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Species Description

Coosa Moccasinshell

Medionidus parvulus


Classification

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

Description

This small freshwater mussel has a fairly elongate shell that rarely measures more than 1.6 inches (40 mm) in length. The periostracum (outer shell surface) is yellowish brown to dark brown in color with delicate green rays. The nacre (inner shell surface) is blue in appearance and occasionally is marked with salmon-colored spots. The Coosa Moccasinshell also has a distinctive, highly rounded posterior ridge.

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 parvulus is presumed to be similar to related species. Male Coosa Moccasinshells release sperm into the water column of moderately flowing waters. Sperm enters females through siphon-like regions and fertilization of eggs occurs. 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 parvulus parasitizes Black-banded Darters and other species of 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.

Natural History

Many of the details about the natural history of Coosa Moccasinshells are not currently known, but they are presumed to be similar to better known, related species. Larvae (glochidia) are parasitic upon fish hosts while completing the metamorphosis to juvenile mussels. Adult mussels are typically sessile and are often found completely buried in the stream bottom. They prefer to attach to stable gravel or sandy-gravel substrates within their clean, moderately flowing water environments. They are filter feeders and usually feed upon plankton and detritus from their aquatic environment. Coosa Moccasinshells 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. Water is then pushed back out another siphon-like region.

Range

Medionidus parvulus can only be found within a few moderately flowing streams and rivers of Georgia , Alabama and Tennessee . In Georgia , it has been found within portions of the Consasuaga and Chattooga Rivers of Gordon, Murray, Chattooga and Whitfield counties.

Conservation Status

The Coosa Moccasinshell is listed as endangered by both state and federal agencies. Like many freshwater mussels, this species is highly susceptible to changes within its habitat. Sedimentation, pollution and habitat degradation are believed to have highly contributed to the dwindling population size of the Coosa Moccasinshell.

Similar Species

The threatened Alabama Moccasinshell (Medionidus acutissimus) is the most similar freshwater mussel found within the same range as the Coosa Moccasinshell. The Coosa Moccasinshell (Medionidus parvulus) can be distinguished by its larger size, rounded posterior ridge and blue inner shell color.