Round pigtoes are more rounded than Wabash pigtoes. Usually, the nacre (shell lining) is white, but in rare individuals it is bright pink. Its is widespread in the Ozarks and the southern half of the state, including lowland rivers, but it also occurs in the Salt River basin.
Externally, the shell of the round pigtoe is relatively thick, heavy, compressed, and rounded (in small streams) or inflated and mostly triangular (in big rivers). The umbo (beak) is low, slightly raised above the hinge line, and tilted forward. The periostracum (thin outer layer) greenish- to reddish-brown or light brown (in juveniles) and chestnut or dark brown (in adults); usually lacking rays.
Inside the shell, the beak cavity is shallow (in medium streams) to moderately deep (in large rivers); the pseudocardinal and lateral teeth are well developed, with the lateral teeth primarily straight; the nacre (mother-of-pearl layer) is white, occasionally pinkish.
For a fuller introduction to Missouri’s native freshwater mussels, and to learn the terms for their anatomy, see their group page.
Similar species: Round pigtoes are easily confused with Wabash pigtoes, which have a pronounced sulcus (a broad, groovelike indentation on the outside of the shell) and a square or triangular appearance. Other similar species include the hickorynut (Obovaria olivaria), Ozark pigtoe (Fusconaia ozarkensis), and ebonyshell.
Adult length: 2–4 inches.
Widespread in the Ozarks and the southern half of the state including lowland rivers, but also found in the Salt River basin.
Habitat and Conservation
Occurs in medium-sized rivers in moderate current with stable gravel, gravel-mud, sand, and cobble.
In this species, the morphology (form and shape) of the shell varies quite a bit, depending on whether the mussel developed in river headwaters or in large, downriver environments.
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.
For its larval hosts, the round pigtoe has been recorded using about 12 species of New World minnows, plus a few others, including the bluegill and the freshwater drum.
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. Mussels in this particular genus were used in the commercial button industry and are valuable for polished chips.
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.
