Curtis' Pearlymussel

Epioblasma florentina curtisii

Description

The oval shell of Curtis' pearlymussel, Epioblasma florentina curtisii, is usually less than 1.5 in (3.8 cm) in length, with males being slightly larger than females. The valve end of the shell is bluntly pointed and biangular, the front smoothly rounded. In both sexes, the shell color is yellow-brown to light brown, sometimes with fine, evenly spaced rays over most of its length. The interior shell surface (nacre) is white to whitish blue.

First described as Truncilla curtisii in 1915 from White River specimens, this species has also been classified as Dysnomia florentina curtisii. Thirteen other species of genus Epioblasma have been federally listed as endangered: southern acornshell (E. othcaloogensis), green blossom (E. torulosa gubernaculum), tubercled blossom (E. torulosa torulosa), turgid blossom (E. turgidula), yellow blossom (E. florentina florentina), catspaw (Epioblasma obliquata obliquata), white catspaw (E. obliquata perobliqua), Cumberlandian combshell (E. brevidens), southern combshell (E. penita), upland combshell (E. metastriata), oyster mussel (E. capsaeformis), northern riffleshell (E. torulosa rangiana) tan riffleshell (E. florentina walkeri).

Behavior

Curtis' pearlymussel is bradytictic—eggs are fertilized in the fall and glochidia (larvae) are released in the spring. For more information about the reproduction and diet of the freshwater mussel, see the Upland Combshell (Epioblasma metastriata ) entry.

Habitat

This pearlymussel is found in transitional zones between swift-flowing stream headwaters and the more leisurely currents of lowland meanders. It buries itself in stable substrates of sand and gravel, or among cobbles or boulders, particularly in shallow water at depths of up to 30 in (76 cm). Populations require clear, unsilted water.

Distribution

Curtis' pearlymussel was identified from scattered locations in the White and St. Francis River basins in southern Missouri and northern Arkansas. A record from 1916 indicates that Curtis' pearly mussel also occurred in the South Fork of the Spring River, a tributary of the Black River in Arkansas.

In 1993, the U. S. Fish and Wildlife Service's Columbia, Missouri Field Office assisted the Missouri Department of Conservation in a search for the Curtis' pearlymussel. After several hours of work, the searchers finally found one live male. Extensive searching in the same pool where the male was discovered, and at other sites on the Little Black River where the species was common about eight years before, resulted in no additional finds.

Since the mid-1970s, Curtis' pearlymussel has been found in southeastern Missouri, in the Castor River, Cane Creek (a Black River tributary), and Little Black River. Only about 6 mi (9.5 km) of the upper Little Black River and 7 mi (11 km) of the Castor River upstream from the Headwater Diversion Channel still support minimal numbers. From 1981-1983 more than 140 probable locations on 26 streams were sampled, but this mussel was found at only six sites. In spite of more than two decades on the federal list of endangered species, Curtis' pearly-mussel is still extremely uncommon and is thought to remain near extinction.

Threats

Much of Curtis' pearlymussel's historic range has been inundated by reservoir construction. Lake Taneycomo, completed in 1913, flooded a long stretch of its habitat. The White River has been dammed repeatedly—in 1952 to create Bull Shoals Reservoir, in 1959 to fill Table Rock Lake, and again in 1966 to create Beaver Reservoir. These impoundments drastically reduced water flows on the river, resulting in stagnant bottom waters and accumulations of silt. Stream channelization and gravel dredging have reduced substrate stability, and poor land management practices have further exacerbated problems of siltation and chemical runoff.

Conservation and Recovery

The Missouri Department of Conservation has conducted low intensity research into the biology of this species, but there are still many unanswered questions. The immediate goal of recovery is to stave off extinction by preventing further loss or damage to the habitat. When the state of research permits, biologists will attempt to transfer mussels from viable reproducing populations to depleted areas to stimulate reproduction. An effort will be made to produce juveniles by artificial culture to assist restocking of suitable habitat within the historic range.

Contacts

U. S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
1 Federal Drive
BHW Federal Building
Fort Snelling, Minnesota 55111
Telephone: (612) 713-5360
http://midwest.fws.gov/

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

References

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

Hudson, R. G., and B. G. Isom. 1984. "Rearing Juveniles of the Freshwater Mussels (Unionidae) in a Laboratory Setting." Nautilus 98 (4): 129-135.

Johnson, R. I. 1978. "Systematics and Zoogeography of Plagiola (=Sysnomia, = Epioblasma ), an Almost Extinct Genus of Freshwater Mussels (Bivalvia: Unionidae) from Middle North America." Bulletin of the Museum of Comparative Zoology 148 (6): 239-321.

U. S. Fish and Wildlife Service. 1986. "Curtis' Pearly Mussel Recovery Plan." U. S. Fish and Wildlife Service, Twin Cities, Minn.