Illinois Natural History Survey - University of Illinois

Epioblasma triquetra (Rafinesque, 1820)



Epioblasma triquetra, INHS 5808Epioblasma triquetra, INHS 5808. Embarras River, Douglas County, Illinois. 
Length: 2.4 inches (6.1 cm) for male (top), 1.9 inches (4.8 cm) for female (bottom).

Other common names None.

Key characters A triangular shell, relatively thick for its size with a sharply defined, broadly expanded posterior slope. Yellow or yellowish green with green rays, blotches, or chevron markings.Epioblasma triquetra distribution

Similar species Elktoedeertoefawnsfoot.

Description Shell small, fairly solid, triangular (males) to somewhat elongate (females) and inflated (particularly in females). Anterior end rounded, posterior end truncated in males, expanded in females. Dorsal and ventral margins straight to slightly curved. Posterior ridge sharply angled, posterior slope wide, expanded, and ribbed (especially in females). Umbos swollen and slightly elevated above the hinge line. Beak sculpture of three or four faint, double-looped bars. Periostracum yellow or yellowish green, with numerous dark green rays, blotches or chevron-shaped markings. Length to 2.5 inches (6.4 cm).

Pseudocardinal teeth elevated, roughened, relatively thin and compressed; two in the left valve, two in the right, with the front one being thinner and much smaller. Lateral teeth very short, slightly curved, serrated, and elevated. Beak cavity fairly deep. Nacre pearly white, iridescent posteriorly.

Habitat Medium to large rivers in clear, gravel riffles.

Status Widespread but rare throughout the Midwest. Federal Candidate (Category 2). Endangered in Illinois, Indiana, Wisconsin, and Michigan. Threatened in Ohio. Rare in Missouri.

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