Cyclonaias tuberculata (Rafinesque, 1820)
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Basis for Listing
Past surveys and shell records indicate that the purple wartyback was once widely distributed in the Mississippi River below St. Anthony Falls and in the Minnesota and St. Croix Rivers, although it was not found in large numbers anywhere (Dawley 1947; van der Schalie and van der Schalie 1950). Today, it is apparently extirpated from the Minnesota River (Bright et al. 1990), extremely rare in the Mississippi River (Thiel 1981), and healthy only in portions of the St. Croix River (Doolittle 1988; Hornbach et al. 1995). The purple wartyback was listed as a threatened species in Minnesota in 1996.
The shell of the purple wartyback can be up to 13 cm (5 in.) long, and it is rounded, compressed, and thick, with numerous pustules (bumps) across the posterior two-thirds of the shell. The outside of the shell is yellowish-brown or greenish-brown, generally without rays, and the inside of the shell is purple. The pseudocardinal and lateral teeth are heavy, and the beak cavity is very deep and compressed. The purple wartyback resembles the pimpleback (Quadrula pustulosa), the mapleleaf (Q. quadruala), and the winged mapleleaf (Q. fragosa), but the pimpleback has a broad green ray on its beak and a white inner shell, the mapleleaf has faint green rays and a white inner shell, and the winged mapleleaf has a well-developed posterior wing and a white inner shell.
The purple wartyback 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. The purple wartyback has been reported to live to 20 years of age (Scavia and Mitchell 1989), reaching sexual maturity by the age of 6 (Jirka and Neves 1992).
Conservation / Management
The viability of remaining purple wartyback populations is jeopardized by the continuing decline in habitat conditions on the Mississippi associated with its management as a navigation canal, and with 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 purple wartyback 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. If the effects of these factors cannot be mitigated, the purple wartyback may become endangered in the future.
Conservation Efforts in Minnesota
A 10-year statewide mussel survey initiated by the Minnesota DNR in 1999 resulted in a better understanding of the purple wartyback's ecology and current status in Minnesota.