Basis for Listing

Past surveys and shell records indicate that the Purple Wartyback (Cyclonaias tuberculata) 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.

  Description

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.

  Habitat

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).

Mussels spend most of their lives buried in the bottom sediments of permanent water bodies, and often live in multi-species communities called mussel beds. They 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 Purple Wartyback has been described by Haggerty et al. (1995) as tachytictic, or a short-term brooder, with females having larvae present in their outer gills between May and late August. 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. The channel catfish (Ictalurus punctatus), yellow bullhead (Ameiurus natalis), flathead catfish (Pylodictis olivaris), and black bullhead (A. melas) have been reported as suitable fish hosts for the glochidia of the Purple Wartyback (Hove 1997; Hove et al. 1997).

  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.

  Authors/Revisions

Bernard E. Sietman (MNDNR), 2018

  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.

Doolittle, T. C. J. 1988. Distribution and relative abundance of freshwater mussels in the Saint Croix National Scenic Riverway. Final report submitted to the Natural Heritage and Nongame Research Program, Minnesota Department of Natural Resouces. Unpaged.

Haggerty, T. M., F. T. Garner, G. H. Patterson, and L. C. Jones, Jr. 1995. A quantitative assessment of the reproductive biology of Cyclonaias tuberculata (Bivalvia: Unionidae). Canadian Journal of Zoology 73:83-88.

Hornbach, D. J., P. Baker, and T. Deneka. 1995. Abundance and distribution of the endangered mussel Lampsilis higginsi in the lower St. Croix River, Minnesota and Wisconsin. Final report submitted to the U.S. Fish and Wildlife Service, Minneapolis, Minnesota. 68 pp.

Hove, M. C. 1997. Ictalurids serve as suitable hosts for the purple wartyback. Triannual Unionid Report 11:4.

Hove, M. C., R. A. Engelking, M. E. Peteler, E. M. Peterson, A. R. Kapuscinski, L. A. Sovell, and E. R. Evers. 1997. Suitable fish hosts for glochidia of four freshwater mussels. Pages 21-25 in K. S. Cummings, A. C. Buchanan, C. A. Mayer, and T. J. Naimo, editors. Conservation and Management of Freshwater Mussels II. Initiatives for the future. Proceedings of a UMRCC Symposium, 16-18 October 1995, St. Louis, Missouri. 293 pp.

Jirka, K. J., and R. J. Neves. 1992. Reproductive biology of four species of freshwater mussels (Mollusca: Unionidae) in the New River, Virginia and West Virginia. Journal of Freshwater Ecology 7(1):35-44.

Scavia, E., and M. Mitchell. 1989. Reoccurrence of Cyclonaias tuberculata in the Huron River, Michigan. The Nautilus 103(1):40-41.

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

Thiel, P. 1981. A survey of unionid mussels in the upper Mississippi River (pools 3-11). Technical Bulletin 124. Wisconsin Department of Natural Resources, Madison, Wisconsin. 24 pp.

van der Schalie, H., and A. van der Schalie. 1950. The mussels of the Mississippi River. American Midland Naturalist 44:448-464.