Basis for Listing

The pistolgrip historically occurred in the Minnesota, Mississippi, and St. Croix rivers in Minnesota (Dawley 1947). It has likely been extirpated from the Minnesota River, as no live specimens have been found in recent surveys (Bright et al. 1990). Its distribution in the Mississippi River has also been greatly reduced, and only a few live specimens have been found in recent decades. The best remaining populations are in the lower St. Croix River, where it is common, but rarely abundant. The pistolgrip was listed as a threatened species in Minnesota in 1996.

  Description

The shell of the pistolgrip reaches up to 20 cm (8 in.) long, is elongate, thick, has numerous pustules (bumps), and has a well-developed posterior ridge. The outside of the shell is greenish to dark brown and rayless. This species is sexually dimorphic, with females being rounded and compressed posteriorly and males being more truncated posteriorly. The pseudocardinal and lateral teeth are well developed, and the inside of the shell is white. The pistolgrip does not closely resemble any other mussel species in Minnesota.

  Habitat

In Minnesota, the pistolgrip is most often found inhabiting larger rivers in areas with moderate current and gravel substrates.

  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 pistolgrip is tachytictic, with females brooding their young short-term, between May and late August, before they are released as glochidia (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. Glochidial host fish for the pistolgrip include flathead catfish (Pylodictis olivaris) (Howells 1996), brown bullhead (Ameiurus nebulosus) (Hove et al. 1998), and yellow bullhead (Ameiurus natalis) (Pepi and Hove 1997).

  Conservation / Management

Degradation of mussel habitat in streams throughout the pistolgrip's known range is a continuing threat to this species. Further survey work in rivers where it was formerly documented is needed to verify its status in that former range. 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 Mississippi river associated with its management as a navigation canal; non-point and point source water and sediment pollution; and the zebra mussel (Dreissena polymorpha) infestation of the Mississippi and St. Croix rivers. 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 pistolgrip'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.

Hove, M. C., J. E. Kurth, and A. R. Kapuscinski. 1998. Brown Bullhead suitable host for Tritogonia verrucosa; Cumberlandia monodonta host(s) remain elusive. Triannual Unionid Report 15:13.

Howells, R. G., R. W. Neck, and H. D. Murray. 1996. Freshwater mussels of Texas. Texas Parks and Wildlife Press, Austin, Texas. 218 pp.

Parmalee, P. W., and A. E. Bogan. 1998. The freshwater mussels of Tennessee. The University of Tennessee Press, Knoxville, Tennessee. 328 pp.

Pepi, V. E., and M. C. Hove. 1997. Suitable fish hosts and mantle display behavior of Tritogonia verrucosa. Triannual Unionid Report 11:5.

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