|Listed||April 5, 1990|
|Family||Unionidae (Freshwater Mussel)|
|Description||Medium-sized, olive brown, elliptical shell with a pearly bluish-white interior.|
|Habitat||Deep pools and backwaters with sand or gravel bottoms.|
|Reproduction||Female stores sperm in gills; glochidia (larvae) are released into streams after hatching.|
|Threats||Impoundments; channel alteration; water pollution.|
The Arkansas fatmucket (Lampsilis powelli ) has an elliptical to long ovate shell with subinflated valves. The umbos (prominences) are moderately full and project slightly above the hinge line. Although usually of medium size, the shell can be as long as 3.9 in (10 cm). The shell's surface is generally smooth; its thin outer layer is shiny olive brown to tawny, with no rays. The inside of the shell is an iridescent bluish-white. There is sexual dimorphism.
The life of mussels is complex, and reproduction often depends upon a stable habitat—unaltered stream conditions, clean water, and an undisturbed stream bottom. The cycle also depends upon the abundance of suitable fish hosts to complete the mussel's larval development.
To reproduce, males discharge sperm, which are dispersed by stream currents. In the process of feeding, females nearby or downstream take in sperm, which fertilize eggs stored in their gills. The gills serve as brood pouches (marsupia), where the glochidia hatch and begin to develop. After a time, these glochidia are released into the stream. A few mussels have inner parts that resemble a tiny minnow and can be manipulated to lure host fish. When a fish gets close to the shell, the mussel expels its glochidia.
Glochidia have tiny bean-or spoon-shaped valves that attach to the gill filaments of host fish. Glochidia can only progress to the juvenile stage while attached to the fish's gills. Those that do not fortuitously encounter a host fish do not survive when released by the female mussel. They sink to the bottom and die.
When the juvenile has developed a shell and is large enough to survive on its own, it detaches from the host fish and falls to the stream bottom, beginning a long association with a single stretch of stream. Maturing mussels bury themselves in riffles and shoals with only the shell margins and feeding siphons exposed to the water. Some mussels live as long as 50 years or more.
The family Unionidae, which includes all of the freshwater mussels in the United States, is separated into two groups based on the length of time the glochidia remain in the female's marsupia. The eggs of the short-term (tachytictic) breeders are fertilized in the spring, and glochidia are released by late summer of the same year. Long-term (bradytictic) breeders hold developing glochidia in the brood pouch over winter and release them in the spring.
Freshwater mussels feed by siphoning phytoplankton and other plant matter from the water. Indigestible particles are expelled from the shell by reverse siphoning. Silt in the water can kill mussels by clogging their feeding siphons.
There are no known interspecific differences in feeding among freshwater mussels. The glochidia are obligate parasites on the gills or fins of fish. Adult mussels are filter-feeders and consume particulate matter in the water column. Identifiable stomach contents almost invariably include desmids, di-atoms, algae, protozoa, and zooplankton.
Most freshwater mussel species display seasonal variations in activity associated with water temperature and reproduction. Metabolic rate is, in part, positively correlated with temperature. Many ectothermic species have the capacity to adjust their metabolic rates in response to long-term changes in temperature. Thus, metabolic rates do not continue to rise as temperatures rise in the summer, and they do not continue to fall during the winter as temperatures decline.
Some freshwater mussels also show diurnal changes in metabolic rates that indicate a tendency toward nocturnal activity patterns. Mussels may move to the surface to feed at night and move deeper into the substrate during the day; this is one way to avoid predators that hunt by visual contact.
Freshwater mussels are nonmigratory.
The Arkansas fatmucket prefers deep pools and backwater areas that possess sand, sand-gravel, sand-cobble, or sand-rock with sufficient flow to periodically remove organic detritus, leaves, and other debris. It is not generally found in riffles, nor does it occur in impoundments. It is frequently found with islands of Justicia americana, where substrate is typically depositional and water depth is about 3.3 ft (1 m).
A 1988 survey of the species' habitat area found 151 individuals. The mussel's range as of the late 1990s was limited to the Quachita, Saline, and Caddo River systems in the state of Arkansas. Specifically, the Arkansas fatmucket occurs in the Quachita River upstream of Lake Quachita in Montgomery and Polk Counties and in the South Fork Quachita River upstream of Lake Quachita in Montgomery County. In the Saline River basin, the species occurs in Alum Fork, the Middle Fork, and the North Fork above their confluence with the Saline River, and in the Saline River from its formation downstream to about the Fall Line. In the Caddo River, Arkansas fatmucket populations are all located in the main stem. The species' probable historic range may have included the Caddo River from Norman downstream to the Quachita River, including at least the lower reach of the South Fork Caddo River. In the Quachita River, the species might have occurred from Malvern upstream to the species' currently known range and throughout the South Fork Quachita River. The species also probably occurred in all four forks in the Saline River drainage and in the main stem from the Fall Line upstream to the extent of permanent flowing water, as well as in Hurricane Creek upstream of the Fall Line.
The construction and operation of impoundments are the primary reasons for the reduction of the species' historic range. Within the historic range there are 16 existing impoundments, one under construction, and plans for constructing another. While these latter two impoundments will not inundate known populations of the Arkansas fatmucket, there are impacts occurring during the building process that may be adverse. During construction, there is increased threat from silt and sediment. After completion, the control of water flows during low flow periods could expose the mussel and result in lowered dissolved oxygen. Channel alterations (for highway construction and maintenance) and gravel operations are adversely altering the substrate.
Water quality degradation continues to threaten this species. A large majority of the watershed for the current range of this species is in timber and agricultural production. There is some conflicting data on the extent of impacts from sedimentation caused by silviculture; however, at least some of the sedimentation is caused by timber production and harvest. In addition, specific portions of the range are adversely affected by runoff from a barite mining operation, improperly treated municipal waste, and acidic runoff from bauxite mines.
Inundation aside, impoundments have also had a negative impact on this species, isolating various populations. The two populations in the Quachita River System are isolated from all other populations and each other by Lake Quachita. The Caddo River population is separated from the Saline River populations by about 200 river mi (321.9 km) and is adversely affected by hypolimnetic (stagnant water) discharges from DeGray Reservoir. If populations still exist in the Caddo River upstream of DeGray Reservoir, they are isolated by that impoundment. The Saline River system populations are not isolated from each other by any apparent barrier. However, if the fish host is not migratory, the exchange of genetic material between these populations would be an uncommon event.
Conservation and Recovery
The U.S. Forest Service (USFS) has funded status surveys of Arkansas fatmucket populations in streams under their jurisdiction. USFS officials have expressed a desire to conduct life-history studies on this species. The U.S. Fish and Wildlife Service is working with the Soil Conservation Service (SCS) to eliminate adverse impacts to the species from the construction and operation of SCS impoundments.
U.S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
1875 Century Blvd., Suite 200
Atlanta, Georgia 30345
Telephone: (404) 679-4000
Harris, J.L., and M.E. Gordon. 1988. "Status Survey of Lampsilis powelli. " A Report to the U.S. Fish and Wildlife Service. 43 pp. + field notes.
U.S. Fish and Wildlife Service. 5 April 1990. "Threatened Status Determined for the Arkansas Fatmucket (Lampsilis powelli )." Federal Register 55 (66): 12797-801.