Ovate Clubshell

Pleurobema perovatum

Description

The ovate clubshell is a small-to medium-sized mussel that rarely exceeds 2 in (5.1 cm) in length. The shell is ovate to elliptical and has nearly terminal inflated umbos (prominences). The posterior ridge is well-developed, broadly rounded, and often concave. The posterior slope is produced well beyond the posterior ridge. Periostracum (shell) color varies from yellow to dark brown and occasionally has broad green rays that may cover most of the umbo and posterior ridge. The inner layer of the shell is white. Due to the nearly terminal umbos in some specimens, P. perovatum may be mistaken for young southern clubshells (P. decisum ). They may be distinguished from the latter by their thinner shells and a gently sloping, well-developed posterior slope.

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, 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.

Habitat

The ovate clubshell inhabits high-quality lotic habitats (living in actively moving water) with stable gravel and sandy-gravel substrates. Little else is known about the habitat requirements of this species.

The habitat of the glochidia is initially in the gills of the female, then in the water column, and finally attached to a suitable host fish. Habitat associations or requirements for the juvenile stage are unknown.

Distribution

This species' historic distribution included the following portions of the Mobile River basin: the Tombigbee River and its tributaries (Buttahatchie and Sipsey Rivers; Luxapalila, Coalfire, and Lubbub Creeks); the Black Warrior River and its tributaries (Locust Fork; Village, Prairie, Big Prairie, Brushy, and Blackwater Creeks); the Alabama River; the Cahaba River and the tributary Buck Creek; Chewacla, Uphapee, and Opintlocco Creeks in the Tallapoosa drainage; and the Coosa River and its tributaries (Conasauga and Etowah Rivers; Holy Creek).

The ovate clubshell is now limited to (1) the Buttahatchie and Sipsey Rivers in the Tombigbee River drainage, (2) Blackwater Creek and Locust Fork in the Black Warrior drainage, and (3) Chewacla Creek in the Tallapoosa drainage. Specimens were last collected in the Coosa drainage in 1974 and in the Cahaba River in 1978. The species was not found in the Coosa River or Cahaba River drainages in a 1991 survey.

Threats

Habitat modification, sedimentation, and water quality degradation represent the major threats to this species. These freshwater mussels do not tolerate impoundments. More than 1,000 mi (1,609 km) of large and small river habitat in the Mobile River drainage has been impounded for navigation, flood control, water supply, and/or hydro-electric production purposes.

Impoundments adversely affect riverine mussels by (1) killing them during construction and dredging, (2) suffocating them with accumulated sediments, (3) lowering food and oxygen availability by reducing water flow, and (4) locally extirpating host fish. Other forms of habitat modification such as channelization, channel clearing and de-snagging, and gravel mining result in streambed scour and erosion, increased turbidity, reduction of groundwater levels, sedimentation, and changes in the aquatic community structure. Sedimentation may cause direct mortality by deposition and suffocation and eliminate or reduce recruitment of juvenile mussels. Suspended sediments can also interfere with feeding. Activities that historically caused sedimentation of streams and rivers in the drainages where this mussel occurs include channel modification, agriculture, forestry, mining, and industrial and residential development. In addition, stream discharge may result in decreased dissolved oxygen concentration, increased acidity and conductivity, and other changes in water chemistry that may impact the mussels and/or fish hosts.

About 230 river mi (370 km) of the Coosa River have been impounded for hydropower by a series of six dams. Water quality degradation caused by textile and carpet mill wastes led to the loss of several known mussel communities in several streams of this river system.

Water quality degradation is a major problem in the Cahaba River system. There are 10 municipal wastewater treatment plants, 35 surface mining areas, a coalbed methane operation, and 67 other permitted discharges in this river system. Siltation from surface mining, road construction, and oil and gas development is also a problem.

Conservation and Recovery

Actions needed for the recovery of freshwater mussels include (1) conducting population and habitat surveys to determine the status and range of the species, (2) determining specific threats to the species and minimizing or eliminating these threats, (3) identifying essential habitat areas in need of protection, and (4) controlling the incidental or illegal take of mussels by commercial and noncommercial collectors.

It is unlikely that the species will recover unless new populations are established by introducing individuals back into the historic range. Methods to accomplish this might include introduction of adult/juvenile mussels, glochidia-infected host fish, and artificially cultured individuals.

Contacts

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/

Ecological Services Field Office
P.O. Box 1190
Daphne, Alabama 36526-1190
Telephone: (334) 441-5181
Fax: (334) 441-6222

Ecological Services Field Office
446 Neal Street
Cookeville, Tennessee 38501-4027
Telephone: (931) 528-6481
Fax: (931) 528-7075

Reference

U.S. Fish and Wildlife Service. 17 March 1993. "Endangered and Threatened Wildlife and Plants; Endangered Status for Eight Freshwater Mussels and Threatened Status for Three Freshwater Mussels in the Mobile River Drainage." Federal Register 58 (50): 14330-14340.