Georgia Wildlife Web

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

Species Description

Threehorn Wartyback

Obliquaria reflexa


Classification

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

Description

The Threehorn Wartyback is a small freshwater mussel that usually measures less than 3.0 inches (76 mm) in length. The shell has a rounded and inflated or swollen shape. The distinctive shell surface is marked with 3 large, elevated knobs or "horns." The periostracum (outer shell surface) is yellowish-brown to green and becomes dark brown in older specimens. The surface is also usually marked with numerous fine, dark green rays. The nacre (inner surface of the shell) is pearly white.

Life Cycle

Many of the specific details about the complex life cycle of this rare mussel are not currently known, but the life history of Obliquaria reflexa is presumed to be similar to related species. Male Threehorn Wartyback mussels release sperm into large rivers or impoundments 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. This species is considered to be tachytictic, which means that spawning typically occurs in the spring and glochidia are released during June, July and August. There is some speculation that metamorphosis may take place without parasitism and this species may not require a fish host to complete metamorphosis from glochidia to a juvenile mussel. If this species does require a fish host, the parasitic glochidia must find and attach to the gills or fins of the appropriate host fish to complete development. However, the host species for the Threehorn Wartyback are not known at this time. The glochidia parasitize a fish host for a variable length of time, likely depending upon water temperature, fish species and other factors. Once transformed into juvenile mussels, the young mussels must find a suitable substrate, often the gravel, mud or sand bottom of a large river.

Natural History

Many of the details about the natural history of the Threehorn Wartyback are not currently known, but they are believed to be similar to better known, related species. There is some speculation that this species may not require a fish host to transform into a juvenile mussel. However; if a host fish is required, larvae (glochidia) are parasitic upon tissue of fish hosts while completing metamorphosis. Adult mussels are typically sessile and are found attached or buried within the sand, mud or gravel bottom of rivers. The Threehorn Wartyback is usually found in water that is only 4-6 feet deep, but it can be found in water up to 20 feet in depth. It can also be abundant in impoundment areas with little or no current. Adult mussels are filter feeders and usually feed upon plankton and detritus from their aquatic environment. Threehorn Wartyback 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

Obliquaria reflexa has a very broad range. Within the southeastern U.S. , it is found in the Coosa and Alabama River systems. It is also found in Mississippi River drainages throughout the Midwest . The range also extends into Lake Erie tributaries in Canada . Within northwestern Georgia , it can be found in Coosa River drainages.

Conservation Status

The Threehorn Wartyback is Rare and Imperiled within Georgia . Although it can tolerate impoundments and can be abundant within these areas, it has presumably been affected by habitat degradation, pollution and excess sedimentation like many freshwater mussels.

Similar Species

Within Georgia , Obliquaria reflexa is rather distinct because of the three large "horns" or knobs on its shell surface.