Yellow Blossom Pearlymussel

Epioblasma florentina florentina

Description

The yellow blossom pearlymussel (Epioblasma florentina florentina ) is a Cumberlandian species with an elliptical shell seldom exceeding 2.4 in (6.1 cm) in length. The slightly inflated valves are of unequal length, and the shell surface is marked by uneven growth lines. The shell is a shiny honey-yellow or tan with numerous green rays uniformly distributed over the surface. The inner shell surface is bluish-white.

E. f. florentina represents the big-river form of this species, which may grade into the smaller head-waters form E. f. walkeri. This species has also been classified as Dysnomia florentina florentina.

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 yellow blossom pearlymussel's natural habitat is in the sand and gravel substrates of shallow, fast-flowing streams and rivers.

Distribution

Cumberlandian mussels are endemic to the southern Appalachian Mountains and the Cumberland Plateau region. Historically, the yellow blossom pearlymussel was widespread in the drainages of the Cumberland and Tennessee rivers. It has been documented from the Flint, Elk, Duck, Holston, Clinch, and Little Tennessee rivers and from other Tennessee River tributaries, including Hurricane, Limestone, Bear, and Cypress creeks (in northern Alabama) and Citico Creek (in Tennessee).

Because this subspecies has not been reliably documented since the first half of the twentieth century, noted malacologist D.H. Stansbery considers it extinct. A report from the General Accounting Office issued in December 1988 also considered the mussel "probably extinct," but specimens that may represent the yellow blossom pearlymussel were collected from Citico Creek in 1957 and from the Little Tennessee River in the mid-1960s. Therefore, the U.S. Fish and Wildlife Service agreed to maintain the current status of endangered until further research can settle the question.

Threats

The single greatest factor in the decline or extinction of this species has been the construction of large dams on its habitat rivers. Since the 1930s the Tennessee Valley Authority (TVA), the Aluminum Company of America, and the Army Corps of Engineers have constructed 51 dams on the Tennessee and Cumberland rivers for flood control, generation of hydroelectric power, and recreation. Many segments of the rivers that once supported large populations of mussels have been permanently flooded. In addition, altered downstream flows have changed water temperatures and increased turbidity. Strip mining, coal washing, farming, and logging have all added loads of silt and pollutants to the streams and rivers of the region. Turbid water clogs the feeding apparatus of mussels, and siltation smothers mussel beds.

Fish species of all kinds, including those that play host to mussel glochidia, have declined, making mussel reproduction problematic. At the beginning of the twentieth century, nearly 80 species of freshwater mussels were documented from the waters of Alabama and Tennessee, but by 1964 the count had declined to only 59. Many of these surviving species are now rare or threatened with extinction.

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 this and other freshwater mussels in the state. Mussel research in Alabama is promoted by the Alabama 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

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

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

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.