Georgia Wildlife Web

Home Glossary Classification Conservation Status Regions of Georgia Fishes of Georgia Make a Donation

Species Description

Roanoke Slabshell

Elliptio roanokensis


Classification

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

Description

The Roanoke Slabshell is a large freshwater mussel that can grow to over 6.0 inches (152 mm) in length. Its shell is compressed or flattened and rather elongated in shape. The periostracum (outer shell surface) is yellow-brown to red-brown in color and darkens with age. Portions of the surface may also be decorated with narrow green rays, but these rays are less distinct in larger, older individuals.

Life Cycle

Some of the details about the complex life cycle of this mussel are not currently known, but the life history of Elliptio roanokensis is presumed to be similar to related species. Male Roanoke Slabshell mussels release sperm into creeks and 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. Parasitic glochidia must find and attach to the gills or fins of the appropriate host fish to complete development. The required host fish have not yet been identified for this species. The largest populations of Roanoke Slabshells are usually found downstream from impoundments, such as dams, and small populations are found in areas beyond barriers and dams. Due to this, there is some speculation that the primary host fish are anadromous. Anadromous fish species usually live within marine waters and migrate to freshwater systems to reproduce. These fish are only capable of traveling within free-flowing, non-impounded areas of rivers. These areas correspond to the locations of the largest populations of Roanoke Slabshells. It is believed that the small populations of Slabshells that are found beyond dams use a less effective, secondary fish host that does not migrate between the ocean and rivers. 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 sand or gravel bottom of a creek or river.

Natural History

Roanoke Slabshell l arvae (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 sand or gravel bottoms of moderate to fast-flowing creeks and rivers. Adult mussels are filter feeders and usually feed upon plankton and detritus from their aquatic environment. Roanoke Slabshell 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

The Roanoke Slabshell was historically found within Atlantic Slope drainages, from the Savannah River along the Georgia/South Carolina border to the Chowa River Basin in Virginia . Today, it has been nearly extirpated from the Chowa River and Roanoke River .

Conservation Status

The status of the Roanoke Slabshell is undetermined in Georgia . This species was once believed to be an abundant and dominant species within its Atlantic Slope drainages. It is still fairly abundant in portions of the Savannah River , but its distribution is greatly reduced and populations are presumably smaller than historic levels. This species has presumably been adversely affected by excess sedimentation, pollution and the construction of impoundments within its range. Impoundments are believed to restrict access and contact with anadromous host fish, causing Slabshells to use a less effective secondary host within portions of its range.

Similar Species

Elliptio species are often quite difficult to differentiate from each other. Elliptio roanokensis is more elongate and larger than many of the Elliptio species within its range in Georgia , but identification should probably be verified by a malacologist (an individual that studies mollusks).