The Wabash pigtoe is a widespread mussel that can have inflated or flattened shells, depending on the size of the stream. In Missouri, it is most common in north-flowing streams south of the Missouri River, but also in the Salt River.
Externally, the Wabash pigtoe's shell is variable, usually thick, square to triangular, somewhat compressed (in creeks and small rivers) to inflated (in large rivers); a broad sulcus (smooth depression) extends from the umbo (beak) to the ventral margin. The umbo is moderately to highly elevated above the hinge line. The periostracum (thin outer layer) is yellowish brown with faint green rays in juveniles, becoming dark brown with age.
Inside the shell, the beak cavity is deep; the pseudocardinal teeth are rough and well developed; the lateral teeth are serrate and straight to slightly curved; the nacre (mother-of-pearl layer) is white, often tinged with 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: Wabash pigtoes are easily confused with round pigtoes, which generally lack a sulcus and have a rounded appearance. Also similar are the hickorynut (Obovaria olivaria) and the deertoe.
Adult length: 2–4 inches.
Widespread; most common in north-flowing streams south of the Missouri River; also in the Salt River.
Habitat and Conservation
Occurs in medium to large rivers in moderate current with a stable mix of coarse sand and 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
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 — in this species, white and black crappie and bluegill, as well as some types of sunfishes, darters, minnows, and others. This mussel releases its larvae in tiny red packages to attract its fish hosts. 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. Large individuals of this species were used in the button industry and have value in the pearl and polished chip 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.
Apparently, the two forms this mussel takes (inflated in larger rivers and downstream sections, and more flattened in smaller or more headwater streams) has to do with the mussel's ability to stay anchored in bottom substrates. A more spherical form may help the mussel keep from being swept away in a larger river's stronger currents. Meanwhile, the flatter, more elongated shells can help mussels burrow down more easily in headwater streams.
