Stirrupshell

Quadrula stapes

Description

The stirrupshell, Quadrula stapes, is a bivalve mollusk, 2.2 in (5.6 cm) long, with a quadrangular shell. The yellowish green shell is marked with zig-zag lines, which are light green on young shells and dark brown on older shells. Its truncated posterior has a sharp ridge and tubercles. The inner shell surface is silvery white.

Behavior

See the Upland Combshell (Epioblasma metastriata ) entry.

Habitat

This mussel is found in the shoals and riffles of fast-flowing rivers, buried in relatively firm, silt-free rubble, gravel, and sand substrates.

Distribution

The stirrupshell was found historically in the Tombigbee River from near Columbus, Mississippi, downstream to Epes, Alabama, and in the Black Warrior, Sipsey, and Alabama Rivers.

The stirrupshell has not been found in the Alabama or Black Warrior Rivers for several decades. It is presently known from only two sites in Alabama-the Sipsey River (Pickens and Green Counties), and the Gainesville Bendway (Pickens County), a meander of the East Fork Tombigbee River that was cut off by construction of the Tennessee-Tombigbee Waterway. A survey and habitat assessment of the Tombigbee River at Gainesville Bendway, Alabama, was conducted in 1988, but no specimens of this species were located.

Threats

When the Tennessee-Tombigbee Waterway was completed to allow barge traffic between the Tennessee and Tombigbee rivers, most of the East Fork Tombigbee River was modified into a series of channels, locks, and impoundments. The dams and locks inundated mussel shoals and slowed the flow of water, increasing siltation, which smothers mussel beds. Dredging to create a navigable channel physically destroyed many mussel beds, and periodic maintenance dredging continues to disturb the river bottom. Bull Mountain Creek, which provided nearly half the water supply of the East Fork, was diverted to feed the waterway. The creek's cooler waters are warmed when routed through the canal, making this part of the river inimical to both mussels and host fishes.

Mussels native to the Tombigbee and Sipsey rivers are threatened by continued flood control and navigation improvement projects. Such activities add silt to waters and increases turbidity, clogging the mussel's feeding apparatus and smothering mussel beds. A currently proposed 84 mi (136 km) channel improvement project would degrade or destroy mussel habitat on the Sipsey River.

The last free-flowing stretch of the East Fork Tombigbee River is threatened by plans to dredge 53 mi (85 km) to improve navigability. Siltation in the Bendway has become more severe in the last few years and may already be smothering surviving mussel beds.

Conservation and Recovery

Under provisions of the Endangered Species Act, federal agencies are required to consult with the U.S. Fish and Wildlife Service (FWS) to ensure that any actions they authorize or fund do not jeopardize Threatened or Endangered species. This rule affects all watershed projects proposed by the Army Corps of Engineers and the Soil Conservation Service of the Department of Agriculture. In the past, similar consultations have resulted in the redesign of projects to preserve significant portions of habitat.

The Recovery Plan calls for maintaining good water flow through the areas the stirrupshell inhabits; creating artificial habitats from gravel bars; prohibiting sand and gravel dredging; and creating land easements to protect remaining habitats.

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

Fuller, S.L.H. 1974. "Clams and Mussels (Mollusca:Bivalvia)." In Hart and Fuller, eds. Pollution Ecology of Freshwater Invertebrates. Academic Press, New York.

Stansbery, D.H. 1976. "Naiad Mollusks." In H.Boschung, ed. "Endangered and Threatened Plants and Animals in Alabama." Bulletin of the Alabama Museum of Natural History 2: 42-55.