Rare Species Guide

  Basis for Listing

The elktoe originally inhabited many rivers in Minnesota, including the Mississippi, Minnesota, and St. Croix (Dawley 1947). However, it is now common only in the St. Croix River and some of its tributaries (Heath and Rasmussen 1990; Hornbach et al. 1995), and less so in the upper Root River system. It is still found occasionally in the Mississippi, upper Iowa, and Zumbro rivers of southeastern Minnesota. Bright et al. (1990) considered the elktoe to have been a minor component of the Minnesota River fauna historically, and it is currently on the verge of extirpation in the Minnesota, Pomme de Terre (Bright et al. 1990, 1995), and Yellow Medicine rivers. It is likely extirpated from the Cedar, Cottonwood, LeSueur, Watonwan, and Blue Earth rivers. The elktoe has recently been found inhabiting only a small number of drainages, making it vulnerable to catastrophic events, and it was listed as a threatened species in Minnesota in 1996.

  Description

The shell of the elktoe is somewhat triangular in shape, thin to stout, and can be up to 14 cm (5.5 in.) long. The outside of the shell is greenish-yellow, with numerous dark green rays and speckles. The shell has a sharply angled posterior ridge and flat posterior slope with numerous fine ridges. The beak sculpture consists of 3-4 heavy double looped ridges. The pseudocardinal teeth are thin, the lateral teeth are absent, and the inner shell is white. The elktoe resembles the snuffbox (Epioblasma triquetra) and the deertoe (Truncilla truncata), both of which lack significant beak sculpture and have well developed teeth.

  Habitat

The elktoe is an inhabitant of medium to large rivers. Suitable habitats include sand and gravel substrates in areas with moderate to fast velocities (Cummings and Mayer 1992; 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. Baker (1928) reports that the elktoe is bradytictic, with females brooding their young long-term, from July through June, before they are released as glochidia. 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. Fish hosts for the elktoe's glochidia include suckers (Moxostoma spp.) and rock bass (Ambloplites rupestris) (Howard and Anson 1922).

  Conservation / Management

The continued persistence of the elktoe in Minnesota is threatened by the hydrologic alteration of streams and their watersheds, and 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 elktoe 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 elktoe's ecology and current status in Minnesota. Ongoing mussel surveys in some of the rivers that make up this species' historical range will further aid in the documentation of the distribution of the elktoe.

  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.

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.

Bright, R. C., C. Gatenby, R. Heisler, E. Plummer, K. Stramer, and W. Ostlie. 1995. A survey of the mussels of the Pomme de Terre and Chippewa rivers, Minnesota, 1990. Final report submitted to the Natural Heritage and Nongame Research Program, Minnesota Department of Natural Resources. 131 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.

Heath, D. J., and P. W. Rasmussen. 1990. Results of base-line sampling of freshwater mussel communities for long-term monitoring and the Saint Croix National Scenic Riverway, Minnesota and Wisconsin. Report submitted to the Wisconsin and Minnesota Department of Natural Resources. 45 pp.

Hornbach, D. J., P. Baker, and T. Deneka. 1995. Abundance and distribution of the endangered mussel Lampsilis higginsi in the lower St. Croix River, Minnesota and Wisconsin. Final report submitted to the U.S. Fish and Wildlife Service, Minneapolis, Minnesota. 68 pp.

Howard, A. D., and B. J. Anson. 1922. Phases in the parasitism of the Unionidae. Journal of Parasitology 9(2):68-82 + 2 plates.

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.