Littlewing Pearlymussel

Pegias fabula

Description

The littlewing pearlymussel (Pegias fabula ) is a small mussel measuring 1.5 in (3.8 cm) in length and 0.5 in (1.3 cm) in width. The shell is light green or dark yellowish-brown with variable dark rays along the anterior portion. The shell surface often has an eroded chalky or ashy white patina.

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 with temperatures 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

This species is restricted to cool, high-to-moderate gradient streams and is usually found only in the narrow zone where riffle flow deepens into pools. It is highly sensitive to alterations of current.

Distribution

At one time, this Cumberlandian mussel was widely distributed in at least 27 of the smaller, cool-water tributaries of the Tennessee and Cumberland Rivers. The species apparently has been extirpated from Alabama. Three populations in Kentucky, nine in Tennessee, and six in Virginia are believed to have died out.

This species is currently found in Kentucky, Tennessee, North Carolina, and Virginia. A 1986 survey of 55 potential and historic habitats located only 17 live specimens. Four of six known populations are threatened by activities associated with exploration for coal.

In the summer of 1990 biologists from the U.S. Fish and Wildlife Service's Asheville (North Carolina) Field Office and the Tennessee Cooperative Fishery Research Unit discovered a new population of the littlewing pearlymussel in a short reach of the Little Tennessee River in North Carolina. At the time of this discovery, the only known littlewing pearlymussel population in North Carolina had been lost. The discovery of this new population in the Little Tennessee River will assist in the species' recovery.

Only seven other populations of this mussel are known, including what is probably the healthiest surviving population: the one in Horse Lick Creek (Jackson and Rockcastle Counties), Kentucky. Smaller populations have been located in the Big South Fork Cumberland River (McCreary County), Kentucky; the Little South Fork Cumberland River (McCreary and Wayne Counties), Kentucky; Cane Creek above Great Falls Lake (Van Buren County), Tennessee (where suitable habitat is limited by downstream siltation); and at two locations in the North Fork Holston River (Smyth County), Virginia—one near Saltville, the others at Nebo.

Threats

Always relatively uncommon because of its more specialized habitat requirements, the littlewing pearlymussel has declined throughout its range because of degradation of water quality. Wastes from coal mining and industrial sites have made many former population sites uninhabitable. Runoff from strip mining, coal washing, and agriculture has clouded waters that were once crystal clear and smothered mussel beds beneath layers of sediment. Toxic chemical releases were apparently responsible for the demise of several mussel populations.

Conservation and Recovery

Part of the mussel's Kentucky watershed lies within the Daniel Boone National Forest, and Horse Lick Creek has been identified as one of Kentucky's Outstanding Resource Waters. State and federal biologists are cooperating to rejuvenate populations at these protected sites.

Ongoing exploration for new coal reserves in the region threatens to degrade the water quality of the remaining habitat.

State laws in Kentucky, Tennessee, and Virginia prohibit the collection of freshwater mussels without a permit. The recovery objectives for this species are to establish 13 viable populations through the protection of existing populations and through the discovery and/or creation of additional populations.

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

Ahlstedt, S.A. 1986. "A Status Survey of the Littlewing Pearly Mussel." Report, Contract No. 14-16-0004-84-927. U.S. Fish and Wildlife Service, Atlanta.

Biggins, R.G. 1989. "Draft Recovery Plan for Littlewing Pearly Mussel." U.S. Fish and Wildlife Service, Asheville, North Carolina.

Soule, M.E. 1980. "Thresholds for Survival: Maintaining Fitness and Evolutionary Potential." In M.E. Soul and B.A. Wilcox, eds., Conservation Biology. Sinauer Associates, Sunderland, Massachusetts.

Stansbery, D.H. 1976. "Status of Endangered Fluviatile Mollusks in Central North America: Pegias fabula." Report, Contract No. 14-166-0008-755.

U.S. Fish and Wildlife Service, Asheville.U.S. Fish and Wildlife Service. 1988. "Determination of Endangered Species Status for the Littlewing Pearly Mussel." Federal Register 53 (219): 45861-65.