Sheepnose (Bullhead)

Plethobasus cyphyus
Unionidae (freshwater mussels) in the phylum Mollusca

Shell thick, oval or oblong, somewhat elongate, and slightly inflated. Anterior end rounded; posterior end bluntly pointed. Dorsal margin straight, ventral margin curved anteriorly, straight posteriorly. Umbos slightly elevated above hinge line. Beak sculpture of two heavy ridges, visible only in young shells. Shell smooth, except for a row of knobs or tubercles on center of valve, running from umbo to ventral margin. A shallow sulcus (furrow) is present between row of tubercles and posterior ridge. Epidermis yellow or light brown in juveniles, becoming chestnut to dark brown in adults.

Pseudocardinal teeth small relative to overall shell size; two in left valve, one in right (with occasional smaller tubercular tooth on either side). Lateral teeth long, straight or slightly curved; two in left valve, one in right. Beak cavity shallow. Nacre white, occasionally tinged with pink or salmon.

Similar species: White wartyback, threehorn wartyback, round pigtoe, Wabash pigtoe.

Adult length: up to 5 inches.
Habitat and conservation: 
Found in medium to large rivers, with a moderate to swift current, with gravel or mixed sand and gravel substrate.
Distribution in Missouri: 
Found only in the east-central part of Missouri, primarily in the Meramec River basin.
Endangered in Missouri; candidate for federal Endangered status.
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—in this species, apparently the sauger. 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. Because of its particularly hard nacre, this shell was not used in the commercial button industry.
Ecosystem connections: 
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