Among all the mussels of Missouri, the threehorn wartyback is perhaps the easiest to recognize: As the shell grows, large knobs are produced, first on one shell and then on the other, in an alternating pattern. It is most common in north-flowing streams south of the Missouri River and in the lower St. Francis River; also in southeastern Missouri, the Salt River, and the Mississippi River.
Externally, the threehorn wartyback's shell is small, moderately thick, and rounded, with 3 or 4 large knobs or horns on each valve (shell half), alternating with those on the opposite valve; there are distinct depressions above each knob. The umbo (beak) is raised above the hinge line. The periostracum (thin outer layer) is light tan to green, becoming darker with age; often, there are fine green rays.
Inside the shell, the beak cavity is shallow to moderately deep; the pseudocardinal teeth are heavy, serrate, and well developed; the lateral teeth are grooved, relatively long, and straight to slightly curved; the nacre (mother-of-pearl layer) is white.
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 and the wartyback are similar, but neither has the fine green rays of the threehorn wartyback, nor such large, distinct knobs with associated depressions. The sheepnose is also similar.
Adult length: 2–3 inches.
Most common in north-flowing streams south of the Missouri River and in the lower St. Francis River; also in southeastern Missouri, the Salt River, and the Mississippi River.
Habitat and Conservation
Occurs in larger streams and rivers with moderate current in gravel, gravel-sand, and gravel-mud; also in reservoirs.
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. The tiny mussel eventually breaks away and floats to the bottom of the stream, and the cycle repeats.
The recorded fish hosts for the threehorn wartyback include the striped and common shiners as well as several other minnows, gizzard shad and skipjack herring, largemouth bass and bluegill, goldeye, walleye, buffalofishes, freshwater drum, channel and blue catfish, and common and bighead carp.
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.
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.
