Freshwater Mussels

Bivalve molluscs in order Unionoida

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Monkey Face Mussel
AJ Hendershott
Other Common Name
Freshwater Pearl Mussels; Naiades; Unionids
Family

In Missouri, most are in Unionidae (river mussels)

Description

Missouri has a spectacular array of freshwater mussels, with nearly 70 species of these secretive aquatic animals within its borders. Learning the names of their shell features and body parts is important if you want to be able to discuss and identify particular species within this group. Since the shell endures long after the animal that created it decomposes, the shell features, especially, are used for identification.

Like all bivalves (such as clams, oysters, and so on), there are two protective shells (also called valves), which are secreted, over time, by the soft, boneless animal inside. Shell shape is important for identification. Typical shapes include rhomboidal (diamond-shaped), round, oval, quadrate (four-sided, like a square or rectangle), elliptical, triangular, and with a concave (indented) or convex (pushed outward) posterior ridge. The shell changes shape gradually as the animal ages. The growth lines, visible on the outside of the shell, show the shapes the mussel had earlier in its life.

The outermost layer of the shell is the periostracum, a thin layer containing proteins; it is often brownish or blackish and often wears away with time or flakes off after the mussel has died. Under the periostracum is a thin layer of vertical prisms of calcite (calcium carbonate). Beneath this is the nacre, or mother-of-pearl layer, made of calcite and aragonite; in most adult mussels, this is the thickest part of the shell.

There is a front (anterior) and back (posterior) to mussels: The anterior is the foot end, and the posterior is the siphon end. The top (dorsal) part of the shell is where the hinge is located, and the bottom (ventral) part is the edge opposite the hinge.

Inside the Shell

The rounded portion near the hinge is the umbo (or beak). The inside portion of this area is the beak (umbonal) cavity; its presence, depth, and width can be important for identifying some species.

The pseudocardinal teeth, anterior to the umbo, usually have 2 teeth on the left shell and 1 in the right. The lateral teeth (toward the posterior side) usually are double in the left shell and single in the right. These sets of teeth fit snugly together and prevent the two shells from shearing apart. In many species, there is a flattened, shelflike area between the two sets of teeth; this is called the interdentum. The sizes and shapes (or presence or absence) of teeth and interdentum are often important for identification.

The two shells are held together by a stout hinge ligament, which is dark brown or black. This ligament constantly exerts pressure on the shells, causing them to be open by default. There are also two strong, columnlike adductor (or retractor) muscles attached to each shell. When relaxed, these muscles allow hinge ligament to pull the two shells open. This is why the shell is open slightly when the animal is siphoning or when it has died. When the adductor muscles are tightened, however, they pull the two shells together. On the inside of a mussel shell, you can see two circular scars where the two adductor muscles were attached.

Another scar that's easy to see on the inside of the shell is the shiny pallial line, which parallels the ventral margin of the shell; this indicates where the mantle of the animal attached to the shell, and this line usually connects to both adductor muscle scars.

Outside the Shell

On the outside surface of the shells, note whether the exterior texture is smooth, or is roughened with wrinkles or any other irregular textures, or has parallel wavelike undulations, knobs, pustules, or warty projections. The concentric growth lines may be only slight ridges or grooves, or they may be more pronounced. External shell coloration is important, too; note the overall hue, plus any stripes, spots, or other markings.

Certain contours on the outside of the shell help identify species, too. The umbo (beak) and hinge have already been mentioned. On the posterior half of the shell, there is often a posterior ridge that extends from the beak to the posterior edge of the shell. Sometimes this ridge adjoins a posterior slope that extends all the way to the dorsal edge of the shell. On the other side of the posterior ridge there can be a sulcus, a valley or furrow-shaped elongated depression, which extends from the beak to the edge of the shell.

The Animal Within

The soft-bodied animal that creates the shell is a mollusk, so it is related to clams, oysters, snails, slugs, squids, and octopus. Bivalves, like mussels, clams, and oysters, are filter feeders. Their mantle secretes the materials that become the shell. A long, muscular mass called the foot can be extended from between the two valves, wedged into the substrate, inflated with fluids to anchor it in the substrate, then retracted, enabling the mussel to move. The incurrent siphon (or aperture) draws water into the shell, and food particles are trapped by mucus in the animal's gills. Debris and waste products are expelled via the excurrent siphon (or aperture).

Similar Species

Freshwater clams are usually much smaller (¾ inch or less), more circular, with the umbo about the same distance from front and back ends, and less flattened. There are several native species of these small native clams, usually called fingernail clams or pea clams.

The Asian clam (Corbicula fluminea) is an exotic, invasive species and is found nearly statewide in a variety of aquatic habitats. It can reach 2 inches in length; its larger size and sculpture of concentric lines, which continue as raised rings, help to distinguish it from our native clams.

The zebra mussel (Dreissena polymorpha) is another exotic, invasive species. The shell is a thin, inflated, elongated triangle. The dorsal edge is straight to broadly rounded and sharp; the posterior ridge is sharply angular; the anterior end is sharply pointed. The periostracum is variable, but it usually has dark brown or black bands.

Size

Shell length varies with species. Most species grow to be 2–7 inches long. Our largest species grow to 10–12 inches long.

Habitat and conservation

Mussels can live in just about any permanent body of water in our state. A few species can live in ponds, lakes, and reservoirs, but most require flowing water, so most are found in rivers and other streams. The downstream sections of stream systems usually have the greatest diversity of mussels, as the tributaries come together and the stream size increases. Adult mussels live partly buried in the substrate. Although some species actively burrow and crawl, others may remain in the same spot for decades. Because most river-dwelling species have similar habitat requirements, as many as 20 or more species can sometimes be found living together in favorable areas, called mussel beds. Usually these stream sections have the most stable substrates, a firm mixture of cobble, gravel, sand, and silt, and are deep enough to be submerged even during drought.

Foods

Mussels are filter-feeders. Primary foods include diatoms and other algae, protozoans, plant and animal debris, eggs and sperm of their own and other species, and molds. Water, which may contain food, is drawn into the mantle cavity via the incurrent siphon. The water passes around the gills, where food, trapped in mucus secreted by the gills, is moved toward the mouth via tiny hairlike structures called cilia. Food is digested in the stomach, and waste products leave via the anus, which is located near the excurrent siphon. Water flowing out of the siphon carries the waste out of the animal. This system of feeding removes particles from water and, over time, tends to make it clearer.

Distribution in Missouri

Statewide. The Ozarks, with its many separated stream systems, holds the greatest variety of freshwater mussels. Some species have narrow ranges and are restricted to a few rivers or locations. Others are widespread and found statewide. The clear, flowing waters of the Gasconade, Meramec, and Osage river systems support the most diverse mussel assemblages in the state.

Status

Freshwater mussels are disappearing at an alarming rate throughout North America. About 69 native mussel species live in Missouri. The number changes a bit, since discoveries are still being made and biologists debate the exact classifications. Our rich diversity of mussels reflects our many different major river systems, each with its own set of native plants and animals. Nearly 42 percent (29 species) of the mussel species in Missouri are Species of Conservation Concern. Of these, 15 species are in danger of extirpation from Missouri or of extinction throughout their entire range. For several endangered species, Missouri rivers are home to some of the last remaining populations of these rare animals. Mussels are the second-most endangered animal species group in Missouri (after crayfishes). Water quality degradation and loss and alteration of physical habitat, dam construction and reservoir formation, sedimentation, loss of streamside vegetation, pollution, and invasive species are just some of the factors that threaten our native mussels.

Life cycle

Male mussels release sperm, which are filtered from the water by females. The fertilized eggs are brooded in the female’s gills, where they develop into tiny larvae called glochidia. The glochidia are parasitic, and to complete their development, they must attach to the gills, fins, or skin of their host, usually a fish. Some mussel species can use a variety of host species, while others can only develop on just a single host species. Mussels have a variety of adaptations that help them attract a host species; for example, some species display lures that resemble small fish or crayfish to attract bass or walleye, which strike at the lure and inhale the larval mussels. Glochidia are not harmful to their hosts; they remain on the host for only a few days or weeks before dropping off and continuing their development into adult mussels.

Human connections

In the past, the shells of mussels were an important economic resource, as many species were harvested and buttons manufactured from their shells.

Native Americans had a multitude of uses for the shells, using them as scraping and digging tools, as jewelry, and much more.

Today, people can collect mussels and use them for bait (permit required; check the current Wildlife Code of Missouri and Area Regulations for specific regulations). Be absolutely certain of your identification, since many species are threatened or endangered, thus prohibited from collection.

You can observe dead shells by simply walking banks during low water periods, and you can see live mussels by snorkeling, using water scopes, or hand-groping in shallow water.

When you return a live mussel to a stream, make sure the hinge, where the two halves of the shell are joined, is pointing up.

Start or join a Stream Team at mostreamteam.org.

Ecosystem connections

Mussels are an important food source for many animals, including fish, small mammals, and some birds.

They filter algae, bacteria, and other particles from the water, improving water quality and cycling nutrients and energy in streams and lakes.

Freshwater mussels depend on healthy populations certain types of fish to complete their life cycle. Although as larvae they temporarily parasitize these fish, the harm to the fish is negligible.

Because mussels are sensitive to habitat disturbance and pollution, they are good indicators of the overall health of aquatic ecosystems and water quality.