Louisiana Pearlshell

Margaritifera hembeli

Description

The Louisiana pearlshell (Margaritifera hembeli ) is a freshwater mussel with a generally elliptical shell that measures about 4 in (10.2 cm) long, 2 in (5 cm) high, and 1.2 in (3 cm) wide. The outer shell surface (periostracum) is dark brown to black, and the inner shell surface (nacre) is white.

Like members of the family Unionidae, the pearl-shell is a filter-feeder. It takes nourishment by siphoning water, drawing it in through the inhalant siphon, and passing it over specialized gills that filter out suspended food particles. Common food materials of freshwater mussels are desmids, diatoms, filamentous algae, detritus, bacteria, and plankton.

This species was reclassified in 1993 from "endangered" to "threatened" because of improvements in habitat management, a new population discovery, and other successful recovery efforts.

Behavior

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 fertilizes 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.

Habitat

Louisiana pearlshells can be found in very shallow flowing streams with gravel and sand substrate. Water depths range from 12-20 in (30.5-50.8 cm). Vegetation in the surrounding watershed is mostly mixed hardwood-loblolly pine forest.

Distribution

This species is thought to have ranged throughout most of the headwater streams of Bayou Boeuf in Rapides Parish, Louisiana.

In 1983, after an extensive search, biologists from the Louisiana Natural Heritage Program found the pearlshell in 11 streams. Two years later the total population was estimated at 10,000 individuals, about 90% of which inhabited four streams: Long Branch, Bayou Clear, Loving Creek, and Little Loving Creek. Much of the pearlshell's range is within the Kisatchie National Forest, administered by the Forest Service.

Since its listing in 1988, the mussel's prospects have improved so much that the species was reclassified from "endangered" to "threatened" in 1993. One reason for the reclassification was the discovery of new populations and an expansion of the known range for the species into the Red River drainage. In the fall of 1991, based upon a report of the Louisiana pearlshell from Moccasin Branch in the Red River drainage, biologists from the U.S. Fish and Wildlife Service (FWS) and Kisatchie National Forest conducted a field survey of streams in and adjacent to the Catahoula District of the National Forest. Twelve populations of the Louisiana pearl-shell were found in three different small drainages that eventually flow into the Red River. One drainage is isolated from the others by the impoundment of Lake Iatt. All of the populations were found in small, shallow, clear streams with gravel or firm sand substrate.

Threats

The pearlshell's range has been reduced by dam construction, stream diversion, and generally degraded water quality. Logging operations in Rapides Parish have included clear-cutting up to stream banks, which has increased erosion and runoff. Freshwater mussels are especially vulnerable to siltation because their feeding siphons are easily clogged. A large population was lost in the early 1980s to natural processes, when beavers constructed a dam that flooded a section of stream habitat.

The Louisiana pearlshell, one of the rarest members of its family, has been avidly sought by both amateur and scientific collectors. As this mussel is already very limited in numbers, any collection can have an adverse affect.

Conservation and Recovery

On September 24, 1993, the FWS determined that the Louisiana pearlshell warranted reclassification from "endangered" to "threatened."

In addition to documenting a larger range than known at the time of its initial listing, surveys found evidence of successful reproduction in most, if not all, populations. Management initiatives at Kisatchie National Forest to benefit the pearlshell have included the control of beavers (whose dams had fragmented the mussel's range and flooded its free-flowing habitat) and the establishment of streamside zones to minimize sedimentation during logging operations.

According to a 1990 FWS draft recovery plan, the main objective for recovery of this species is increasing population numbers in each of Long Branch, Bayou Clear, Loving Creek, and Little Loving Creek to greater than 2,000 individuals and increasing population numbers in the Mack Branch, Castor Creek, and Brown Creek to over 1,000 individuals. These minimal levels are to be maintained for a period of at least 10 years with evidence of successful reproduction and recruitment. In the future, all streams will be surveyed to determine population status.

Because this mussel occurs within a national forest and on land administered by the U.S. Air Force, the Forest Service and the air force are required to formally consult with the FWS concerning any proposed actions that would potentially harm the pearlshell or its habitat.

Other recommended recovery efforts include the reduction of off-road vehicle activity. Better timber management could reduce siltation in the streams. In addition, public education and information on the hazards of siltation and waste runoff (such as motor oil, sewage, and agricultural pesticides) are necessary. The reclassification to "threatened" status is a clear sign that recovery efforts are on the right track.

Contact

U.S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
1875 Century Blvd., Suite 200
Atlanta, Georgia 30345
Telephone: (404) 679-4000
http://southeast.fws.gov/

References

Athearn, H.D. 1970. "Discussion of Dr. Heard's Paper (Eastern Freshwater Mollusks, the South Atlantic and Gulf Drainages)." Malacologia 20 (1): 1-56.

French, III, John R.P. November 1990. "The Exotic Zebra Mussel: A New Threat to Endangered Freshwater Mussels." Endangered Species Technical Bulletin 15 (11).

Johnson, R.E. 1983. "Margaritifera marrianae, A New Species of Unionacea (Bivalvia: Margaritiferidae) from Mobile-Alabama-Coosa and Escambia River Systems, Alabama." Occasional Papers on Mollusks, Museum of Comparative Zoology, Harvard University 4 (62): 299-304.

U.S. Fish and Wildlife Service. 1990. "Endangered and Threatened Species Recovery Program, Report to Congress." U.S. Fish and Wildlife Service.