Freshwater Mussels

M. Lynne Corn 1
Specialist in Natural Resources Policy
Environment and Natural Resources Policy Division

July 14, 1994

94-560 ENR

COMMON NAMES; Mussels, clams, naiades, or unionids

SCIENTIFIC NAMES: Families Margaritiferidae and Unionidae

CURRENT RANGE: Worldwide distribution with the greatest diversity in North America, chiefly east of the Mississippi River, but with a few species established along the Pacific Coast.

HABITAT: While typically found in streams, some species may be found in ponds or lakes. In order to reproduce, they require a stable, reasonably undisturbed habitat of clean water. Fish of particular species are required as hosts for mussel larval development. After fertilization and development within the female mussel, the larvae are released and attach themselves to fish hosts. The larvae mature further, drop off the host, and settle into the stream bottom.

The United States is home to approximately one-third of the world's freshwater mussel species. Freshwater mussels, regarded as important indicators of aquatic ecosystem health, are assigned tremendous ecological importance by many freshwater biologists. Of the approximately 300 U.S. species, 18 are thought to be extinct, and nearly 60 listed as endangered or threatened under the Endangered Species Act (50 Code of Federal Regulations 17.11 & 17.12). Major threats to mussels include loss of habitat (from channel modifications, dams, etc.) and habitat deterioration from pollution. Efforts to increase populations include habitat improvement, artificial propagation, reintroduction, and development of captive populations in hatcheries and farm ponds.

MAJOR THREATS: Habitat destruction from dams, channel modification, siltation, and pollution, and loss of fish hosts are the principal causes for population declines. In some cases declines may be due to overharvesting for commercial purposes, illegal poaching, or the introduction of nonindigenous species such as the zebra mussel (Dreissena polymorpha).

While zebra mussels have only begun to invade areas of high mussel diversity, the threat to native mussels may become severe as the zebra mussel expands its range. The zebra mussel--native to the Black, Caspian, and Aral seas--was first found in Lake St. Clair of the Great Lakes basin in the late 1980s and has since expanded its range to most of the Great Lakes, the Illinois River, and segments of the Ohio, Tennessee, Mississippi, Susquehanna, and Hudson Rivers. Fast reproduction (as many as five million eggs per year per female) and transport by human activities such as shipping, fishing, and boating contribute to its rapid spread. Zebra mussels attach themselves to native mussels, and in sufficient quantities, may smother the native mussels. Zebra mussels also compete with native mussels for food.

BACKGROUND; Historically, Native Americans used freshwater mussels for food, ornamental objects, and tools. Mussels were used more recently in the manufacture of buttons, until the plastic industry was established. Today, the cultured pearl industry uses small pieces of mussel shell in the production of seed pearls. Freshwater mussels are also used today in biomedical research: they seem to be resistant to cancer and scientists would like to discover why. Furthermore, many animal species (such as muskrat, mink, and otter) feed on freshwater mussels.

Mussels can be relocated within their historic range to reestablish wild populations, but many habitats remain degraded. Experimental farm ponds are being used to develop the technology to maintain captive mussel populations, and laboratories are being used to propagate listed species for return to native habitat. For example, Richard Neves, a U.S. Fish and Wildlife Service biologist based at Virginia Polytechnic Institute, is culturing mussels from the Clinch River watershed (Tennessee and Virginia) in hopes of saving them. 2

MAJOR AFFECTED AREAS: Diversity of freshwater mussels is highest in the Tennessee River drainage (which includes parts of Tennessee, Virginia, Kentucky, North Carolina, Alabama, Georgia, and Mississippi) where there are approximately 100 species. In the drainage, 14 species are candidates for listing under the Endangered Species Act, and 5 species are extinct. In the State of Tennessee alone, 22 species are endangered. North of the Tennessee River drainage, mussel diversity was reduced by the advance of glaciers, but a substantial number of species survived in the Tennessee Basin, which was not glacially affected. Diversity in the Tennessee Basin is also due to fluctuating interglacial water levels that encouraged speciation.

There are several causes for freshwater mussel population declines in this region; however, the effect of dams on mussels may be the most significant. For example, beginning in the 1930s, the Tennessee Valley Authority began building dams for hydroelectric power and flood control. Currently there are 9 dams on the Tennessee River and 24 on its tributaries. For those species adapted to free-flowing waters, dams contributed to their decline by slowing or stopping the flow of water. Gravelly or sandy, fast or free-flowing creeks were converted to quiet water with silted substrate behind the dam. In the altered habitat, many mussel species starve or smother. Alternatively, their fish hosts may be displaced as a result of the alteration in the habitat. Eventually mussel species may become extinct.

The U.S. Fish and Wildlife Service has launched numerous projects to protect mussels in the Tennessee River drainage. One project is the zebra mussel removal program. This program is developing methods to remove zebra mussels from live native mussels. Such a study is valuable in the event large numbers of native mussels must be removed from infested waters. Another project is studying oxygen tolerances of various mussels. The information will be used to set effluent limits for oxygen to protect mussels.

References

1 Under the supervision of M. Lynne Corn, Cynthia Marcum, B.Sc. candidate at Bowling Green State University, researched and contributed to this report.

2 Kuznik, Frank. "America's Aching Mussels." National Wildlife. Oct-Nov. 1993, p. 34-38.