further research into the species’ ecology, life history and genetic diversity, the implementation of a monitoring programme, and working to improve habitat quality (3) (4). Techniques for the artificial propagation of laboratory- or hatchery-reared mussels are also being developed, with the ultimate aim of reintroducing the oyster mussel into parts of its historical range (3) (4) (8).

View information on this species at the UNEP World Conservation Monitoring Centre.

To find out more about the oyster mussel and its conservation see:

• U.S. Fish and Wildlife Service: Oyster Mussel Epioblasma capsaeformis:
  http://ecos.fws.gov/docs/life_histories/F01T.html
• U.S. Fish and Wildlife Service: Oyster Mussel in North Carolina:
  http://www.fws.gov/nc-es/mussel/oystermuss.html

For more information on the conservation of freshwater mussels in North America, see:

• U.S. Fish and Wildlife Service - Discovery Freshwater Mussels: America’s Hidden Treasure:
  http://www.fws.gov/news/mussels.html
• Freshwater Mollusk Conservation Society (FMCS):
  http://ellipse.inhs.uiuc.edu/FMCS/