Stirrupshell


Description and biology

The stirrupshell is a small freshwater mussel with a shell measuring 2.2 inches (5.6 centimeters) long. Its yellowish-green shell is marked with a pattern of zig–zag lines. These lines are light green on young stirrupshells and dark brown on older ones. The inner surface of the shell is silvery white.

Like other North American freshwater mussels, the stirrupshell must have a river habitat that is clean and undisturbed. The stirrupshell eats plant material it removes from the water by a feeding tube called a siphon.

Any silt (mineral particles) or sediment (sand and stone) in the water can pose a serious threat to the stirrupshell. Its siphon can become clogged, causing it to suffocate to death.

A stable habitat is also necessary for the stirrupshell to reproduce, which, like other mussels, it does in a unique way. In the spring, males release sperm, which is carried away by river currents.

Females feeding downstream take in this sperm, which then fertilizes the eggs stored in their gills. When the eggs hatch, the glochidia (pronounced glow –KID–ee–a) or larval forms of the mussel continue to develop in the gills.

After some time, the glochidia are released and attach themselves to the gill of a host fish (those glochidia unable to attach to a host fish drift to the bottom of the river and die).

The glochidia remain attached to the host fish until they become young stirrupshells, meaning they have developed a shell and are large enough to survive on their own. The young stirrupshells detach from the host fish and fall to the riverbed, burying themselves with only their shell margins (edges) and feeding siphons exposed.

Habitat and current distribution

The stirrupshell is found only in Alabama in areas of the Sispey River and the Gainsville Bendway (a part of the East Fork Tombigbee River cut off by the construction of the Tennessee–Tombigbee Waterway).

Its total population number is unknown, but the U.S. Fish and Wildlife Service noted in the Endangered Species database in the early 2000s that this species may well be extinct in the wild, as no populations have been found for some time.

History and conservation measures

This species of mussel was once found in the Tombigbee River from Columbus, Mississippi, to Epes, Alabama. It was also found in the Alabama and Black Warrior Rivers.

The stirrupshell has declined mainly because of the construction of the Tennessee–Tombigbee Waterway (a series of channels, locks, and impoundments or reservoirs built to provide a link for barge traffic between these two rivers). The dredging (digging out) of river bottoms to create channels destroyed many mussel beds.

To maintain these channels, this dredging continues periodically. Dams and locks built for the waterway caused mussel beds to become flooded. The water flowing over these areas was also slowed, causing silt to build up. Many stirrupshells suffocated as a result.

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