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Species Description

Flat Floater

Anodonta suborbiculata


Classification

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

Description

The Flat Floater is a large freshwater mussel. Older specimens can measure up to 8.0 inches (203 mm) in length. Its shell is nearly circular in shape and rather compressed or flattened in appearance. The distinctive umbos (rounded, often raised structures on shell surface that are also called beaks) are low and flattened. The smooth and shiny periostracum (outer shell surface) is light yellow to dark brown and is sometimes marked with fine green rays. The nacre (inner surface of the shell) is white.

Life Cycle

Some of the details about the complex life cycle of this mussel are not currently known, but the life history of Anodonta suborbiculata is presumed to be similar to related species. Male Flat Floater mussels release sperm into the slow current of creeks, lakes and backwater areas of rivers. 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 is believed to be bradytictic (a long-term brooder). Bradytictic species usually spawn during the summer and release mature glochidia during late winter or spring. Parasitic glochidia must find and attach to the gills or fins of the appropriate host fish to complete development. Recent laboratory studies have determined that there are several suitable hosts for the widely distributed Flat Floater. The Golden Shiner (Notemigonus crysoleucas), Warmouth (Lepomis gulosus), White Crappie (Pomoxis annularis) and Largemouth Bass (Micropterus salmoides) are all suitable hosts. The glochidia parasitize a fish host for a variable length of time, likely depending upon water temperature, fish species and other factors. Larvae transform into juvenile mussels on the fish and then release from the host to find a suitable substrate, often the muddy bottom of slow-moving creeks and backwater areas of large rivers.

Natural History

Flat Floater l arvae (glochidia) are parasitic upon tissue of fish hosts while completing the metamorphosis into juvenile mussels. This freshwater mussel can grow quite rapidly and sometimes grows to more than 4 inches (100 mm) in length during its first two years. Despite being called the Flat Floater, adult mussels are typically sessile and are found attached or buried in the mud bottoms of lakes, creeks and backwater areas with slow currents. This species is often found in water that is less than 3 feet in depth and sometimes thrives in impounded areas. Adult mussels are filter feeders and usually feed upon plankton and detritus from their aquatic environment. Flat 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 its gills and food particles are trapped and eventually digested.

Range

Historically, this species was found in Gulf Coast drainages from the Trinity River in Texas to the Escambia River system of Alabama and Florida . It was also found in the Mississippi River Basin from the Gulf Coast up to Indiana . Its range has recently expanded into portions of Georgia , Tennessee and northern portions of the Midwest . It has recently expanded its range in northwestern Georgia , near the Georgia/Alabama border.

Conservation Status

The Flat Floater is listed as Rare and Imperiled in Georgia . However, this species has apparently only recently been established in the state and may eventually become more widely distributed. The range of the Flat Floater has expanded with the construction of impoundments, which have created areas with slower currents and mud substrates. The distribution of this species is rather sporadic, but it is often quite abundant when it is found.

Similar Species

Within its narrow range in Georgia , the Flat Floater is rather distinct. Its combination of large size, low umbos and circular shell distinguishes it from most other freshwater mussels in creeks and backwaters.