Oyster Mussel in North Carolina

Epioblasma capsaeformis (Lea 1834)

FAMILY: Unionidae

STATUS:  Endangered, Federal Register, January 10, 1997

DESCRIPTION:

The oyster mussel is a small (maximum about 70 millimeters or 2.1 inches) freshwater mussel (Neves 1991). Its periostracum (outer shell surface) has a dull to sub-shiny yellowish to green colored shell with numerous narrow dark green rays. The shells of females are slightly inflated and quite thin and fragile towards the shell's posterior margin. The nacre (inside shell surface) is whitish to bluish-white in color [see Johnson (1978) for a more complete description of the species].

Gordon (1991) provides the following diagnostic characters: "The pronounced development of the posterior-ventral region in females distinguishes Epioblasma from similarly shaped species. Capsaeformis is recognized by the typically dark coloration and fragility of the marsupial expansion and the lack of development of the posterior ridge (e.g., not angular, no knobs). Males in comparison to similar Epioblasma tend to be more elliptical, have a moderately developed posterior ridge and accompanying sulcus, and have a regularly curved ventral margin. The ventral margin in species such as E. florentina (Lea 1857) and E. turgidula (Lea 1858) often exhibit an emargination of the ventrum just anterior to the terminus of the posterior ridge. Yellowish specimens of E. capsaeformis have been mistaken for E. walkeri (Wilson and Clarke 1914) (including records in Johnson [1978: as E. florentina]). Males of E. walkeri tend to be broader and have a rounded posterior ridge; females lack the distinctive darkening of the marsupial expansion."

HISTORIC AND CURRENT RANGE:

The oyster mussel historically occurred throughout much of the upper Tennessee and Cumberland River drainages in Alabama, Kentucky, North Carolina [French Broad River, Buncombe County (Ortmann 1918)], Tennessee, and Virginia [Gordon 1991 and U.S. Fish and Wildlife Service (USFWS) 1998]. Ortmann (1918) considered the species to be very abundant in the upper Tennessee River drainage.

Currently, within the Cumberland River, the oyster mussel survives as a very rare component of the benthic community in Buck Creek, Pulaski County, Kentucky; and in a few miles of the Big South Fork Cumberland River, McCreary County, Kentucky, and Scott County, Tennessee (Bakaletz 1991 and USFWS 1998). Within the Tennessee River system, only small populations survive at a few sites in the Powell River, Lee County, Virginia; and Hancock and Claiborne Counties, Tennessee; in the Clinch River system, Scott County, Virginia, and Hancock County, Tennessee; Copper Creek (a Clinch River tributary), Scott County, Virginia; and Duck River, Marshall County, Tennessee. Although not seen in recent years, the species may still persist at extremely low numbers in the lower Nolichucky River, Cocke and Hamblem Counties, Tennessee, and in the Little Pigeon River, Sevier County, Tennessee (Gordon 1991 and USFWS 1998). We have no current records of the species in North Carolina; it was last collected in North Carolina in 1913 (Ortmann 1918).

Habitat:

The oyster mussel inhabits small to medium rivers (Dennis 1985) in areas with coarse sand to boulder substratum (rarely in mud) and moderate to swift currents (Gordon 1991). It is sometimes found associated with water-willow (Justicia americana) beds (Gordon and Layzer 1989) and in pockets of gravel between bedrock ledges in areas of swift current (Neves 1991). Gordon (1991) reports that this species, like other freshwater mussels, can bury itself below the substratum surface. Females have been observed to lie on top of the substratum while releasing glochidia (larvae).

REPRODUCTION:

Males release sperm into the water column, which are taken in by females through their siphons during feeding and respiration. The eggs are fertilized and retained in specialized portion of the gills (marsupium) until the larvae (glochidia) fully develop. After the glochidia are released into the water, they attach and encyst on the gills and/or fins of a host fish. When metamorphosis (change to a young mussel) is complete, they drop to the streambed as juvenile mussels. Spawning probably occurs in late summer; glochidia have been observed in the marsupia during October, May, June, and July; and the glochidia are likely released in early summer (Gordon and Layzer 1989). Four fish have been identified as host--spotted darter (Etheostoma maculatum), redline darter (Etheostoma rufilineatum), dusky darter (Percina sciera), and banded sculpin (Cottus carollinae) (Yager and Saylor 1995).

REASONS FOR CURRENT STATUS:

The oyster mussel, like most mussels in the Ohio River basin, has been directly impacted by impoundments, siltation, channelization, and water pollution. Reservoir construction is the most obvious cause of the loss of mussel diversity in the basin's larger rivers. Most of the main stem of both the Tennessee and Cumberland River and many of their tributaries are impounded. For example: over 2,300 river miles or about 20 percent of the Tennessee River and its tributaries with drainage areas of 25 square miles or greater are impounded (Tennessee Valley Authority 1971). In addition to the loss of riverine habitat within impoundments, most impoundments also seriously alter downstream aquatic habitat; and mussel populations upstream of reservoirs may be adversely affected by changes in the fish fauna essential to a mussel's reproductive cycle.

Coal mining related siltation and associated toxic runoff have adversely impacted many stream reaches. Numerous streams have experienced mussel and fish kills from toxic chemical spills, and poor land use practices have fouled many waters with silt. Runoff from urban areas has degraded water and substrate quality. Because of the extent of habitat destruction, the overall aquatic faunal diversity in many of the basins' rivers has declined significantly (USFWS 1998).

MANAGEMENT AND PROTECTION:

A technical draft recovery plan for the species was published in June 1998 (USFWS 1998). The downlisting criteria (reclassifying the species to threatened) objectives in the recovery plan call for: (1) the existence of at least five distinct viable populations--two in the upper Cumberland River system and three in the Tennessee River system, (2) one distinct naturally reproduced year class within each of the species' viable populations, (3) studies of the species' biological and ecological requirements have been completed and any required recovery measures developed from these studies are beginning to be successful, and (4) no foreseeable threats exist that would likely impact the survival of the species over a significant portion of its range. The delisting (removing the species from the protection of the Endangered Species Act) objectives call for: (1) the existence of at least seven distinct viable populations--two in the upper Cumberland River system and five in the Tennessee River system, (2) two distinct naturally reproduced year class within each of the species' viable populations, (3) studies of the species' biological and ecological requirements have been completed and any required recovery measures developed from these studies are successful, and (4) no foreseeable threats exist that would likely impact the survival of any of the viable populations.

Currently, we are developing artificial propagation and reintroduction technology for this and other federally listed mussels. We are considering reintroducing the oyster mussel into historic habitats in two reaches in the Tennessee River basin. We are also working with various partners to improve habitat quality for existing populations.

AUTHOR:

Richard G. Biggins (see address below)

REFERENCES:

Bakaletz, S. 1991. Mussel survey of the Big South Fork National River and Recreation area. Unpublished M.S. thesis, Tennessee Technological University, Cookeville, Tennessee. 62 pp.

Dennis, S.D. 1985. Distributional analysis of the freshwater mussel fauna of the Tennessee River system, with special reference to possible limiting effects of siltation. Tennessee Wildlife Resources Agency Report No. 85-2, Nashville. 171 pp.

Gordon, M.E. 1991. Species accounts for Cumberland elktoe (Alasmidonta atropurpurea), oyster mussel (Epioblasma capsaeformis) Cumberlandian combshell (Epioblasma brevidens), purple bean (Villosa perpurpurea), and rough rabbitsfoot (Quadrula cylindrica strigillata). Unpublished report to The Nature Conservancy, Boston,Massachusetts, 75 pp.

Gordon, M.E., and J.B. Layzer. 1989. Mussels (Bivalvia: Unionidae) of the Cumberland River: review of life histories and ecological relationships. U.S. Department of the Interior, Fish and Wildlife Service Biological Report 89(15). 99 pp.

Johnson, R.I. 1978. Systematics and zoogeography of Plagiola (=Dysnomia =Epioblasma), an almost extinct genus of mussels (Bivalvia: Unionidae) from middle North America. Bulletin of the Museum of Comparative Zoology 148:239-320.

Neves, R.J. 1991. Mollusks. Pp. 251-319 in Virginia's endangered species. Proceedings of a symposium coordinated by K. Terwilliger. McDonald & Woodward Publishing Co., Blacksburg, Virginia.

Ortmann, A.E. 1918. The nayades (freshwater mussels) of the upper Tennessee drainage with notes on synonymy and distribution. Proceedings of the American Philosophical Society 77:521-626.

Parmalee, P.W., and A.E. Bogan. 1998. The Freshwater mussels of Tennessee. The University of Tennessee Press, Knoxville, Tennessee. 328 pp.

Tennessee Valley Authority. 1971. Stream length in the Tennessee River Basin. Tennessee Valley Authority, Knoxville, Tennessee. 25 pp.

U.S. Fish and Wildlife Service. 1984. Technical draft recovery plan for Cumberland elktoe, oyster mussel, Cumberlandian combshell, Purple bean, and rough rabbitsfoot. Atlanta, GA. 119 pp.

Yeager, B.L., and C.F. Saylor. 1995. Fish host for four species of freshwater mussels (Pelecypoda: Unionidae) from the upper Tennessee River drainage. American Midland Naturalist 133(1):1-6.

For more information contact: Mr. Richard G. Biggins U.S. Fish and Wildlife Service 160 Zillicoa Street Asheville, North Carolina 28801 Telephone: (828)258-3939 ext. 228

Map: Show the species as being historic from Buncombe County, NC.

Species Distribution from known occurrences. Species may occur in similar habitats in other counties.Green counties indicate observed within 20 years. Yellow counties indicate an obscure data reference to the species in the county. Red counties indicate observed more than 20 years ago.

For additional information regarding this Web page, contact John Fridell, in Asheville, NC, at john_fridell@fws.gov

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