The elktoe is one of many Missouri freshwater mussels with a declining population. It is found mostly in the southern half of the state.
Externally, the elktoe's shell is elongate, nearly rhomboidal (like a stretched diamond-shape), inflated, and relatively thin. The anterior end is rounded; the posterior end is sharply angled, ending in a blunt, squared point. The posterior ridge is sharply angled and prominent; the posterior slope is broad, flat, and covered with fine ribs. The umbo (beak) is large, located near the center of the shell, and elevated above the hinge line. The beak sculpture consists of three or four heavy, double-looped ridges. The periostracum (thin outer layer) is smooth and dull, yellowish green or bright green, with numerous rays and dark green spots. The posterior slope is often lighter-colored than the rest of the shell.
Inside the shell, the beak cavity is moderately deep. The teeth are poorly developed. The pseudocardinal teeth are thin and elongate; one in the right valve (shell), occasionally two in the left. The lateral teeth are reduced to a thickened swelling along the hinge line. The nacre (mother-of-pearl layer) is bluish white, occasionally with salmon near the beak.
For a fuller introduction to Missouri’s native freshwater mussels, and to learn the terms for their anatomy, see their group page.
Similar species: Compare to the snuffbox, slippershell mussel (Alasmidonta viridis), and deertoe.
Adult length: up to 4 inches.
Sporadic in distribution. Generally in the southern half, in rivers draining the Springfield and Salem plateaus. Also has been found in the Salt River and Cuivre River.
Habitat and Conservation
Medium-sized streams in gravel 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
Uncommon; a Missouri species of conservation concern. The elktoe is one of many Missouri mussels with a declining population. It is usually not abundant where found. It had been been quite numerous in the Big River before two major intrusions of mud and finely ground rock changed the stream. These incidents were caused by improperly handled wastes from nearby mining activity.
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; this species uses several members of the sucker family, including white sucker, northern hog sucker, shorthead redhorse, smallmouth buffalo, and quillback, as well as some types of minnows, killifish, and sculpins. 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.
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.
