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Species Description

Gulf Moccasinshell

Medionidus penicillatus


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


This small freshwater mussel has a fairly elongate and inflated shell that measures less than 2.2 inches (55 mm) in length. The yellowish to greenish-brown periostracum (outer shell surface) is marked with fine, interrupted green rays. The inner shell surface (nacre) is dark purple or greenish.

Life Cycle

Many of the specific details about the life cycle of this endangered mussel are not currently known, but the life history of Medionidus penicillatus is presumed to be similar to related species. Male Gulf Moccasinshells release sperm into the water column of medium-sized streams and large rivers with 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. Glochidia must find and attach to the gills or fins of the appropriate host fish to complete development. Recent research suggests that Medionidus penicillatus may parasitize Blackbanded Darters and Brown Darters for varying lengths 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 gravel or sandy bottoms of medium-sized streams and large rivers within its limited range.

Natural History

Many of the details about the natural history of the Gulf Moccasinshell are not currently known, but they are presumed 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 gravel bottom of slight to moderate-flowing streams and rivers. Adult mussels are filter feeders and usually feed upon plankton and detritus from their aquatic environment. Gulf 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, Gulf Moccasinshells were found at a number of sites within the Appalachicola-Chattahoochee-Flint (ACF) River System of Georgia, Florida and Alabama . However, today the Gulf Moccasinshell is only found at a few sites within Georgia and Florida , including a number of sites within the Flint and Chattahoochee Rivers of Georgia. Recent surveys suggest that Medionidus penicillatus is likely extirpated in Alabama .

Conservation Status

The Gulf Moccasinshell is listed as endangered by state and federal agencies. Recent surveys at several sites within its limited range have found drastic declines in the population size of Medionidus penicillatus and it is now considered one of the rarest mussels within the ACF (Appalachicola-Chattahoochee-Flint) River Basin. Several factors are believed to have contributed to the diminishing numbers of Gulf Moccasinshells, including sedimentation, decline in required fish hosts and habitat degradation.

Similar Species

There is not another freshwater mussel that occurs within the same limited range of the Gulf Moccasinshell that possesses the same yellowish or greenish-brown shell with fine green rays and dark purple or greenish nacre.