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Fusconaia ebena (I. Lea, 1831)
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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.
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), rouhd 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.
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).
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.
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 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.