Synonyms

Fusconaia ebenus

  Basis for Listing

Historically the most abundant mussel in the upper Mississippi River, the ebonyshell was prized by button makers for its thick white shell (Coker 1919). Populations in Minnesota occurred in the lower Minnesota, lower St. Croix, and Mississippi rivers. The ebonyshell is presently restricted to the lower St. Croix River above Lakeland and at Prescott, where less than 40 individuals have been found in recent decades (Kelner and Sietman 2000), and it is on the verge of extirpation from Minnesota. Overharvest during the pearl button era, pollution, and dams, which block the migration of its primary host fish, skipjack herring (Alosa chrysochloris), are the reasons for the species' demise. Its extreme rarity and narrow distribution make it vulnerable to catastrophic events. The ebonyshell was originally listed as a special concern species in Minnesota in 1984, but given continued habitat degradation and concerns over the availability of its host fish, it was reclassified as an endangered species in 1996.

  Description

The ebonyshell has a round, heavy shell with a beak that curves forward. The shell is brown to black in color, and can reach up to 10 cm (4 in.) long. The teeth of the ebonyshell are well developed, with the pseudocardinal teeth parallel to the lateral teeth. The beak cavity is deep, and the inner shell is white. Similar species include the Wabash pigtoe (Fusconaia flava), round pigtoe (Pleurobema sintoxia), and hickorynut (Obovaria olivaria), but the combination of the ebonyshell's round shape, forward curved beak, deep beak cavity, and parallel pseudocardinal and lateral teeth, distinguish it from all of these species.

  Habitat

The ebonyshell mussel primarily inhabits large rivers in sand or gravel (Cummings and Mayer 1992).

  Biology / Life History

Mussels are long-lived animals. Members of many species may live for several decades and in some instances, a century or more. They spend most of their lives buried in the bottom sediments of permanent water bodies, and often live in multi-species communities called mussel beds (Sietman 2003).

Mussels are primarily sedentary, but they can move around with the use of their "foot", which is a hatchet shaped muscle that can be extended out between the valves (shells). A mussel will burrow its foot into the sediment and then contract it to pull itself slowly along the bottom of its aquatic habitat (Sietman 2003).

Mussels eat by filtering bacteria, protozoans, algae, and other organic matter out of the water. They draw water into their body through their incurrent siphon, remove food and oxygen with their gills, and then expel the filtered water through their excurrent siphon. Food particles are carried to the mussel's mouth by tiny hairlike cilia located on the gills. Waste is expelled through the excurrent siphon (Sietman 2003).

Mussels have a complex and distinctive reproductive cycle. Males release sperm into the water, which are drawn in by females through their incurrent siphon. Fertilized eggs are brooded in the female's gills, where they develop into tiny larvae called glochidia. The ebonyshell mussel is tachytictic, with females brooding their young short-term, from May to early fall, before they are released as glochidia (Parmalee and Bogan 1998). Once the glochidia are expelled from the female's gills, they attach to fish gills or fins by clamping onto them with their valves. The glochidia live as parasites on the host fish until they develop into juvenile mussels, at which point they detach from the fish and fall to the streambed as free-living mussels. The primary host fish for the ebonyshell is the skipjack herring (Watters 1994).

  Conservation / Management

The primary threats to the continued persistence of the ebonyshell in Minnesota are the blockage of its migratory host fish, the skipjack herring, by dams (Kelner and Sietman 2000), and the species' small population size. The ebonyshell is also threatened by the continuing decline in habitat conditions on the Mississippi River, which are associated with its management as a navigation canal, and by non-point and point source water and sediment pollution. Dams, channelization, and dredging increase siltation and physically alter habitat conditions. The ebonyshell is also being impacted by the infestation of non-native zebra mussels (Dreissena polymorpha) in the Mississippi River and its tributaries. Zebra mussels can attach themselves in large numbers to the shells of native mussels, eventually causing death by suffocation.

  Conservation Efforts in Minnesota

A 10-year statewide mussel survey initiated by the Minnesota DNR in 1999 resulted in a better understanding of the ebonyshell mussel's ecology and current status in Minnesota. Efforts are also underway to propagate juveniles for restocking into areas where habitat conditions have improved.

  References and Additional Information

Coker, R. E. 1919. Fresh-water mussels and mussel industries of the United States. Bulletin of the U.S. Bureau of Fisheries 36:13-89.

Cummings, K. S., and C. A. Mayer. 1992. Field guide to freshwater mussels of the Midwest. Illinois Natural History Survey Manual No. 5. 194 pp.

Dawley, C. 1947. Distribution of aquatic mollusks in Minnesota. American Midland Naturalist 38:671-697.

Kelner, D. E., and B. E. Sietman. 2000. Relic populations of the ebony shell, Fusconaia ebena (Bivalvia: Unionidae), in the Upper Mississippi River drainage. Journal of Freshwater Ecology 15(3):371-377.

Parmalee, P. W., and A. E. Bogan. 1998. The freshwater mussels of Tennessee. The University of Tennessee Press, Knoxville, Tennessee. 328 pp.

Sietman, B. E. 2003. Field guide to the freshwater mussels of Minnesota. Minnesota Department of Natural Resources, St. Paul, Minnesota. 144 pp.

Watters, G. T. 1994. An annotated bibliography of the reproduction and propagation of the Unionoidea (Primarily of North America). Ohio Biological Survey Miscellaneous Contributions No. 1, Columbus, Ohio. 158 pp.