|Listed||April 7, 1987|
|Family||Unionidae (Freshwater Mussel)|
|Description||Greenish-brown subtriangular shell.|
|Habitat||Sand and gravel substrate in flowing rivers.|
|Reproduction||Female stores sperm in gills; glochidia (larvae) are released into streams after hatching.|
|Threats||Low numbers; restricted range; dredging; siltation.|
The shell of the black clubshell (Pleurobema curtum ) is about 2 in (5.1 cm) long and varies in color from light green in young mussels to a dark greenish-brown in older ones. The shell is subtriangular and inflated in front. The thin inner shell surface is an iridescent bluish-white. This species was first classified as Unio curtus.
The identity of species within the genus Pleurobema is currently the focus of debate among malacologists. The U.S. Fish and Wildlife Service (FWS) has adopted the majority view but acknowledges that further research may warrant reclassification of the black clubshell and other mussels of the genus.
The life of mussels is complex, and reproduction often depends upon a stable habitat—unaltered stream conditions, clean water, and an undisturbed stream bottom. The cycle also depends upon the abundance of suitable fish hosts to complete the mussel's larval development.
To reproduce, males discharge sperm, which are dispersed by stream currents. In the process of feeding, females nearby or downstream take in sperm, which fertilizes eggs stored in their gills. The gills serve as brood pouches (marsupia), where the glochidia hatch and begin to develop. After a time, these glochidia are released into the stream. A few mussels have inner parts that resemble a tiny minnow and can be manipulated to lure host fish. When a fish gets close to the shell, the mussel expels its glochidia.
Glochidia have tiny bean-or spoon-shaped valves that attach to the gill filaments of host fish. Glochidia can only progress to the juvenile stage while attached to the fish's gills. Those that do not fortuitously encounter a host fish do not survive when released by the female mussel. They sink to the bottom and die.
When the juvenile has developed a shell and is large enough to survive on its own, it detaches from the host fish and falls to the stream bottom, beginning a long association with a single stretch of stream. Maturing mussels bury themselves in riffles and shoals with only the shell margins and feeding siphons exposed to the water. Some mussels live as long as 50 years or more.
The family Unionidae, which includes all of the freshwater mussels in the United States, is separated into two groups based on the length of time the glochidia remain in the female's marsupia. The eggs of the short-term (tachytictic) breeders are fertilized in the spring, and glochidia are released by late summer of the same year. Long-term (bradytictic) breeders hold developing glochidia in the brood pouch over winter and release them in the spring.
Freshwater mussels feed by siphoning phytoplankton and other plant matter from the water. Indigestible particles are expelled from the shell by reverse siphoning. Silt in the water can kill mussels by clogging their feeding siphons.
There are no known interspecific differences in feeding among freshwater mussels. The glochidia are obligate parasites on the gills or fins of fish. Adult mussels are filter-feeders and consume particulate matter in the water column. Identifiable stomach contents almost invariably include desmids, di-atoms, algae, protozoa, and zooplankton.
Most freshwater mussel species display seasonal variations in activity associated with water temperature and reproduction. Metabolic rate is, in part, positively correlated with temperature. Many ectothermic species have the capacity to adjust their metabolic rates in response to long-term changes in temperature. Thus, metabolic rates do not continue to rise as temperatures rise in the summer, and they do not continue to fall during the winter as temperatures decline.
Some freshwater mussels also show diurnal changes in metabolic rates that indicate a tendency toward nocturnal activity patterns. Mussels may move to the surface to feed at night and move deeper into the substrate during the day; this is one way to avoid predators that hunt by visual contact.
Freshwater mussels are nonmigratory.
The black clubshell is found in clean, swift-flowing rivers where the bottom is formed of firm rubble, gravel, or sand. This mussel prefers shallow riffles and shoals, where the current is strong enough to keep the bottom scoured of silt.
The black clubshell has been found in the East Fork and mainstream of the Tombigbee River and has been collected from only five locations. Reports of this species from the Big Black River in Mississippi are probably erroneous.
The species is thought to survive in an unmodified segment of the East Fork Tombigbee River (Itwamba and Monroe Counties, Mississippi). Only two living specimens have been found since 1974. An extensive survey of the river conducted in 1987 failed to turn up any living or recently dead specimens. The FWS believes that the black clubshell mussel survives but that its population is critically low.
When the Tennessee-Tombigbee Waterway was constructed 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 new 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.
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 this portion of the river has become more severe since the mid-1990s and may already be smothering the surviving mussel beds. The riverbeds are also exposed to continuing runoff of fertilizers and pesticides, which can produce stream eutrophication. Filter-feeders such as mussels ingest these chemicals, which alters their siphoning period and metabolic rate.
The black clubshell may also be adversely affected by the loss of its fish host as a result of habitat alteration.
Conservation and Recovery
Under provisions of the Endangered Species Act, federal agencies are required to consult with the FWS to ensure that any actions they authorize or fund do not jeopardize an endangered species. This rule affects current and proposed flood control and navigation projects sponsored by the Army Corps of Engineers and watershed projects proposed by 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.
Recovery of the black clubshell would require construction of sediment basins and selective dredging to limit siltation.
U. S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
1875 Century Blvd., Suite 200
Atlanta, Georgia 30345
Telephone: (404) 679-4000
Stansbery, D.H. 1983. "The Status of Pleurobema curtum. " Unpublished Report. U.S. Fish and Wildlife Service, Atlanta.