The wartyback is a Missouri species of conservation concern. Its favored habitat is large streams or rivers in sand to fine gravel. The bumps on the shell may help to anchor it in the river bottom. In Missouri, it mostly occurs in rivers of the Bootheel, northeast Missouri, and along the Mississippi River.
Externally, the wartyback's shell is thick anteriorly to thin posteriorly; rounded to squared-off, and moderately inflated. Two poorly defined rows of elongated knobs diverge from the umbo (beak) to the ventral margin. The umbo is rounded and raised high above the hinge line. The periostracum (thin outer layer) is generally yellowish to light brown, becoming darker with age.
Inside the shell, the beak cavity is deep; the pseudocardinal teeth are large, grooved, and roughened; the lateral teeth are moderately long, grooved, and 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 pimpleback is similar but has randomly distributed small knobs and may have a green ray from the umbo to halfway down the shell. The monkeyface has a distinct groove. The threehorn wartyack, purple wartyback, and mapleleaf are also similar.
Adult length: 2–3 inches.
Most often found in rivers of the Mississippi Lowland region, northeast Missouri, and Mississippi River; may be locally abundant.
Habitat and Conservation
Medium to large rivers in sand to fine gravel.
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 Missouri species of conservation concern. Vulnerable. Found only in a very few regions, but locally abundant in those areas. Relatively recently, it was found in the South Grand River (an Osage tributary, now inundated by the Truman Reservoir) and in the Salt River (much of which is now Mark Twain Lake), but damming destroyed their habitat in these waterways.
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, white crappie, channel and flathead catfish, bluegill, and more. 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 were an important resource historically for button manufacture. Some species are still commercially important in the cultured pearl and jewelry industries. This species has been used in the cultured pearl and button-making industries.
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.
