The pimpleback is usually bumpy, but some individuals are perfectly smooth. It is widespread in Missouri, occurring in small streams to large rivers in coarse gravel and sand or mud. The only substrate it cannot tolerate is loose, shifting sand.
Externally, the pimpleback's shell is thick, rounded to squared, moderately inflated, and covered with few to many pustules (bumps) in no discernable pattern; the anterior third is smooth. The umbo (beak) is elevated above the hinge line and turned forward. The periostracum (thin outer layer) is yellowish green to light brown in juveniles, becoming chestnut to brown with age; there may be a broad green ray on the umbo.
Inside the shell, the beak cavity is deep; the pseudocardinal teeth are well developed and serrate; the lateral teeth are moderately short, heavy, serrate, and straight to slightly curved; the nacre (mother-of-pearl layer) is white, iridescent posteriorly.
For a fuller introduction to Missouri’s native freshwater mussels, and to learn the terms for their anatomy, see their group page.
Similar species: The wartyback has knobs arranged in two weakly defined rows and the green umbo ray is not present. The purple wartyback is more compressed with a purple nacre.
Adult length: 2–4 inches.
Widespread; statewide.
Habitat and Conservation
Small streams to large rivers in coarse gravel and sand or mud. The only substrate it cannot tolerate is loose, shifting sand.
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
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; this species uses shovelnose sturgeon, black and brown bullheads, flathead and channel catfish, and others. The tiny mussel eventually breaks away and floats to the bottom of the stream, and the cycle repeats.
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, including this species, were an important resource historically for button manufacture. Some species are still commercially important in the cultured pearl and jewelry industries.
Ecosystem 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. This species is one that was used to make buttons. Some species are still commercially important in the cultured pearl and jewelry industries.
