Pleurobema sintoxia    (Rafinesque, 1820)

Round Pigtoe 

MN Status:
special concern
Federal Status:


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Minnesota range map
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North American range map
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Pleurobema coccineum

  Basis for Listing

The round pigtoe was historically found in the Zumbro, Cannon, Minnesota, and St. Croix rivers, as well as the Mississippi River below St. Anthony Falls (Doolittle 1988; Bright et al. 1988, 1990; Davis 1987). However, Dawley (1947) considered the species not to be abundant anywhere in the state. Today, the round pigtoe is apparently extirpated from the Minnesota River (Bright et al. 1990), extremely rare in the Cannon and Zumbro rivers (Bright et al. 1988; Davis 1987), rare in the Mississippi River, and common only in the St. Croix River (Doolittle 1988; Heath 1990). Hornbach et al. (1995) reported that the round pigtoe comprised only 3% of the specimens they collected from the St. Croix River. It has recently been found alive in only a small number of drainages, making it vulnerable to catastrophic events. Given its limited distribution and degradation of its habitat, the round pigtoe was listed as a threatened species in Minnesota in 1996.


The shape of the round pigtoe's shell is highly variable depending on habitat. The shell can be triangular (large river form) or rounded (small stream form), compressed or inflated, with valves that are moderately thick to heavy. The shell reaches up to 13 cm (5 in.) long and the outside is chestnut to dark brown, sometimes with fine green rays. The beak sculpture consists of 2 or 3 small ridges, which are visible only in young specimens. The pseudocardinal and lateral teeth are heavy, and the beak cavity is moderately shallow. The inside of the shell is usually white, but it can occasionally be pink or salmon colored. The round pigtoe resembles several other Minnesota mussels. The ebonyshell (Fusconaia ebena) can be distinguished by its more circular shape, forward curved beak, and parallel pseudocardinal and lateral teeth. The sheepnose (Plethobasus cyphyus) can be distinguished by its row of low, broad knobs that extend down the center of the valve. The Wabash pigtoe (F. flava) can be distinguished by its distinct posterior ridge and deep beak cavity.


The round pigtoe is found primarily in medium to large rivers but occasionally occurs in smaller rivers. Preferred habitats include fast current areas dominated by coarse sand and gravel substrates. Round pigtoes can be found in waters 0.9 m (3 ft.) to greater than 6.1 m (20 ft.) deep (Parmalee and Bogan 1998).

  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 round pigtoe is tachytictic, with females brooding their young short-term, from mid-May through July, before they are released as glochidia (Baker 1928). 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 bluegill (Lepomis macrochirus) and several species of minnows are suitable fish hosts for the glochidia of the round pigtoe (Coker et al. 1921; Hove 1995).

  Conservation / Management

The round pigtoe is jeopardized by the continuing decline of habitat conditions associated with the management of the Mississippi River as a navigational canal, and from 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 round pigtoe 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 in large numbers to the shells of native mussels, eventually causing death by suffocation. Continued survey work in rivers where the round pigtoe 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 round pigtoe's ecology and current status in Minnesota. Additionally, over 50 round pigtoes were collected from zebra mussel infested habitats in the Mississippi River in 2000 and translocated into areas of the Mississippi River south of the Twin Cities, where habitats were devoid of zebra mussels.

  References and Additional Information

Baker, F. C. 1928. The fresh water mollusca of Wisconsin: part II: Pelecypoda. Wisconsin Geological and Natural History Survey Bulletin No. 70, Part II. University of Wisconsin, Madison. 495 pp.

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.

Bright, R. C., E. Plummer, and D. Olson. 1988. A survey of the mussels of the Zumbro River drainage, southeastern Minnesota. Report submitted to Minnesota Department of Natural Resources. 27 pp. + tables, figures, and appendices.

Coker, R. E., A. F. Shira, H. W. Clark, and A. D. Howard. 1921. Natural history and propagation of fresh-water mussels. Bulletin of the Bureau of Fisheries. 37 (1919-1920):77-181 + 17 plates.

Davis, M. 1987. Freshwater mussels (Mollusca: Bivalvia: Unionidae) of the Cannon River drainage in southeastern Minnesota. Final report submitted to the Nongame Wildlife Program, Minnesota Department of Natural Resources. 21 pp. + figures and original data sheets.

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.

Heath, D. J. 1990. Identification of distribution, abundance, and critical habitat for Lampsilis higginsi and Category 2 species of mussels - performance report - Ocober 1, 1987 to September 30, 1988. Wisconsin Endangered Resources Report #65. 11 pp. + tables and figures.

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. 1995. Host research on Round Pigtoe glochidia. Triannual Unionid Report 8:8.

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

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

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