Basis for Listing

The wartyback historically occurred in the Minnesota and Mississippi rivers in Minnesota (Dawley 1947; van der Schalie and van der Schlalie 1950), where populations have since declined. Bright et al. (1990) found only 7 live individuals at 4 sites in the Minnesota River. The species is still rare and sporadically distributed in the Mississippi River, although there is recent evidence of some recovery in the Twin Cities area (Kelner and Davis 2002). Given the wartyback's restricted range and the small number of recent live specimens, it was listed as an endangered species in Minnesota in 1996.

  Description

The shell of the wartyback is rounded, thick, truncated posteriorly, moderately inflated, and up to 8 cm (3 in.) long. The outside of the shell is straw colored to brown, and rayless. The shell has large pustules that are usually paired, but are sometimes single, forming 2 rows that extend from the beak to the ventral margin. The shell also has a small posterior wing that is usually present, and an obscure beak sculpture. The pseudocardinal and lateral teeth are well-developed, and the inside of the shell is white. The wartyback is similar to the purple wartyback (Cyclonaias tuberculata), the winged mapleleaf (Quadrula fragosa), the pimpleback (Q. pustulosa), and the mapleleaf (Q. quadrula), but its 2 rows of large pustules, lack of green rays, and straw color distinguish it from these species.

  Habitat

The wartyback is found in large rivers in Minnesota, and it can be found in fine or coarse substrates in areas of slow or moderate current.

  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 wartyback is tachytictic, with females brooding their young short-term before they are released as glochidia. Female wartybacks may be gravid in June and July (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. Fish hosts for the glochidia of the wartyback include black crappie (Pomoxis nigromaculatus), white crappie (P. annularis), bluegill (Lepomis macrochirus), channel catfish (Ictalurus punctatus), and largemouth bass (Micropterus salmoides) (Watters 1994).

  Conservation / Management

Degradation of mussel habitat in streams throughout the wartyback's known range is a continuing threat to this species. Populations in Minnesota are vulnerable to further decline because of hydrologic alteration of streams and their watersheds; the continuing decline in habitat conditions on the Minnesota and Mississippi rivers; non-point and point source water and sediment pollution; and 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. Further survey work in rivers where the wartyback was formerly documented is needed to verify its status in that former range.

  Conservation Efforts in Minnesota

A 10-year statewide mussel survey initiated by the Minnesota DNR in 1999 resulted in a better understanding of the wartyback mussel's ecology and current status 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.

Dawley, C. 1947. Distribution of aquatic mollusks in Minnesota. American Midland Naturalist 38:671-697.

Kelner, D., M. Davis. 2002. Final report: mussel (Bivalvia:Unionidae) survey of the Mississippi National River and Recreation Area Corridor, 2000-01. Final report submitted by the Minnesota Department of Natural Resources. 44 pp. + appendices.

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.

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.