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Species Description

Apalachicola Floater

Anodonta heardi


Classification

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

Description

The Apalachicola Floater is a medium-sized freshwater mussel that measures less than 4.4 inches (113 mm) in length. It is oval in shape and very inflated (the width and height of the shell are nearly identical). The periostracum (outer shell surface) is yellowish olive to light brown in color and may be marked with olive to brown-colored concentric bands. The nacre (inner shell surface) is white with a slight pinkish color.

Life Cycle

Many of the specific details about the complex life cycle of this imperiled mussel are not currently known, but the life history of Anodonta heardi is presumed to be similar to related species. Male Apalachicola Floater mussels release sperm into the shallow water of rivers with little or no 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 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 muddy bottom of shallow rivers.

Natural History

Many of the details about the natural history of the Apalachicola Floater 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 or buried in the muddy bottom of shallow rivers. Adult mussels are filter feeders and usually feed upon plankton and detritus from their aquatic environment. Apalachicola Floater 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

The Apalachicola Floater was described in 1995 from the Apalachicola River System of Florida and it has also recently been found in one tributary of the Flint River in Georgia and 1 tributary of the Chattahoochee River in Alabama .

Conservation Status

The Apalachicola Floater is listed as critically imperiled within Georgia because of its extreme rarity and very small range. This species is very susceptible to changes within its habitat (shallow rivers with muddy bottoms and little or no current). The factors that are believed to have contributed to the declining numbers of this species are pollution, habitat degradation, habitat loss due to construction of impoundments and introduction of exotic species, such as the Asiatic Clam (Corbicula fluminea) and Zebra Mussel (Dreissena polymorpha).

Similar Species

There are several species that are fairly similar in appearance to the very rare Apalachicola Floater, but the similar species do not have the combination of very inflated appearance, yellowish-olive to light brown periostracum and medium-length size.