The ellipse and the bleedingtooth mussel are closely related and are very similar in appearance, but they occur in different streams.
Although both occur mostly south of the Missouri River, in streams arising from the Springfield and Salem plateaus:
- The ellipse occurs in northward-flowing streams;
- The bleedingtooth mussel occurs in southward- and westward-flowing streams.
Externally, in both species, the shell is small, relatively thick, elliptical to elongate, compressed to moderately inflated with a bluntly pointed (ellipse) to acutely rounded (bleedingtooth) posterior end and a thicker anterior end (ellipse). The umbo (beak) is low and slightly raised above the hinge line. The periostracum (thin outer layer) is yellowish brown to brown in adults with many fine dark green rays.
Inside the shell, the beak cavity is shallow; the pseudocardinal teeth are triangular, heavy, and roughened; the lateral teeth are short, thick, and straight to slightly curved; the teeth are sometimes rusty red; the nacre (mother-of-pearl layer) is bluish white and often tinged with rusty 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: Juvenile muckets are shorter, stouter, uniformly thick, and lack the fine rays. The Arkansas brokenray is shorter, more rounded, and thinner, and females often have an indentation in the posterior margin. The rainbow (Villosa iris) is also similar.
Adult length: 1–3 inches.
Both species occur mostly south of the Missouri River, in streams arising from the Springfield and Salem plateaus. The ellipse occurs in northward-flowing streams, and the bleedingtooth mussel occurs in southward- and westward-flowing streams.
Habitat and Conservation
Small to relatively large rivers in noticeable current with firm sand or mixed 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
Fairly common, although degrading water quality and watershed destabilization interfere with the survival of these 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 (darters are the host fish for the ellipse). The tiny mussel eventually breaks away and floats to the bottom of the stream, and the cycle repeats.
For its larval hosts, these species use darters. Though they might be able to use different species of darters, it appears that the ellipse and bleedingtooth may use different species of darters.
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.
