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Species Description

Brook Floater

Alasmidonta varicose


Classification

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

Description

The Brook Floater is a small freshwater mussel that measures less than 2.8 inches (70 mm) in length. The shell is kidney-shaped and the outer surface is marked with numerous wavy ridges that give it a rough texture. The periostracum (outer shell surface) of adult specimens is brownish with dark green rays, whereas juvenile specimens are yellowish with green rays. The nacre (inner shell surface) is glossy, bluish-white in appearance, especially near the margins of the shell.

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 Alasmidonta varicose is presumed to be similar to related species. Male Brook Floater mussels release sperm into clean, swiftly flowing rivers and streams. 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. This species has been found to be bradytictic. This means that spawning usually occurs during the summer and glochidia are released the following spring. When released, the parasitic glochidia must find and attach to the gills or fins of the appropriate host fish to complete development. Recent studies have determined that the suitable fish hosts for parasitic Brook Floater glochidia are the Golden Shiner (Notemigonus crysoleucas), Margined Madtom (Noturus insignis) and the Pumpkinseed (Lepomis gibbosus). 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 gravel or sandy gravel bottoms of swiftly-flowing rivers.

Natural History

Many of the details about the natural history of the Brook 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 to in the gravel or sandy-gravel bottom of clean streams and rivers. Adult mussels are filter feeders and usually feed upon plankton and detritus from their aquatic environment. Brook 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

Historically, the Brook Floater was found throughout Atlantic Slope river basins from the Savannah River along the Georgia/South Carolina border northward to the St. Lawrence River basin in Canada . However; the current distribution is quite spotty, especially in the southern portion of the range. Today it is rarely found within the Savannah River basin along the Georgia/South Carolina border at isolated locations.

Conservation Status

The Brook Floater is listed as Imperiled in Georgia because it is rare within its limited range within the state. There have been significant declines throughout much of the broad range of the Brook Floater, especially within the southern portion of the range where the distribution is very spotty. The primary factors that are believed to have contributed to the decline of this mussel species are: pollution, habitat destruction and degradation, sedimentation and introduction of exotic species; such as the Asiatic Clam (Corbicula fluminea) and the Zebra Mussel (Dreissena polymorpha).

Similar Species

Within the very limited Georgia portion of the Brook Floater's range, there is not another mussel that possesses wavy ridges, kidney shape and a bluish-white nacre.