The sheepnose is classified as endangered in Missouri and federally. It is found only in the east-central part of Missouri, primarily in the Meramec River basin.
Externally, the shell of the sheepnose is thick, oval or oblong, somewhat elongate, and slightly inflated. The anterior end is rounded; the posterior end is bluntly pointed. The dorsal margin is straight; the ventral margin is curved anteriorly, straight posteriorly. The umbo (beak) is slightly elevated above the hinge line. The beak sculpture consists of two heavy ridges, visible only in young shells. The shell is smooth, except for a row of knobs or tubercles on the center of the valve (shell), running from the umbo to the ventral margin. A shallow sulcus (furrow) is present between the row of tubercles and the posterior ridge. The periostracum (thin outer layer) is yellow or light brown in juveniles, becoming chestnut to dark brown in adults.
Inside the shell, the beak cavity is shallow. The pseudocardinal teeth are small relative to the overall shell size; there are 2 teeth in the left valve and 1 in the right (with an occasional smaller tubercular tooth on either side). The lateral teeth are long, straight or slightly curved, with 2 in the left valve and 1 in the right. The nacre (mother-of-pearl layer) is white, occasionally tinged with pink or salmon.
For a fuller introduction to Missouri’s native freshwater mussels, and to learn the terms for their anatomy, see their group page.
Similar species: Mucket, threehorn wartyback (Obliquaria reflexa), and wartyback.
Adult length: up to 5 inches.
Found only in the east-central part of Missouri, primarily in the Meramec River basin.
Habitat and Conservation
Found in medium to large rivers, with a moderate to swift current, with gravel or mixed sand and gravel substrate.
Food
Freshwater mussels consume algae, bacteria, and fine particles of decaying organic matter. They extract nutrients and oxygen from water drawn into the body cavity through a specialized structure called the incurrent siphon; sediment, undigested food, and waste (called pseudofeces) are expelled through the excurrent siphon.
Status
A species of conservation concern: endangered in Missouri and federally.
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.
For its larval hosts, the sheepnose has been recorded using more than 24 species of New World minnows, as well as the sauger and a few species of killifish and topminnows, and the western mosquitofish.
Human Connections
Mussels play important roles in maintaining the health of Missouri’s water resources:
- As food for fish, they are important for Missouri’s fisheries. Small mammals and some birds eat them, as well.
- They filter algae, bacteria, and other particles from the water, improving water quality and cycling nutrients and energy in streams and lakes. Clean water is necessary for people and nature.
- Because mussels are sensitive to habitat disturbance and pollution, they are good indicators of the overall health of aquatic ecosystems and water quality.
The shells of several types of mussels were an important resource historically for button manufacture. Some species are still commercially important in the cultured pearl and jewelry industries. Because of its particularly hard nacre, this shell was not used in the commercial button industry.
Ecosystem Connections
Native freshwater mussels provide vital functions in aquatic ecosystems:
- They are an important food source for many species of fish, wildlife, and other invertebrates.
- Their shells, both while the animal is alive and after it dies, provide habitat for other organisms.
- As filter feeders, individual mussels can filter a significant amount of water per day, but not all of what they remove is eaten. What they don’t eat is combined with mucus into packets. These pseudofeces packets are eaten by some fish and invertebrates.
Freshwater mussels depend on healthy populations of certain types of fish to complete their life cycle. Although as larvae they temporarily parasitize these fish, the harm to the fish is negligible.
