Tar River Spinymussel

Elliptio steinstansana

Description

The Tar River spinymussel (also known as Tar spinymussel), Elliptio steinstansana, reaches a mature length of 2.4 in (6 cm), and the rhomboidal shell is distinguished by having several short spines. The shell surface is smooth and shiny, marked with fine concentric rings. The inequilateral valves are regularly rounded, becoming slightly wider at the hinges and ending in a blunt point. The inner shell nacre is yellowish or pinkish, and young specimens have an orange-brown outer scale with greenish rays.

Aside from the Tar River spinymussel, only two other freshwater spinymussels are known to exist: a small-shelled and short-spined species (Fusconaia collina ) found only in the James River in Virginia and considered Endangered, and a large-shelled and long-spined species (Elliptio spinosa ) collected from the Altamaha River system in Georgia. The shell size and spine length of the Tar River mussel is intermediate between these two. It has been alternatively classified as Canthyria steinstansana.

Behavior

See the Upland Combshell (Epioblasma metastriata ) entry.

Habitat

This spinymussel has been collected on sand and mud substrates. The mussel's spines help it maintain an upright position as it works its way through the soft streambed.

Distribution

The Tar River spinymussel was first discovered in the Tar River (Edgecombe County), North Carolina, in 1966. Records suggest that the species inhabited the mainstream Tar River from Nash County downstream through Edgecombe County to Pitt County near the town of Falkland, North Carolina.

As of 1992, when the most recent Recovery Plan revision for the species was published, there were only three known remaining populations of this spinymussel—two extremely small, apparently non-reproducing populations in the main stem of the Tar River and a third, larger population, in Swift Creek, a tributary of the Tar River. Existing population estimates have ranged from a low of fewer than 100 individuals to a high of about 500 individuals.

Threats

The Tar River spinymussel may have always been rare, but its recent reduction in range and small population size make it vulnerable to extinction from a single catastrophic event, such as a tank-truck accident involving a toxic chemical spill— a real possibility, since an Interstate highway bridge passes directly over its habitat. Water quality is also a problem. The North Carolina Department of Natural Resources and Community Development reports levels of nutrients and pesticides are above average in the river.

Wastewater treatment plants are another serious threat to the spinymussel. During surveys conducted in the mid-1980s, a decline in mussel populations was noted for approximately one mile below the Tar River Regional Wastewater Treatment Plant operated by the City of Rocky Mount. Mussels were abundant above the plant and again several miles below, but the river immediately below the outfall was devoid of mussels, although the habitat appeared suitable. The wastewater plant at Rocky Mount was constructed in 1982 and has had a continuous history of National Pollution Discharge Elimination System Permit compliance problems since it opened. Several other wastewater treatment facilities that discharge into the Tar River or its tributaries, above the existing population of spinymussels in the Tar River, have also been implicated as violating water quality standards, including the plants at Franklinton, Littleton, Louisburg, Oxford, Scotlandneck and Tarboro.

Some of these plants have been upgraded, and the North Carolina Division of Environmental Management plans to upgrade and incorportae uniform, up-to-date pollution reduction requirements at all sewage treatment facilities within the Tar and Pamlico River basin. Although spinymussel populations may be reestablished as a result of improved water quality, recolonization will probably take many years, and the species rarity makes natural reestablishment unlikely.

As a further threat, the Tar River has become infested by the Asiatic clam, considered a pest. The Asiatic clam feeds in densities estimated at 1,000 individuals per square meter in some places, reducing the availability of phytoplankton needed as a food source for the Tar River spiny mussel.

Because this species has only recently been described and its approximate range located, notoriety for such a unique and rare mussel could increase collection pressure from shell dealers and collectors. And because the population is small, the unlawful removal of any individuals could seriously affect the species' survival. North Carolina State law prohibits collecting wildlife without a state permit.

Conservation and Recovery

State law does not protect the species' habitat from the potential impact of large-scale construction projects. Federal listing protects the Tar River spiny mussel by requiring federal agencies to consult with the Fish and Wildlife Service when projects they fund, authorize, or carry out may affect the species. Three specific projects have been identified that could affect the spiny mussel a hydroelectric project on the Tar River at Rocky Mount, a navigation and flood control project on the Tar River, and a stream obstruction removal project on Tar River tributaries. These projects may have to be redesigned to protect this mussel.

The 1992 revision of the 1987 Recovery Plan for the species notes that the species' extremely low population levels and restricted distribution may preclude full recovery; therefore, the recovery goal for the foreseeable future is dowlisting from Endangered to Threatened. Downlisting could occur when a number of recovery criteria are met, including the requirement that all three existing populations show evidence of reproduction, including at least two juvenile (ages 3 or younger) age classes; and that two new, distinct, viable populations must be discovered or reestablished within the species' historic range. Other downlisting criteria include the protection of all populations and their habitats from present and foreseeable threats, and the stabilization or increase over a 15 to 20-year period of all populations.

Contact

U.S. Fish and Wildlife Service
Division of Endangered Species
1875 Century Blvd., Suite 200
Atlanta, Georgia 30345

References

French, III, John R.P. 1990. "The Exotic Zebra Mussel: A New Threat to Endangered Freshwater Mussels." Endangered Species Technical Bulletin 15(11).

Johnson, R. I., and A. H. Clarke. 1983. "A New Spiny Mussel, Elliptio (Canthyria ) steinstansana (Bivalvia: Unionidae), from the Tar River, North Carolina." Occasional Papers on Mollusks 4(6): 289-298.

Shelley, R. M. 1972. "In Defense of Mollusks."Wildlife in North Carolina 36: 4-8, 26-27.

U.S. Fish and Wildlife Service. 1987. "Recovery Plan for Tar River Spiny Mussel." U.S. Fish and Wildlife Service, Atlanta.

U.S. Fish and Wildlife Service. 1992. "Tar Spiny-mussel Recovery Plan First Revision." U.S. Fish and Wildlife Service, Atlanta.