Turgid Blossom Pearlymussel

Epioblasma turgidula

Description

The turgid blossom pearlymussel (Epioblasma turgidula ) is a small Cumberlandian species, seldom exceeding 1.6 in (4.1 cm) in shell length. The species is strongly dimorphic—males and females differ in shape and structure. Shells of the male tend to be more elliptical or oval, while females tend to be more rounded. Valves are inequilateral, solid, and slightly inflated. The outer shell is a shiny yellowish-green with numerous fine green rays over the entire surface. The shell surface is marked by irregular growth lines that are especially strong on females. The inner shell surface is bluish-white.

Male and female specimens were originally described as two separate species. The turgid blossom pearlymussel was previously classified as Dysnomia turgidula.

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 turgid blossom pearlymussel buries itself in sand and gravel substrates of shallow, fast-flowing streams. Clear unpolluted water is required for healthy freshwater mussel populations.

Distribution

Cumberlandian mussels are endemic to the southern Appalachian Mountains and the Cumberland Plateau. This species was relatively widespread within this region and was also found in the Ozarks. It is documented from the Tennessee River and its tributaries, including Elk, Duck, Holston, Clinch, and Emory Rivers (in Tennessee) and Shoals and Bear Creeks (in Alabama). Large numbers were found in the Cumberland River and its tributaries. In the Ozark Mountains it occurred in Spring Creek, Black River, and White River (in Arkansas and Missouri).

The turgid blossom pearlymussel was last reported in the mid-1960s from the Duck River near Normandy, Tennessee. D.H. Stansbery and J.J. Jenkinson, the experts on Cumberlandian mussels, consider this species extinct. A General Accounting Office report released in December 1988 included this mussel among those species "believed to be extinct but not yet officially declared so."

Threats

The construction of dams and reservoirs by the Tennessee Valley Authority (TVA), the Aluminum Company of America, and the Army Corps of Engineers completely and abruptly changed the character of the Tennessee and Cumberland Rivers, endangering many freshwater mussels in the process. Since the 1930s, 51 dams have been constructed on the Tennessee and Cumberland Rivers for flood control, generation of hydroelectric power, and recreation.

Segments of these rivers that once supported healthy populations of mussels have been permanently inundated. In addition, altered water flows and random water releases have changed water temperatures, increased turbidity, and contributed to problems of siltation. Strip mining, coal washing, farming, and logging have all added silt and pollutants to the streams of the region. Turbid water and siltation clog the feeding apparatus of mussels and smother mussel beds.

Many fish species, including those that play host to mussel glochidia, have declined in numbers, making mussel reproduction problematic.

Conservation and Recovery

Recovery strategies for this subspecies cannot be developed until a viable breeding population is discovered. The Tennessee Wildlife Resources Agency, the Tennessee Heritage Program, and the TVA continue to support research into the status of mussels in the state. Mussel research in Alabama is promoted by the State Department of Conservation and Natural Resources.

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

General Accounting Office. 1988. "Endangered Species: Management Improvements Could Enhance Recovery Program." GAO/ RCED-89-5. General Accounting Office, Washington, D.C.

Stansbery, D.H. 1971. "Rare and Endangered Mollusks in Eastern United States." In S.E. Jorgenson and R.E. Sharp, eds., Proceedings of a Symposium on Rare and Endangered Mollusks (Naiades). U.S. Fish and Wildlife Service, Twin Cities.

U.S. Fish and Wildlife Service. 1985. "Recovery Plan for the Tubercled-Blossom Pearly Mussel, Turgid-Blossom Pearly Mussel, and Yellow-Blossom Pearly Mussel." U.S. Fish and Wildlife Service, Atlanta.