Rough Pigtoe

Pleurobema plenum

Description

The shell of the rough pigtoe, Pleurobema plenum, is somewhat triangular in outline with mature dimensions of 2.5 in (6.5 cm) long, 2.8 in (7.1 cm) high, and 1.7 in (4.3 cm) wide. The inflated valves are solid and heavy. The shell surface is marked by irregular, concentric growth marks and has a clothlike texture. It is a slightly glossy, yellowish to reddish brown. The inner shell surface (nacre) varies in color from white to pinkish, reddish, or orange.

Behavior

The family Unionidae is separated into two groups based on the length of time glochidia (larvae) remain in the female. The rough pigtoe pearly mussel is probably "tachytictic," a short-term breeder—one that breeds in spring and releases glochidia by late summer of the same year. Long-term breeders hold developing glochidia in a brood pouch over winter and release them in spring. The fish hosts for this species are thought to include the rosefin shiner and possibly the bluegill.

For more on the reproduction and diet of freshwater mussels, see See the Upland Combshell (Epioblasma metastriata ) entry.

Habitat

The rough pigtoe is a big-river shoal species, and is found in deeper waters of streams 66 ft (20 m) wide or wider. It buries itself in the gravel or sandy bottom with only the posterior margin of the shell and siphons exposed to the water.

Distribution

First discovered in the Ohio River near Cincinnati in 1840, this mussel was subsequently documented from four major regions—the basins of the Cumberland and Tennessee Rivers (Virginia, Tennessee, Alabama, and Kentucky), the Ohio River drainage (Ohio, Indiana, and Illinois), the Ozarks (Kansas, Missouri, and Arkansas), and the mainstream Mississippi River (Arkansas).

As of the late twentieth century, the rough pigtoe was known from near the confluence of the Green and Barren Rivers (Warren County, Kentucky), a river system that empties into the Ohio River near Evansville, Indiana. It survived in the Clinch River near Kyles Ford (Hancock County), Tennessee, and in the Tennessee River below Guntersville Dam (Marshall County, Alabama), Wilson Dam (Lauderdale County, Alabama), and Pickwick Dam (Hardin County, Tennessee). No population estimates were available. The U. S. Fish and Wildlife Service lists the species as endangered in Alabama, Indiana, Kentucky, Pennsylvania, Tennessee, and Virginia.

Threats

The reasons for the decline of the rough pigtoe are not fully understood, but the longevity of most mussel species—up to 50 years—and their sedentary nature make them especially vulnerable to habitat alterations caused by dam construction, dredging, siltation, and pollution.

Since the early 1930s and 1940s, the Tennessee Valley Authority (TVA), the U. S. Army Corps of Engineers, Aluminum Company of America, and other water authorities have constructed more than 50 dams in the Tennessee and Cumberland water-sheds alone. The Ohio River has been extensively modified by a series of dams and locks along its length. Heavy siltation, caused by poor agricultural practices, has rendered large portions of this mussel's historic habitat unsuitable.

Agricultural chemicals and industrial wastes have generally degraded the water quality of major rivers, such as the Ohio, Missouri, and Mississippi.

Conservation and Recovery

The states of Tennessee and Alabama have designated portions of the Tennessee and Cumberland Rivers as freshwater mussel sanctuaries; the head-waters, however, originate in Virginia, where pollutants introduced by strip mining and coal washing have affected these rivers throughout the drainage. The recovery of this species will depend upon the success of regional efforts to improve water quality. The TVA, which administers the operation of dams on the Tennessee and Cumberland Rivers, is currently developing a comprehensive water management plan. This plan would guarantee constant minimum flows in all rivers in the region by timing water discharges from TVA dams. Such an effort might mollify many of the negative effects of dams and reservoirs on remaining stretches of mussel habitat.

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
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

Bates, J. M., and S. D. Dennis. 1978. "The Mussel Fauna of the Clinch River, Tennessee and Virginia." Sterkiana 69-70: 3-23.

Isom, B. G. 1974. "Mussels of the Green River, Kentucky." Proceedings of the Kentucky Academy of Science 35 (1-2): 55-7.

U. S. Fish and Wildlife Service. 1984. "Rough Pigtoe Pearly Mussel Recovery Plan." U. S. Fish and Wildlife Service, Atlanta.