Elliptio crassidens (Lamarck, 1819)
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Basis for Listing
The elephant-ear was historically an uncommon resident of the Mississippi, St. Croix, and Minnesota rivers (Dawley 1947), and for this reason, van der Schalie and van der Schalie (1950) did not consider it to be a normal component of the Mississippi River fauna. Presently, the only known extant population in Minnesota occurs in the lower St. Croix River at Prescott (Kelner and Sietman 2000), and it is on the verge of extirpation from the state. Only 1 other live specimen has been reported from the entire Upper Mississippi River main-stem in recent decades (Illinois Natural History Survey Mollusk Collection). Its extreme rarity and narrow distribution make it vulnerable to catastrophic events. The elephant-ear was originally listed as a special concern species in Minnesota in 1984, but given its extremely limited distribution and concerns over the availability of its fish host, it was reclassified as endangered in 1996.
The elephant-ear has a shell that is triangular, thick to heavy, and up to 15 cm (6 in.) long. The outside of the shell is brown to black and is usually rayless. The pseudocardinal and lateral teeth are well developed. The inner shell is usually light purple. Similar species include the spike (Elliptio dilatata) and the mucket (Actinonaias ligamentina), but the spike is more elongate, and the mucket is usually rayed and not as dark, with a white inner shell.
The elephant-ear mussel primarily inhabits large rivers in mud, sand or fine 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 elephant-ear in Minnesota are blockage of its migratory host, the skipjack herring, by dams (Kelner and Sietman 2000), and the species' small population size. It is also threatened by the continuing decline in habitat conditions on the Mississippi River 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 elephant-ear 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 elephant-ear mussel's ecology and current status in Minnesota. Efforts are also underway to propagate juveniles for restocking into areas where habitat conditions have improved.
Baker, F. C. 1928. The fresh water mollusca of Wisconsin. Part II Pelecypoda. Bulletin of the Wisconsin Geological and Natural History Survey 70:1-495.
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.
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.
van der Schalie, H., and A. van der Schalie. 1950. The mussels of the Mississippi River. American Midland Naturalist 44:448-464.