Round Pigtoe

Pleurobema sintoxia


round pigtoe
Jim Rathert

Unionidae (freshwater mussels) in the phylum Mollusca


Shell is relatively thick, heavy, compressed and rounded (small streams) or inflated and mostly triangular (big rivers). Umbo is low, slightly raised above hinge line and tilted forward. Greenish- to reddish-brown or light brown epidermis in juveniles and chestnut or dark brown in adults; usually without rays. Inside shell beak cavity is shallow (medium streams) to moderately deep (large rivers); pseudocardinal and lateral teeth well developed, with the lateral primarily straight; nacre (lining) white, occasionally pinkish.

Similar species: Round pigtoes are easily confused with Wabash pigtoes, which have a pronounced sulcus (a broad, groovelike indentation on the outside of the shell) and a square or triangular appearance.


Adult length: 2-4 inches.

Habitat and conservation

Medium-sized rivers in moderate current with stable gravel, gravel-mud, sand and cobble. In this species, the morphology (form and shape) of the shell varies quite a bit, depending on whether the mussel developed in river headwaters or in large, downriver environments.


Algae and fine particles of decaying organic matter; extracts nutrients and oxygen from water drawn into the body cavity through a specialized gill called the incurrent siphon; sediment and undigested waste are expelled through the excurrent siphon.

image of Round Pigtoe Distribution Map
Distribution in Missouri

Widespread in the Ozarks and the southern half of the state including lowland rivers, but also found in the Salt River basin.


Common, although degrading water quality and watershed destabilization interfere with the survival of this and all freshwater mussels.

Life cycle

Males release sperm directly into water. Females downstream siphon sperm into the gill chamber, where eggs are fertilized. Eggs mature into larvae (called glochidia), which discharge into the water and attach to host fish. The tiny mussel eventually breaks away and floats to the bottom of the stream, and the cycle repeats.

Human connections

Mussels are excellent biological indicators of water quality because they are long-lived and relatively immobile, accumulating contaminants in water that can be scientifically analyzed. Mussels in this particular genus were used in the commercial button industry and are valuable for polished chips.

Ecosystem connections

Mussels act as nature's “vacuum cleaners,” filtering and cleansing polluted waters. They are also an important food source for other species in the aquatic environment.