Quadrula nodulata (Rafinesque, 1820)
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.
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.
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).
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.