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.

  Description

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.

  Habitat

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).

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 elephant-ear mussel is tachytictic, with females brooding their young short-term, from June to July, before they are released as glochidia (Baker 1928). 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 elephant-ear is the skipjack herring (Alosa chrysochloris) (Watters 1994).

  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.

  References and Additional Information

Baker, F. C. 1928. The fresh water mollusca of Wisconsin: part II: Pelecypoda. Wisconsin Geological and Natural History Survey Bulletin No. 70, Part II. University of Wisconsin, Madison. 495 pp.

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.

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

van der Schalie, H., and A. van der Schalie. 1950. The mussels of the Mississippi River. American Midland Naturalist 44:448-464.

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.