Medionidus simsponianus is a small freshwater mussel with a slightly elongated shell that usually measures less than 2.2 inches (55 mm) in length. The Ochlockonee Moccasinshell has a broadly curved ventral margin and a posterior ridge that is heavily marked with irregular ridges. The outer shell surface (periostracum) is light brown to yellow green in color and sculptured with dark green rays. The nacre (inner shell surface) is bluish white in appearance.
Many of the specific details about the complex life cycle of this endangered mussel are not currently known, but the life history of Medionidus simpsonianus is presumed to be similar to related species. Male Ochlockonee Moccasinshells release sperm into the water column of rivers with a moderate current. 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 the fish host(s) for a variable length of time, likely depending upon water temperature, fish species and other factors. The larvae metamorphose into juvenile mussels on the fish and then release from the host to find a suitable substrate, often the sandy or rocky bottom of rivers with a moderate current.
Many of the details about the natural history of the Ochlockonee Moccasinshell 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 to the sandy or rocky bottom of slight to moderate-flowing rivers. Adult mussels are filter feeders and usually feed upon plankton and detritus from their aquatic environment. Ochlockonee Moccasinshell 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.
Historically, this endangered mussel was found at a number of sites within the Ochlockonee River system of Georgia and Florida . However, today its range within the Ochlockonee River is greatly reduced and in Georgia it is limited to a small number of sites in Grady and Thomas counties.
The Ochlockonee Moccasinshell is currently listed as endangered within its greatly reduced range by state and federal agencies. Like many freshwater mussels, the Ochlockonee Moccasinshell is highly sensitive to changes within its habitat. Due primarily to sedimentation, pollution, introduction of the Asiatic Clam (Corbicula sp.) and habitat degradation through the construction of impoundments, this small mussel is one of the rarest species in the eastern Gulf region.
There are no similar species within the very limited range of the Ochlockonee Moccasinshell.