Tubercled Blossom Pearlymussel

Epioblasma torulosa torulosa

Description

The tubercled blossom pearlymussel (Epioblasma torulosa torulosa ) is a medium-sized freshwater mussel, reaching about 3.6 in (9.1 cm) in shell length. The shell is irregularly egg shaped or elliptical, slightly sculptured, and corrugated with distinct growth lines. The outer surface is smooth and shiny; tawny, yellowish-green, or straw colored; and usually has numerous green rays. The inner shell surface is white to salmon-red. Females are generally larger than males and display a large, rounded marsupial swelling, which is often a darker green than the rest of the shell.

The tubercled blossom is a more inflated big-river form of the federally endangered green blossom pearlymussel (E. t. gubernaculum ), a subspecies found in the headwater tributaries of the Tennessee River above Knoxville. The tubercled blossom was first described from the Ohio and Kentucky rivers as Amblema torulosa and later as Dysnomia torulosa torulosa.

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, diatoms, 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 tubercled blossom pearlymussel inhabited the larger rivers within its range, preferring to bury itself into sand and gravel shoals.

Distribution

This mussel was once fairly abundant and widespread throughout all the major rivers of the eastern United States and southern Ontario, Canada. These rivers included in particular the Tennessee, Cumberland, Ohio, and St. Lawrence.

One specimen, thought to be freshly dead, was collected in 1969 from the Kanawha River below Kanawha Falls in West Virginia, but no other recent collections have been made, and this subspecies may well be 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."

Studies of the Kanawha River in 1982-83 found no further evidence of the tubercled blossom pearlymussel. A detailed scuba search below the Kanawha Falls turned up nothing, and scientists concluded that the species no longer occurred in the drainage.

Threats

Mussels have declined steadily in the major rivers because of increased turbidity and siltation triggered by deforestation and the spread of intensive agriculture throughout the East. The decline of the genus Epioblasma may have begun in earnest when settlers crossed the Appalachians to farm the rich Ohio and Tennessee river valleys.

More recently, major rivers have suffered from extensive chemical pollution caused by both agricultural and industrial runoff. Because mussels filter many gallons of water to extract food, pollutants build up in their tissues. The health of mussel populations in major rivers can be used as a general indicator of the health of the ecosystem.

Conservation and Recovery

Understandably, this mussel has been accorded a low priority in the allocation of research funds. If a viable population can be located, further recovery strategies will be designed and implemented.

Contacts

U.S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
1 Federal Drive
BHW Federal Building
Fort Snelling, Minnesota 55111
Telephone: (612) 713-5360
http://midwest.fws.gov./

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

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. General Accounting Office, Washington, D.C.

Jenkinson, J.J. 1986. "Endangered or Threatened Aquatic Mollusks of the Tennessee River System." Bulletin of the American Malacological Union 2: 63-65.

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.