Cumberland Bean Pearlymussel

Villosa trabalis

Description

The Cumberland bean pearlymussel (Villosa trabalis ) is a small-to medium-sized Cumberlandian freshwater species with solid, elongated oval valves. The shell is unsculptured except for concentric growth marks and ridges on the beak. The surface is somewhat glossy; olive-green, yellowish-brown, or blackish; and covered with many narrow, wavy, dark green or blackish rays. The inner shell surface is white except for an iridescent blue-green posterior. This species was formerly classified as Micromya trabalis.

Behavior

The life of mussels is complex, and reproduction often depends upon a stable habitat—unaltered stream conditions, clean water, and an undisturbed stream bottom. The cycle also depends upon the abundance of suitable fish hosts to complete the mussel's larval development.

To reproduce, males discharge sperm, which are dispersed by stream currents. In the process of feeding, females nearby or downstream take in sperm, which fertilizes eggs stored in their gills. The gills serve as brood pouches (marsupia), where the glochidia hatch and begin to develop. After a time, these glochidia are released into the stream. A few mussels have inner parts that resemble a tiny minnow and can be manipulated to lure host fish. When a fish gets close to the shell, the mussel expels its glochidia.

Glochidia have tiny bean-or spoon-shaped valves that attach to the gill filaments of host fish. Glochidia can only progress to the juvenile stage while attached to the fish's gills. Those that do not fortuitously encounter a host fish do not survive when released by the female mussel. They sink to the bottom and die.

When the juvenile has developed a shell and is large enough to survive on its own, it detaches from the host fish and falls to the stream bottom, beginning a long association with a single stretch of stream. Maturing mussels bury themselves in riffles and shoals with only the shell margins and feeding siphons exposed to the water. Some mussels live as long as 50 years or more.

The family Unionidae, which includes all of the freshwater mussels in the United States, is separated into two groups based on the length of time the glochidia remain in the female's marsupia. The eggs of the short-term (tachytictic) breeders are fertilized in the spring, and glochidia are released by late summer of the same year. Long-term (bradytictic) breeders hold developing glochidia in the brood pouch over winter and release them in the spring.

Freshwater mussels feed by siphoning phytoplankton and other plant matter from the water. Indigestible particles are expelled from the shell by reverse siphoning. Silt in the water can kill mussels by clogging their feeding siphons.

There are no known interspecific differences in feeding among freshwater mussels. The glochidia are obligate parasites on the gills or fins of fish. Adult mussels are filter-feeders and consume particulate matter in the water column. Identifiable stomach contents almost invariably include desmids, di-atoms, algae, protozoa, and zooplankton.

Most freshwater mussel species display seasonal variations in activity associated with water temperature and reproduction. Metabolic rate is, in part, positively correlated with temperature. Many ectothermic species have the capacity to adjust their metabolic rates in response to long-term changes in temperature. Thus, metabolic rates do not continue to rise as temperatures rise in the summer, and they do not continue to fall during the winter as temperatures decline.

Some freshwater mussels also show diurnal changes in metabolic rates that indicate a tendency toward nocturnal activity patterns. Mussels may move to the surface to feed at night and move deeper into the substrate during the day; this is one way to avoid predators that hunt by visual contact.

Freshwater mussels are nonmigratory.

Habitat

The Cumberland bean pearlymussel is found in clean, fast-flowing water in gravel and sand shoals that have been swept free of silt by the action of the current.

Distribution

This species was widely distributed in many of the larger tributary streams of the upper Cumberland River and was considered rare in the Tennessee River drainage. In the 1920s it was reported in the Tennessee River and its tributaries—South Chicamauga Creek (northern Georgia), Paint Rock and Flint Rivers (northern Alabama), and the Hiwassee and Clinch Rivers (Tennessee). It has been reported more recently from the Cumberland River and its tributaries—Buck and Beaver Creeks and the Obey and Rockcastle Rivers (Kentucky).

The U. S. Fish and Wildlife Service has declared this species endangered in Alabama and Virginia.

Threats

The Cumberland bean pearlymussel was probably eradicated from the Tennessee River water-shed by the construction of major dams on the river for flood control and hydroelectric power production. Since the 1930s the Tennessee Valley Authority (TVA) has erected 36 dams in the Tennessee River basin. Five major dams have been located on the Cumberland River and six others on its tributaries. The cold tailwaters from these dams have made long upstream portions of these rivers uninhabitable for both mussels and host fishes. Siltation caused by strip mining, coal washing, logging, and poor agricultural practices within the water-sheds have buried gravel and sand shoals and smothered mussel beds. Because mussels must siphon gallons of water each day to feed, the effects of water pollutants such as herbicides and pesticides are intensified.

Conservation and Recovery

Buck Creek and the Little South Fork Cumberland River are eligible for Scenic River status under the National Wild and Scenic Rivers Act. Such a designation would provide additional protection for the species and its habitat. The TVA is working on a water management plan to guarantee constant minimum flows in all rivers in the Tennessee and Cumberland basins by timing water discharges from its dams. Such an effort may ease many of the negative effects of dams and reservoirs on remaining stretches of mussel habitat.

Contact

U. S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
1875 Century Blvd., Suite 200
Atlanta, Georgia 30345
Telephone: (404) 679-4000
http://southeast.fws.gov/

References

Branson, B.A. 1974. "Stripping the Appalachians."Natural History 38 (9): 53-60.

Jenkinson, J.J. 1986."The Tennessee Valley Authority Cumberlandian Mollusk Conservation Program." Bulletin of the American Malacological Union 2: 62-63.

U.S. Fish and Wildlife Service. 1984. "Cumberland Bean Pearly Mussel Recovery Plan." U.S. Fish and Wildlife Service, Atlanta.