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).
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.