Basis for Listing

Reported in the literature to be a species common only to large rivers, the sheepnose mussel, also known as the bullhead mussel, was considered abundant in the Mississippi River below Lake Pepin and rare in the Minnesota and St. Croix rivers by Dawley (1947). Shells of the sheepnose can be found in pearl hunter middens near Red Wing, Minnesota (M. Davis, Minnesota DNR, pers. comm.), and museum and literature records exist from other areas of the Mississippi such as Lake Pepin (Grier 1922; van der Schalie and van der Schalie 1950). Bright et al. (1990) did not find any sheepnose mussels in the Minnesota River. The only recent live records of sheepnose mussels are of an old individual collected by a fisheries biologist near Minnieska, Minnesota in 1991 (M. Davis, pers. comm.), and 3 live specimens found in the lower St. Croix River in connection with a bridge replacement project (Heath 1989). Given its extreme rarity, the sheepnose mussel was listed as an endangered species in Minnesota in 1996 and as federally endangered in 2012.

  Description

The shell of the sheepnose is roundly triangular or oblong, thick, and can reach 13 cm (5 in.) long. The exterior of the shell is yellow to dark brown, and rayless. The shell has a row of low, broad knobs (tubercles) that extend down the center of the valve, which can be faint and obscure. The sulcus (depression or channel on outside of shell) between the tubercles and the posterior ridge is shallow. The pseudocardinal and lateral teeth are well-developed, and the inner shell is white with a shallow beak cavity. The sheepnose resembles the hickorynut (Obovaria olivaria), round pigtoe (Pleurobema sintoxia), threehorn wartyback (Obliquaria reflexa), and Wabash pigtoe (Fusconaia flava).

  Habitat

The sheepnose has historically inhabited large rivers such as the Mississippi, Ohio, and Tennessee (Parmalee and Bogan 1998). Suitable substrates for this species are sand and gravel (Cummings and Mayer 1992) in water depths up to 4.5 m (15 ft.) (Parmalee and Bogan 1998).

  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 sheepnose is tachytictic, with females brooding their young short-term before they are released as glochidia. Females may be gravid in the earlier part of the summer (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 sauger (Stizostedion canadense) is a fish host for the glochidia of the sheepnose mussel (Fuller 1974).

  Conservation / Management

Extirpation of the sheepnose may be imminent because of the continuing decline in habitat conditions on the Mississippi River associated with its management as a navigation canal, and from non-point and point source water and sediment pollution. Dams, channelization, and dredging increase siltation, physically alter habitat conditions, and block the movement of fish hosts. The sheepnose 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 current status of the sheepnose mussel in Minnesota.

  References and Additional Information

Bright, R. C., C. Gatenby, D. Olson, and E. Plummer. 1990. A survey of the mussels of the Minnesota River, 1989. Final report submitted to the Natural Heritage and Nongame Research Program, Minnesota Department of Natural Resources. 106 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.

Fuller, S. L. H. 1974. Clams and mussels (Mollusca: Bivalvia). Pages 215-273 in C. W. Hart, Jr., and S. L. H. Fuller, editors. Pollution Ecology of Freshwater Invertebrates. Academic Press, New York.

Grier, N. M. 1922. Final report on the study and appraisal of mussel resources in selected areas of the Upper Mississippi River. American Midland Naturalist 8:1-33.

Heath, D. J. 1989. Saint Croix River U.S. Highway 10 bridge freshwater mussel relocation project at Prescott, Wisconsin. Phase 1: mussel removal. Unpublished report prepared for Ayres Associates, Madison, Wisconsin. 19 pp.

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.

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