Georgia Wildlife Web

Home Glossary Classification Conservation Status Regions of Georgia Fishes of Georgia Make a Donation

Species Description

Oval Pigtoe

Pleurobema pyriforme


Classification

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

Description

This small-sized freshwater mussel is highly variable in appearance and infrequently measures more than 2.4 inches (60 mm) in length. The Oval Pigtoe varies from a compressed, yellow form to an inflated, dark brown form, but it usually has distinct growth lines. The nacre (inner shell surface) is also highly variable and can be either white or salmon in appearance.

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 pyriforme is presumed to be similar to related species. Male Oval Pigtoes release sperm into the water column of rivers with a slow to moderate current flow. 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 research suggests that Pleurobema pyriforme parasitizes the Sailfin Shiner (Pteronotropis hypselopterus) and Eastern Mosquitofish (Gambusia holbrooki) for varying lengths 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 muddy, sandy or gravel bottom of rivers with slow to moderate currents.

Natural History

Many of the details about the natural history of the Oval 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 often found attached or buried within the sandy, muddy or rocky bottom of slight to moderately flowing rivers. Adult mussels are filter feeders and usually feed upon plankton and detritus from their aquatic environment. Oval 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, the Oval Pigtoe was one of the most common and widely distributed freshwater mussels within the Appalachicola-Chattahoochee-Flint (ACF) River system of Georgia , Florida and Alabama . However, it apparently has been extirpated from two-thirds of its historic range. Today it can be found in smaller numbers within the Flint , Ochlockonee and Chattahoochee Rivers in the southwestern and central sections of Georgia .

Conservation Status

The Oval Pigtoe is currently listed as endangered by both federal and state agencies. Like many freshwater mussels, the Oval Pigtoe is extremely sensitive to changes within its habitat (muddy, sandy or gravel-bottomed rivers with a slow to medium current). Due primarily to habitat degradation, pollution and the introduction of the Asiatic Clam (Corbicula sp.), this once common mussel has been extirpated from approximately two-thirds of its historical range.

Similar Species

There are no similar small-sized freshwater mussels within the limited range of the Oval Pigtoe that possess the distinct growth lines, white or salmon nacre and yellow or dark brown outer shell color.