Oyster Mussel

Epioblasma capsaeformis

Description

The Epioblasma capsaeformis (oyster mussel) described in 1834, 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 towards the shell's posterior margin. The inside of the shell is whitish to bluish white in color.

The pronounced development of the posterior-ventral region in females distinguishes Epioblasma capsaeformis from similarly shaped species. Epioblasma 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 Epioblasma fiorentina and Epioblasma turgidula often exhibit an emargination of the ventrum just anterior to the terminus of the posterior ridge. Yellowish specimens of Epioblasma capsaeformis have been mistaken for Epioblasma walkeri. Males of Epioblasma walkeri tend to be broader and have a rounded posterior, ridge; females lack the distinctive darkening of the marsupial expansion.

There are color differences in female oyster mussel mantle pads, which is presumably a host fish attractant. The mantle color appears to be bluish or greenish white in the Clinch River, greyish to blackish in the Duck River, and nearly white in Big South Fork population. Varying mantle coloration may be an indication that Epioblasma capsaeformis is a complex species.

Behavior

The lampsiline oyster mussel appears to be bradytictic. Spawning probably occurs in late summer, as glochidia have been observed in the marsupia during May, June, and July. In the Powell River, 58% of the females were gravid, but specimens were gravid only in May at a water temperature from 59-64°F (15-18°C). The age of gravid females, using the external growth ring method, was estimated at 7-10 years. The glochidia are likely released in early summer. Four fish species have been identified as hosts-the wounded darter (Etheostoma vulneratum), redline darter (Etheostoma rufilineatum), dusky darter (Percina sciera), and banded sculpin (Cottus carolinae). Transformation took from 19-34 days, at 60.4-62.4°F (15.8-16.9°C).

Habitat

This species inhabits small to medium-sized rivers, and sometimes large rivers, in areas with coarse sand to boulder substrata (rarely in mud) and moderate to swift currents. It is sometimes found associated with water-willow (Justicia americana) beds and in pockets of gravel between bedrock ledges in areas of swift current. This species, like other freshwater mussels, can bury itself below the substratum surface, but females have been observed to lie on top of the substratum while displaying and releasing glochidia.

Distribution

This species historically occurred throughout much of the Cumberlandian region of the Tennessee and Cumberland river drainages in Alabama, Kentucky, Tennessee, and Virginia; in 1918, Ortmann considered the species to be very abundant in the upper Tennessee River drainage. Much of the oyster mussel's historic range has been impounded by the Tennessee Valley Authority and the U.S. Army Corps of Engineers. Other populations were lost due to various forms of pollution and siltation.

Only five populations of the oyster mussel remain. The oyster mussel still survives within the Cumberland River as a very rare component of the benthic community in Buck Creek, Pulaski County, Kentucky; and it still occurs in a few miles of the Big South Fork Cumberland River in McCreary County, Kentucky and Scott County, Tennessee. The Tennessee River system populations are also small in size and few in number, occurring at 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.

Threats

The present populations are threatened by the adverse impacts of coal mining, poor land-use practices, and pollution, primarily from nonpoint sources. The Duck River population could be lost if the proposed Columbia Dam on the Duck River at Columbia, Tennessee is completed as presently planned. All the known populations are small and could be decimated by episodic events such as toxic chemical spills.

Conservation and Recovery

The oyster mussel is considered endangered by the States of Kentucky and Virginia, and by the Fish and Wildlife Service. It conservation requires the protection of the five stream reaches where it persists in small, isolated populations. These critical habitats must be protected from proposed impoundments, and from pollution associated with coal mining (including acid-mine drainage) and poor land-use practices in the watershed that cause erosion and inputs of pesticides and other chemicals. The populations of the rare mussel must be monitored, and research undertaken into its basic biology and ecological requirements.

Contacts

U. S. Fish and Wildlife Service
Asheville Ecological Services Field Office
160 Zillicoa Street
Asheville, North Carolina 28801-1082
Telephone: (828) 258-3939
Fax: (828) 258-5330

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 Canter Dr.
Hadley, Massachusetts 01035-9589
Telephone: (413) 253-8200
Fax: (413) 253-8200
http://www.northeast.fws.gov/

Reference

U.S. Fish and Wildlife Service. 10 Jan 1997. "Endangered and Threatened Wildlife and Plants; Determination of Endangered Status for the Cumberland Elktoe, Oyster Mussel, Cumberlandian Combshell, Purple Bean, and Rough Rabbitsfoot." Federal Register 62 (7): 1647-1658