Pale Lilliput Pearlymussel

Toxolasma cylindrellus

Description

The shell of the pale lilliput pearlymussel (Toxolasma cylindrellus ) measures 1.8 in (4.6 cm) in length, 1 in (2.5 cm) in height, and 0.64 in (1.6 cm) in width. The valves are solid, elongated, and appear nearly cylindrical. The smooth shell surface is yellowish-green. The inner shell surface (nacre) varies from white to light yellow with metallic tints of blue and purple. This Cumberlandian species was formerly classified as Carunculina cylindrellus.

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

This mussel inhabits narrow streams and prefers clean, shallow, fast-flowing water with a firm, silt-free rubble, gravel, or sandy bottom.

Distribution

The pale lilliput pearlymussel probably ranged in the narrower tributaries of the Tennessee River in Tennessee and Alabama. It was documented from the Flint and Elk Rivers in the 1920s, from the Sequatchie and Little Sequatchie Rivers in the 1950s, from the Buffalo River in 1973, and from the Duck River as recently as 1976. Of its reported occur-rences, it continues to survive only in the Paint Rock River watershed in northern Alabama.

Threats

The pale lilliput pearlymussel has been rare at least since its discovery but has disappeared from much of its former range because of dam construction, stream siltation, and pollution. Siltation caused by strip-mining, coal washing, dredging, clear-cutting, and poor agricultural practices have buried gravel and sand shoals and smothered mussel beds in many watersheds. Because mussels must siphon gallons of water each day to feed, the effects of water pollutants such as herbicides and pesticides are intensified. All these factors have resulted in the reduction in range and numbers of most Cumberlandian pearlymussels, including the pale lilliput.

Conservation and Recovery

Recovery of this species will depend on the larger effort of rehabilitating pearlymussel habitat in the Tennessee Valley.

The Tennessee Valley Authority is currently developing a comprehensive water management plan for the region that would establish minimum year-round flows in all rivers by carefully timing water discharges from its many dams. In addition, the Paint Rock River may be eligible for "scenic river" status under the National Wild and Scenic Rivers Act, a designation that would provide additional protection for this species and its remaining habitat. Pending the results of ongoing research, the U.S. Fish and Wildlife Service may attempt to reintroduce the pale lilliput to suitable stretches of habitat within its historic range.

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/

Ecological Services Field Office
446 Neal Street
Cookeville, Tennessee 38501-4027
Telephone: (931) 528-6481
Fax: (931) 528-7075

References

Bogan, A., and P. Parmalee. 1983. "Tennessee's Rare Mollusks." In Tennessee's Rare Wildlife, Final Report. Tennessee Heritage Program, Department of Conservation, University of Tennessee Press, Knoxville.

Isom, B.G. 1969. "The Mussel Resources of the Tennessee River." Malacologia 7 (2-4): 397-425.

U.S. Fish and Wildlife Service. 1984. "Pale Lilliput Pearly Mussel Recovery Plan." U.S. Fish and Wildlife Service, Atlanta.