Flat Pigtoe

Pleurobema marshalli

Description

The oval or obliquely elliptical shell of the flat pigtoe mussel, Pleurobema marshalli, is about 2.4 in (6 cm) long, 2 in (5 cm) high, and 1.2 in (3 cm) thick. It has a shallow cavity and very low pustules or welts on the post-ventral surface. Older shells are a dark brown with irregular concentric black growth lines. The thin, inner shell surface (nacre) is bluish-white. The flat pigtoe is also known as Marshall's mussel.

In the late twentieth century, the identity of species within the genus Pleurobema was the focus of debate among malacologists. The U. S. Fish and Wildlife Service (FWS) adopted the majority view but acknowledged that further research could warrant reclassification of the flat pigtoe and other mussels of the genus.

Behavior

See the Upland Combshell (Epioblasma metastriata ) entry.

Habitat

The flat pigtoe is found in clean, fast-flowing water in relatively shallow stretches where the bottom is composed of firm rubble, gravel, or sand, swept free of silt.

Distribution

This mussel was known from the main stem of the Tombigbee River from Tibbee Creek near Columbus (Lowndes County), Mississippi, downstream to Epes (Sumter County), Alabama, above the confluence of Noxubee River. The absence of specimens from anywhere except the Tombigbee River suggests that this species was historically restricted to this single stretch of river.

This mussel was collected alive in Sumter and Pickens Counties, Alabama, in 1972. The Pickens County site and former sites in Mississippi have suffered from heavy sedimentation since that time and are no longer considered viable habitat. The only remaining viable habitat for this species in the Tombigbee River is a gravel bar in the Gainesville Bendway in Sumter County, Alabama. The bend-way is a remnant of the old riverbed that was bypassed when a navigable channel was constructed.

Threats

Low numbers and very restricted occurrence make the flat pigtoe one of the most immediately endangered of the freshwater mussels. Completion of the Gainesville Dam and other structures of the Tennessee-Tombigbee Waterway (a navigable canal built to connect the Tennessee and Tombigbee Rivers) effectively eliminated much of the historic habitat of the flat pigtoe except for the gravel bars in the Gainesville Bendway. A 1987 survey conducted in the bendway documented extensive siltation and found only a few common mussel species. Few of the more uncommon varieties were found. A recently completed dam on Bull Mountain Creek, the source of most of the water for the East Fork Tombigbee River, slowed currents and reduced water flow throughout the system, causing the sedimentation. The condition of the river also caused a decline of common fish hosts.

Conservation and Recovery

Because the condition of the Gainesville Bend-way has deteriorated so abruptly, the FWS may be forced to attempt an emergency relocation of mussel populations, if other suitable habitat can be found. There is some question whether enough of the mussels survive to justify such a drastic measure or whether the relocation attempt would, in itself, result in extinction for the flat pigtoe.

Contact

U. S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
1875 Century Blvd., Suite 200
Atlanta, Georgia 30345
http://southeast.fws.gov/

References

Stansbery, D. H. 1983. "The Status of Pleurobema marshalli." Unpublished report. U. S. Fish and Wildlife Service, Atlanta.

U. S. Army Corps of Engineers. 1981. "Water Resources Development in Alabama." Report. Mobile District Office, Mobile, Ala.

U. S. Department of Agriculture. 1983. "Soil Conservation Service Watershed Progress Report Mississippi." U. S. Department of Agriculture, Washington, D.C.