Tritogonia verrucosa (Rafinesque, 1820)
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.
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.
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).
Conservation / Management
Degradation of mussel habitat in streams throughout the pistolgrips'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.