Purple Bean

Villosa perpurpurea

Description

The purple bean mussel (Villosa perpurpurea ), described by Lea 1861, has a small-to medium-sized shell. The shell's outer surface is usually dark brown to black with numerous closely spaced fine green rays. The inside of the shell is purple, but the purple may fade to white in dead specimens. Villosa perpurpurea most closely resembles V. trabalis. The most obvious difference is the purple nacre of the former in comparison to the white nacre of the latter. However, this character is somewhat variable and the purple color may fade rapidly in dead specimens. With regards to other shell characters, V. perpurpurea tends to be more compressed, thinner, slightly broader, the beak is less developed, and the emargination of the ventral margin in female shells is not as pronounced. The base color of the periostracum in V. trabalis is greenish. The perpurpurea is less exaggerated in its particular characters than V. trabalis. The glochidia of the two species are also shaped differently. V. vanuxemii (V. v. vanuxemensis ) may be sympatric with perpurpurea but it tends to be a bit larger. In V. vanuxemii, the nacre is shiny purple but tends to be reddish or brownish in the area of the beak cavity and may be lighter around the periphery of the shell, the base color of the periostracum is brown, and raying is rather obscure. Female shells are strongly truncated, often with a distinct notch just ventral to the terminus of the posterior ridge which runs approximately parallel to the dorsal margin.

Behavior

The purple bean, another lampsiline species, appears to be bradytictic, as gravid females have been observed in January. Three host fish species have been identified—the fantail darter (Etheostoma fiabellare), greenside darter, and mottled sculpin (Cottus bairdi) and/or banded sculpin. Transformation took from 11 to 25 days, at 70.7 to 76.1 F.

Habitat

This species inhabits small headwater streams to medium-sized rivers. It is found in moderate to fast-flowing riffles with sand, gavel, and cobble substrata and rarely occurs in pools or slack water. It is sometimes found out of the main current adjacent to water-willow beds and under flat rocks.

Distribution

The purple bean historically occupied the upper Tennessee River basin in Tennessee and Virginia upstream of the confluence of the Clinch River. In 1918 it was considered "not rare" in Virginia. The purple bean populations in the lower Clinch, Powell, and Holston Rivers were extirpated by reservoirs; now only three populations remain.

The purple bean now survives in limited numbers at a few locations. In Virginia, it occurs in the upper Clinch River basin in Scott, Tazewell, and Russell Counties; as well as in the Clinch River tributaries of Copper Creek in Scott County and Indian Creek in Tazewell County. This last location is the same reach of river where the federally listed tan riffleshell mussel has also been found. In Tennessee, the purple bean occurs in the Obed River in Cumberland and Morgan Counties, in the Emory River just below its confluence with the Obed River in Morgan County, and Beech Creek in Hawkins County.

Threats

This taxon likely declined throughout its range until its current meager numbers due to the deleterious effects of reservoir construction, coal mining, poor land-use practices, and non-point pollution. These are still the primary threats to its survival. The population centers that remain are so limited that they are very vulnerable to random events such as toxic chemical spills.

Conservation and Recovery

The purple bean is considered endangered by the States of Tennessee and Virginia, as well as at the federal level. Its most pressing conservation need is the prevention of further deterioration of its limited areas of critical habitat, and the mitigation of existing stresses where possible. Specific actions include the prevention of: harvesting of specimens for commercial sale; destruction or alteration of the known habitat by dredging, channelization, or the discharge of fill material; significant habitat dewatering by water withdrawal; threatening discharges of toxic chemicals, organic matter, or other pollutants; or reservoir construction. The known populations of the purple bean should be monitored for size and the occurrence of reproduction, and research should be undertaken into its ecological needs, with a view of designing management practices to maintain and improve its habitat.

Contacts

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 Center Dr.
Hadley, Massachusetts 01035-9589
Telephone: (413) 253-8200
Fax: (413) 253-8308
http://www.northeast.fws.gov/

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

Reference

U. S. Fish and Wildlife Service. 10 January 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.