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Species Description

Southern Clubshell

Pleurobema decisum


Classification

Phylum: Mollusca
Subphylum:
Class: Bivalvia
Order: Unionoida
Family: Unionidae

Description

The Southern Clubshell is a medium-sized freshwater mussel that averages about 2.8 inches (70 mm) in length. This thick-shelled species is more or less rectangular in shape and the periostracum (outer shell surface) is yellow to yellow-brown in color. Young specimens have green rays or spots on portions of the outer shell surface.

Life Cycle

Some of the specific details about the complex life cycle of Pleurobema decisum are not currently known, but the life history of this endangered mussel is presumed to be similar to related species. Male Southern Clubshell mussels release sperm into the shallow water of streams and rivers with moderate currents. Sperm enters females through siphon-like regions and fertilization of eggs occurs within female shells. These fertilized eggs develop into special larva called glochidia. Glochidia continue to develop and are released into the water column when fully matured. The parasitic glochidia must find and attach to the gills or fins of the appropriate host fish to complete development. Recent studies have determined that females release glochidia in the form of conglutinates. A conglutinate is a mass of glochidia that resembles a fish or another organism that is used to attract fish. Fish will be attracted to this lure and this increases the probability that fish will come in contact or consume the mass of larvae, thus allowing the glochidia to attach to the gills of the proper host fish. Recent research suggests that the required fish host for the Southern Clubshell is the Blacktail Shiner (Cyprinella venusta). The glochidia parasitize the fish host for a variable length of time, likely depending upon water temperature, fish species and other factors. Larvae metamorphose into juvenile mussels on the fish and then release from the host to find a suitable substrate, usually the gravel or sandy bottoms of streams and rivers with moderate currents.

Natural History

Many of the details about the natural history of the Southern Clubshell are not currently known, but they are believed to be similar to better known, related species. Larvae (glochidia) are parasitic upon tissue of fish hosts while completing the metamorphosis into juvenile mussels. Adult mussels are typically sessile and are found attached to the sand or gravel bottom of moderately flowing rivers. Adult mussels are filter feeders and usually feed upon plankton and detritus from their aquatic environment. Southern Clubshell mussels bring water from their habitat into their shells through specialized regions that are similar to the true siphons of clams. The water is then filtered over the gills and food particles are trapped and eventually digested.

Range

Historically, the Southern Clubshell was found in nearly every major stream and river of the Mobile River Basin region of Georgia , Alabama and Mississippi . This region included the Alabama River , Black Warrior River and the Coosa River and its tributaries. However, today it has been eliminated or greatly reduced from much of its range. Within Georgia , it is only found in the Conasauga and Coosa Rivers of Gordon, Murray and Whitfield counties.

Conservation Status

The Southern Clubshell is currently listed as endangered by both state and federal agencies. This once widespread species has been greatly reduced or eliminated from its historical range. Recent surveys suggest that Pleurobema decisum is limited to about 6 viable populations within its range. Like many of the endangered freshwater mussels, the Southern Clubshell is highly susceptible to habitat change. The primary factors that are believed to have contributed to the decline of this species are pollution, sedimentation and habitat degradation.

Similar Species

The most similar species to the Southern Clubshell is the Black Clubshell (Pleurobema curtum). However, this endangered mussel species does not occur within the Georgia portion of the limited range of the Southern Clubshell .