Southern Combshell

Epioblasma penita

Description

The southern combshell, or penitent mussel, (Epioblasma penita ) is a bivalve mollusk about 2.1 in (5.3 cm) long. The rhomboid shell is yellowish, greenish-yellow, or tawny, sometimes with dark spots. The surface is characterized by irregular growth lines and a radially sculptured posterior. The inner shell surface (nacre) is white or straw-colored. Females have a large, grooved swelling at the rear of the shell. First described as Unio penitus in 1834, this species has been variously classified since then, most commonly as Dysnomia penita.

Behavior

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, diatoms, 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.

Habitat

The penitent mussel is found in the shallow reaches of larger streams and rivers where there is a moderately strong current. It prefers riffle runs or shoals with a stable substrate composed of sandy gravel or cobbles. Specimens have rarely been found in waters deeper than 2.3 ft (0.7 m).

Distribution

The species was known from the Tombigbee, Alabama, Buttahatchie, Cahaba, and Coosa Rivers in Alabama, Georgia, and Mississippi, all of which are part of the Mobile River basin. In the Tombigbee River, it was found from the confluence of Bull Mountain Creek above Amory, Mississippi, downstream to Epes, Alabama. The Alabama River supported populations at Claiborne and Selma. A population was known from the stretch of the Cahaba River below Centreville, Alabama.

The penitent mussel has not been collected from the Alabama and Cahaba Rivers since the 1800s. It was last collected from the mainstream Tombigbee River in 1972. It disappeared from the Coosa River after 1974, when a new dam inundated its habitat there.

The penitent mussel survives in the Gainesville Bendway of the Tombigbee River (Sumter County, Alabama). This cut-off section of the river provides only marginal habitat, which is subject to siltation, reduced water flows, and degraded water quality. The species also survives in sections of the Buttahatchie River and the East Fork Tombigbee River.

All mussel populations in the lower reaches of the Buttahatchie River have declined dramatically. More than 1,200 individual mussels in the family Union-idae, including 92 specimens of E. penita, were sampled from two reaches in the lower portion of the river in 1977, but in 1989 only 75 unionids (including 3 penitent mussels) were found. Fortunately, the numbers of penitent mussels collected from two reaches immediately above the gravel mines approached or exceeded the 1977 survey in 1989.

Threats

The southern combshell has declined because of loss of habitat. The once free-flowing Tombigbee River has been modified into a series of locks and channels to form a barge canal. Physical destruction of mollusks during dredging and construction and the resulting increase of siltation, reduction of water flow, and disturbance of host fish movements have all but eliminated the mollusk from this river. Further siltation in the Gainesville Bendway would probably eliminate the southern combshell there.

Bull Mountain Creek contributes nearly half of the flow of the East Fork of the Tombigbee River. During canal construction, the creek was diverted and its cool waters redirected into the warm canal, resulting in warmer water temperatures in the East Fork. Warmer waters stress the mussels and diminish their food supply. Changes in current and river flow interfere with reproduction, since this species depends on currents to carry mature ga-metes downstream to other mussels for fertilization. Mussel beds also suffer from degraded water quality from the runoff of fertilizers and pesticides.

Impoundment of the Tombigbee River in Mississippi and Alabama for the Tennessee-Tombigbee Waterway has affected the lower 1.9 mi (3.1 km) of the Buttahatchie River by reducing the current and scouring capacity of the river. Abandoned kaolin (clay) mines and abandoned and active gravel mines also appear to be causing problems. Above the portion of the river affected by the impoundment of the Tombigbee River, the Buttahatchie River channel has broken into gravel mines excavated adjacent to the channel, eroding former riffle areas and creating ponds. Farther north, an estimated 17,000 tons (12,690.2 metric tons) of sediment are eroding annually from abandoned kaolin mines and moving through the Buttahatchie system.

Conservation and Recovery

Both Mississippi and Alabama require permits to take freshwater mussels. It is difficult to detect and apprehend violators, however, and regulations do not prevent habitat degradation. Recovery of the penitent mussel will require developing some means to ensure adequate water flow in remaining, unmodified sections of the river. Flood control projects and canal maintenance activities will need to consider the presence of mussel beds.

The range of the penitent mussel is threatened by two proposed channel improvement projects. If implemented, these projects would dredge and straighten 59 mi (95 km) of the Buttahatchie and 53 mi (85 km) of the East Fork.

Contact

U.S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
1875 Century Blvd., Suite 200
Atlanta, Georgia 30345
Telephone: (404) 679-4000
http://southeast.fws.gov/

References

Clench, W.J., and R.D. Turner. 1956. "Freshwater Mollusks of Alabama, Georgia, and Florida from the Escambia to the Suwannee River." Bulletin of the Florida State Museum (Biological Sciences) 1: 97-239.

Johnson, R.I. 1978. "Systematics and Zoogeography of Plagiola, an Almost Extinct Genus of Freshwater Mussels." Bulletin of the Museum of Comparative Zoology 148: 239-320.

Stansbery, D.H. 1976. "Naiade Mollusks." In H.Boschung, ed., Bulletin of the Alabama Museum of Natural History no. 2.