Pearlymussel, little-wing

Pegias fabula

phylum: Mollusca

class: Bivalvia

order: Unionoida

family: Unionidae

status: Critically endangered, IUCN Endangered, ESA

range: USA (Kentucky, Tennessee, Virginia)

Description and biology

The shell of the little-wing pearlymussel measures up to 1.5 inches (3.8 centimeters) long and 0.5 inch (1.3 centimeters) wide. The outer side is light green or yellow-brown in color; the inner side is much lighter. Dark rays run along the shell's front edge. A white, chalky film often flakes off its surface. Like other freshwater mussels, the little-wing pearlymussel feeds on plankton (microscopic plants and small animals) and other plant matter it removes from the water through a siphon or tube.

The little-wing pearlymussel breeds in the spring in a unique way. A male releases sperm that is carried downstream by currents. While eating, a female takes in this sperm, which fertilizes eggs stored in her gills. The eggs hatch, and the glochidia (pronounced glow-KID-ee-a) or larval forms of the mussel continue to develop in her gills.

After a certain period, the glochidia are released. Floating away, they attach themselves to the gills of a host fish (those glochidia unable to attach themselves sink to the bottom of the river and die). When they have grown and developed a shell, the young pearlymussels detach and fall to the riverbed. Here, they bury themselves, leaving only their shell margins (edges) and siphons exposed.

Habitat and current distribution

In the mid-1980s, biologists (people who study living organisms) conducted a survey of this pearlymussel's habitat. They found this species in only five locations: Horse Lick Creek, the Big South Fork Cumberland River, and the Little South Fork Cumberland River in Kentucky; Great Falls Lake in Tennessee; and the North Fork Holston River in Virginia. During this study, biologists found only 17 live pearlymussels.

Little-wing pearlymussels inhabit rivers with cool waters and moderately to steeply inclined riverbeds. Because of the way the pearlymussel feeds, these rivers must have a low current and very little silt (mineral particles). Too much silt in the water can plug the pearlymussel's siphon and kill it.

History and conservation measures

The little-wing pearlymussel once existed in at least 27 cool water tributaries of the Tennessee and Cumberland Rivers. Biologists believe the pearlymussel is now extinct in North Carolina and Alabama. A total of 18 populations have disappeared in Kentucky, Tennessee, and Virginia.

Water pollution has been the main reason for the pearlymussel's decline. Toxic (poisonous) runoff from farms, strip mining operations, and industries has clouded many rivers that were once clear. Increased amounts of sediment (sand and stones) have also built up in these rivers, settling on the riverbeds and suffocating the pearlymussels.

In Kentucky, Tennessee, and Virginia, laws have passed banning the harvesting (gathering) of freshwater mussels without a permit. In Kentucky, part of the pearlymussels' remaining habitat is bounded by the Daniel Boone National Forest. Despite these conservation measures, pollution from coal exploration threatens to pollute the little-wing pearlymussels' habitat in unprotected areas.