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Species Description

Southern Pigtoe

Pleurobema georgianum


Classification

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

Description

This small to medium-sized freshwater mussel usually measures less than 2.4 inches (60 mm) in length. The Southern Pigtoe is oval to elliptical in shape with a yellow to yellow-brown periostracum (outer shell surface). The periostracum is often marked with numerous dark brown growth lines and smaller specimens may also have green spots. The nacre (inner shell surface) is white in color.

Life Cycle

Many of the specific details about the complex life cycle of this endangered mussel are not currently known, but the life history of Pleurobema georgianum is presumed to be similar to related species. Male Southern Pigtoe mussels release sperm into the high quality water of creeks and rivers. 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. The required fish hosts are not currently known for this species. The glochidia parasitize a 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, often the sand or gravel bottoms of high quality creeks and rivers.

Natural History

Many of the details about the natural history of the Southern Pigtoe 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 clean rivers and creeks. Adult mussels are filter feeders and usually feed upon plankton and detritus from their aquatic environment. Southern Pigtoe 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, this mussel was found throughout the Coosa River tributaries and drainage of Georgia , Alabama and Tennessee . However, recent surveys did not find the Southern Pigtoe throughout most of its historic range and only a small handful of specimens were found within the Conasauga River .

Conservation Status

The Southern Pigtoe is currently listed as endangered by both state and federal agencies. Like most freshwater mussels, it is highly susceptible to habitat changes. Pollution, sedimentation and habitat degradation are believed to be the primary factors for the declining numbers of Southern Pigtoe mussels throughout its limited range.

Similar Species

The most similar species to the Southern Pigtoe is the Alabama Clubshell (Pleurobema troschelianum). The two species are quite difficult to differentiate due to very similar shell characteristics. However, the Alabama Clubshell appears to likely be extirpated in Georgia because only dead specimens have been found in recent years.