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Species Description

Southern Acornshell

Epioblasma othcaloogensis


Classification

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

Description

The Southern Acornshell is a small freshwater mussel that rarely exceeds 1.2 inches (30 mm) in length. It has a round to oval shape and is sexually dimorphic. Female Southern Acornshell mussels have a distinctive, swollen posterior ridge that can be used to differentiate females from males. The periostracum (outer shell surface) is shiny, smooth and yellow 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 Epioblasma othcaloogensis is presumed to be similar to related species. Male Southern Acornshell mussels release sperm into the shallow water of streams and rivers with moderate to strong 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. 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 fine gravel bottoms of streams with moderate to strong currents.

Natural History

Many of the details about the natural history of the Southern Acornshell 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 fine gravel bottom of moderate to strong flowing rivers. Adult mussels are filter feeders and usually feed upon plankton and detritus from their aquatic environment. Southern Acornshell 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, Epioblasma othcaloogensis was found in streams of the Coosa River system and Conasauga River of Georgia, Alabama and Tennessee . However, when it was last collected in 1974, it was only found within a small portion of the Upper Coosa River drainage in Georgia and Alabama . Recent surveys did not confirm the presence of the Southern Acornshell within its limited range and it is possibly extinct.

Conservation Status

The Southern Acornshell is currently listed as endangered by both state and federal agencies. However, recent surveys have not confirmed the presence of the Southern Acornshell within its historic range and it may possibly be extinct. Potentially suitable habitat still exists within the Upper Coosa River , offering hope that it may be found in future surveys. Like many endangered freshwater mussels, Epioblasma othcaloogensis is highly susceptible to changes within its habitat. Sedimentation, pollution and habitat degradation are believed to be the primary factors that have contributed to its current status.

Similar Species

The Upland Combshell (Epioblasma metastriata) and Southern Combshell (Epioblasma penita) are the freshwater mussels most likely to be confused with the endangered Southern Acornshell . The Southern Acornshell can be distinguished from the similar species by its smaller size and smooth yellow shell.