The scaleshell is a rarely seen, endangered freshwater mussel. It has a thin and delicate shell that is strikingly beautiful inside. It occurs only in the Gasconade and Meramec river basins.
Externally, the scaleshell's shell is relatively small, elongate, thin, and compressed. The anterior end is rounded; the posterior end is pointed. The dorsal margin is straight; the ventral margin is straight to broadly curved. The umbo (beak) is small and low, about even with the hinge line. The beak sculpture, if visible, consists of 4 or 5 double-looped ridges. The periostracum (thin outer layer) is olive to brown, covered with fine, wavy rays.
Inside the shell, the beak cavity is very shallow or absent. The pseudocardinal teeth are reduced to a small thickened ridge; the lateral teeth are moderately long, with 2 low, indistinct lateral teeth in the left valve, and 1 fine tooth in the right. The nacre (mother-of-pearl layer) is pinkish white (rose-colored) in the cavity, bluish or light purple elsewhere, and highly iridescent.
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 fragile papershell and the paper pondshell are most similar.
Adult length: up to 4 inches.
Found only in the Gasconade and Meramec river basins.
Habitat and Conservation
Occurs in clear, nonpolluted riffles with moderate current and firm gravel, cobble, or sand substrates.
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
Listed as endangered by both the Missouri Department of Conservation and the U.S. Fish and Wildlife Service. Current threats include loss of habitat and competition from nonnative, invasive zebra 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 freshwater drum is the only known host species for the scaleshell. 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.
