Birdwing Pearlymussel

Conradilla caelata

Description

The birdwing pearlymussel (Conradilla caelata ) is a relatively small Cumberlandian mussel, seldom over 2 in (5 cm) in width. The valves are solid, slightly inflated (especially in females), and triangular or egg shaped. The surface of the shell is marked by strong, irregular growth lines, and the outer coloring is olive green or dark green. Inside coloring of the shell is always white. Fish hosts for this mussel are thought to be the shiner (Notropis galacturus ) and at least one darter. This species is sometimes referred to as Lemiox rimosus, as described in 1834 from the Cumberland River.

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

Cumberlandian freshwater mussels are found in clean, fast-flowing streams and rivers in riffles and shoals where the bottom consists of firm rubble, gravel, or sand.

Distribution

The birdwing pearlymussel once was widely dispersed in small numbers within the Tennessee River and its major tributary streams. Collectors have always considered it a rare shell. Although a few records have located the birdwing pearlymussel in other watersheds, these records are now considered in error.

The birdwing pearlymussel is presently found only in larger tributaries of the Tennessee River— the Duck, Elk, Clinch, and Powell rivers. The mussel is abundant in Duck River but limited to a 40-mi (64.4-km) reach between Lillard Mill Dam and the former location of the Columbia Dam. The population there has been estimated at 20,000-30,000 individuals

Threats

Major water control projects flood upstream valleys, reduce downstream flows, alter temperature gradients, cause extreme water level fluctuations, increase turbidity and silting, and create seasonal oxygen deficits. These factors can eliminate mussels that are fixed to a single locality. The Columbia Dam on the Duck River was at one time a threat to the species, but demolition on it began in June 1999.

Conservation and Recovery

The U.S. Fish and Wildlife Service Recovery Plan for the birdwing pearlymussel stresses 1) the need to seek agreements with landowners along the rivers to preserve streambank habitat and 2) the need to develop a public education program to discuss the uniqueness of this river system and the rarity of the resources at risk. As suitable sites are identified within the historic range, reintroduction of the species will be attempted.

The TVA is currently working on a comprehensive water management plan that would guarantee constant minimum flows in all rivers in the Tennessee and Cumberland basins by timing water discharges from its dams. Such an effort might mollify many of the negative effects of dams and reservoirs on remaining stretches of mussel habitat.

Contacts

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/

U. S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
300 Westgate Center Dr.
Hadley, Massachusetts 01035-9589
Telephone: (413) 253-8200
Fax: (413) 253-8308
http://northeast.fws.gov/

References

Bates, J.M., and S.D. Dennis. 1978. "The Mussel Fauna of the Clinch River, Tennessee and Virginia." Sterkiana 69/70: 3-23.

Pardue, J.W. 1981. "A Survey of the Mussels (Unionidae) of the Upper Tennessee River, 1978." Sterkiana 71: 41-51.

U.S. Fish and Wildlife Service. 1983. "Birdwing Pearly Mussel Recovery Plan." U.S. Fish and Wildlife Service, Atlanta.