Missouri's Freshwater Mussels

By Sue Bruenderman | August 2, 1999
From Missouri Conservationist: Aug 1999

Missouri's freshwater pearly mussels live in the bottoms of rivers and streams near you. They sit quietly in the water and never utter a sound. They offer culinary and cleaning services, without lifting a finger. (They don't have any.) They tell us if our water is clean enough for people and livestock to drink. They can live longer than most humans.

Nearly 300 species of freshwater mussels live in North America, approximately 65 of which live in Missouri waters. The iridescent whites, brilliant purples and beautiful pinks of the nacre lining the inside of freshwater mussel shells are as colorful as the common names of mussels--washboard, pimpleback, elephant ear, rabbitsfoot, ladyfinger and pink heelsplitter, to name just a few.

Freshwater mussels live throughout Missouri in a variety of aquatic habitats. Some species, like the giant floater, are widespread and likely to be found throughout Missouri. Other species, like the purple wartyback, may live in several regions but are found in greatest numbers in areas with particular physiographical characteristics. Four main types of these areas or "aquatic regions" are found in Missouri.

More commonly known as clams or shellfish, freshwater mussels have been an important source of food, tools, buttons and jewelry. Native Americans relied heavily on mussel meat during the winter months when other food was scarce. They carved mussel shells and shaped them into tools. They also made pottery and jewelry from the mother-of-pearl shell interior, also called nacre (nay-ker).

In the early 1900s, the only type of button available was one punched from the shell of a freshwater mussel. The button industry originated here in the Midwest because a rich mussel fauna existed in the Mississippi River. At the time, it seemed an unlimited resource, but as barge-loads of mussels were plucked daily from the river bottoms, mussels began to decline rapidly. The long-lived, thick-shelled commercial freshwater mussel species couldn't reproduce as quickly as they could be harvested. In the 1940s, however, plastic was invented. The shell button industry eventually folded, and the mussels began to recover.

It wasn't long, though, before humankind discovered another commercial use for freshwater mussel shells. The pearly white nacre of some North American mussel species makes excellent starting nuclei for what will eventually become cultured pearls. Small blanks are cut from shells, formed into beads and tucked into oysters. Because it is an irritant, the oyster lays down a thick coating of secretions over the mussel bead. After two to four years, a lustrous, "cultured" pearl is removed from the oyster. Today, the cultured pearl industry is a multimillion dollar business.

Freshwater mussels are ecologically important. Mussels are eaten by raccoons, mink, otters, waterfowl and fish. Muskrats may subsist on freshwater mussels during winter months, leaving behind piles of shells or "middens" on stream banks--remnants of a tasty dinner.

Mussels are biological indicators of water quality because they are long-lived and relatively immobile, and they accumulate contaminants present in water and sediment that can be scientifically analyzed. They are nature's "vacuum cleaners," filtering and cleansing polluted waters.

Mussel hunt

Snorkeling and scuba diving are the most common methods used to search for live freshwater mussels. Water scoping is another method used when the water is cold or polluted.

With your face or bucket close to the stream bottom, familiarize yourself with your aquatic surroundings. Watch for the slightest movements. Perhaps you'll see a puff of sand or silt. Where did it come from? Look closer. If it's a freshwater mussel, the first thing you'll notice are the two siphons extending from between the shell valves. If you reach out and touch it, a live mussel will quickly clamp shut. They are sensitive to shadows and will react as if you are a predator about to eat them.

You can pick up a live mussel without hurting it if you treat it with care. They are so interesting and beautiful, it's hard to resist. Note its shell patterns and long foot as you pull the mussel from the stream bottom. What color is it? The part of the shell where the foot is can be thought of as the "head" (anterior) end.

Actually, mussels do not have a head. Instead, they have a long muscular foot that protrudes from their protective house-of-shell. The mussel walks through the substrate with its foot. The siphons are located at the rear (posterior) end and are sometimes visible sticking out of the substrate.

When replacing the mussel, rebury the foot end. If you accidentally rebury the siphon end, instead, you could suffocate the mussel. If unsure, leave the mussel close by in the same habitat, on its side, behind a boulder or in quiet water and let it rebury itself. If left unburied in swift current, it can easily be swept away to unfavorable habitat. Be sure to always put mussels back where you found them.

A number of Missouri's freshwater mussels are hard to identify. Nevertheless, with a little practice you can learn to identify the more common species near your own home.

Shells of freshly dead mussels, picked clean and discarded in a pile by a muskrat or raccoon, are the best to start with. Some features to inspect include shape and thickness of shell, color of the shell exterior and interior, presence of bumps, pustules or ridges on the shell exterior and shape and size of internal teeth.

A good reference book is Missouri Naiades: A Guide to the Mussels of Missouri by Ronald D. Oesch (Copyright 1984, 1995). This manual contains excellent line drawings and detailed descriptions of each species. It is available from Nature Shop, Missouri Department of Conservation, P.O. Box 180, Jefferson City, Missouri 65102-0180 for $6. Shipping and handling is $2. Missouri residents must include 6.225 percent sales tax.

Feeding and reproduction

The bulk of a freshwater mussel consists of a long muscular foot that contracts and withdraws into its shell if pulled from the substrate. On either side of the foot is a pair of thin, specialized gills that allow the mussel to breathe and filter-feed.

Water is drawn into the body or mantle cavity through the incurrent siphon and passes over the gills, which extract critical oxygen and food (algae and fine particles of decaying organic matter). While food travels to the mussel's stomach, sediment and undigested wastes travel and exit the body through its excurrent siphon. Upon exiting, these "pseudofeces" are in a form other aquatic animals can eat.

Freshwater mussels have a complex life cycle in which the larval stage is parasitic on a host, typically a fish. During the breeding season, males release sperm into the water, which enter the female via her incurrent siphon and fertilize her eggs. Her modified gills serve as a brooding chamber for developing embryos that mature into larvae called glochidia (glo-kid-ee-ah).

Glochidia, which have specialized teeth or hooks on the inside of their microscopic shells, are often contained in a gelatinous packet called a conglutinate, which takes on a shape and size unique to each mussel species. Various mussel species have unique tactics for attracting particular host fishes that share their same habitat. The mantle flap of female pocketbooks, for example, mimics a young fish. She contracts this "lure" to draw the attention of a nearby bass, its host. When the bass attempts to eat the "fish," it gets a mouthful of glochidia!

The glochidia attach to the fish's gills, (which usually doesn't harm the fish) become encysted, develop into juveniles and then drop to the stream bottom. If they land in suitable habitat, they will grow into adults and repeat the cycle. If they land in poor habitat, they will die. Similarly, if glochidia fail to encounter a specific host fish, they will not develop vital organs and slough from the fish and die.

Mussels threatened

Freshwater mussels are declining at an alarming rate throughout North America. They are sensitive to disturbance, because they are relatively immobile organisms, sometimes staying in a single spot for their entire lives. They have a complex life cycle that is easily disrupted, causing reproductive failure. Habitat alteration and loss, illegal and overharvesting, and competition from introduced species are the primary reasons for their decline.

Many Missouri watersheds have been destabilized and water quality has been degraded from poor land-use practices and urbanization. Freshwater mussels cannot tolerate a shifting, unstable stream bottom. They need stable habitat consisting of rocks, sand, cobbles and boulders for securing themselves in an otherwise turbulent environment.

Excessive silt and gravel loads go hand-in-hand with excessive land disturbance. Heavy silt loads interfere with the filtering and feeding activities of adults and smother young juvenile mussels. Mussels can close their shells to avoid temporary slugs of pollutants coming downstream, but eventually they have to open up to breathe and feed, so long-term water quality problems in a watershed will eventually kill them.

Mussel invaders

Two non-native freshwater mussel species--Asian clam (Corbicula fluminea) and the zebra mussel (Dreissena polymorpha) --have found their way into Missouri.The Asian clam was introduced into the western U.S. in the 1930s and quickly spread eastward. Since 1968 it has spread rapidly throughout Missouri and is most abundant in streams south of the Missouri River. Asian clams are believed to compete with native mussels for food and habitat.

In the mid-1980s Zebra mussels hitched a ride in the ballast water of freighter ships traveling from Europe to the Great Lakes. It has rapidly spread throughout the Mississippi River basin and westward to Oklahoma. In Missouri, the zebra mussel is thus far restricted to the Mississippi River, but its expansion into inland lakes and streams is anticipated. In April biologists confirmed a sighting of a zebra mussel in the Missouri River at Sioux City, Iowa.

Asian clam and zebra mussel larvae don't require a fish host to reach a juvenile stage, so they are capable of reproducing at a much faster rate than our native freshwater mussels. Zebra mussels attach to any solid surface, including industrial pipes, native mussels and snails, and to other zebra mussels. They form dense clumps that suffocate and kill native mussel species by restricting feeding, breathing and other life functions.

The zebra mussel has a slightly elongated, triangular D-shaped shell. Its shell color is distinctive, with alternating bands of light and dark colors. This mussel species rarely gets larger than 2 inches in length.The Asian clam has a greenish yellow to black shell that is thick, rounded to somewhat triangular and inflated. The shell is covered with heavy, concentric ridges, originating from the hinge area. Adults are generally 1.5 inches long.

Clams in the Kitchen

With a fishing license, you may take five non-endangered mussels a day for your use. However, because all native freshwater mussel populations are declining, we suggest that you help our natives by using Asian clams (Corbicula) instead. Native clams are tough and tasteless, anyway. Popular in Pacific Rim countries, Asian clams may be taken and possessed in any number, although you still need a license. Because they are filter feeders they should be taken only from unpolluted waters and cooked thoroughly. Use only clams that have tightly closed shells. Put live clams in a bowl with fresh water to cover and let stand for 12-24 hours to clear them of sand and possible pollutants before cooking. Asian clams are easily distinguished from native species by the concentric ridges on their shells.

Asian Clam Chowder

  • 4 to 5 dozen steamed Asian clams, chopped
  • 1 1/2 cups clam broth from steam pot (or substitute 1cup chicken broth, if desired)
  • 3 cups milk
  • 2 slices of bacon, chopped
  • 1 large onion, chopped
  • 2 large potatoes, peeled and diced
  • 2 cups celery, chopped
  • salt and pepper
  • 1 bay leaf, crushed
  • 2 tablespoons butter
  • parsley
  • thyme
  • Optional, to taste: cayenne pepper, paprika, Tabasco(TM) sauce

Scrub clams thoroughly. Place cleaned clams in large saucepan, cover with water and cook over low heat until the shells open. Increase the heat and cook 5-10 minutes more or until you can easily lift or take apart the half shells. Remove clam meat and chop into bite-sized pieces. While clams are cooking, sauté bacon until crisp. Drain and set aside. In same sauté pan, combine onion, 1 tablespoon butter, celery, bay leaf, parsley and thyme (and cayenne pepper if desired) and sauté several minutes until tender. In large saucepan, combine the clam broth reserved from steaming, milk, potatoes, and ingredients from sauté pan. Simmer for 20 to 30 minutes until vegetables are soft and broth has thickened a bit. Stir in the clams and remaining butter and add more liquid, if necessary. Simmer slowly for 10 additional minutes. Garnish with chopped parsley. Serves four to six.

Asian Clam Appetizers

  • 4 to 5 dozen steamed Asian clams
  • 1/2 cup soy sauce
  • 5 garlic cloves, mashed
  • 1 hot red pepper, sliced diagonally
  • 1 tsp. white vinegar
  • 1 tsp. sugar
  • 3 Tbs. cooking wine or sherry
  • 2 Tbs. cold water

Prepare and cook clams as in the chowder recipe, except do not chop the clams. Mix all other ingredients to make a marinade. Remove clams from the shell, saving half of the shells, and marinate clams in the refrigerator for 24 hours. Arrange half shells on a plate and place a marinated clam in each shell. Serve with toothpick skewers. These are popular in bars in parts of Asia where they are snacked on the same way we snack on peanuts in this country.

This Issue's Staff

Editor - Tom Cwynar
Assistant Editor - Charlotte Overby
Managing Editor - Jim Auckley
Art Editor - Dickson Stauffer
Designer - Tracy Ritter
Artist - Dave Besenger
Artist - Mark Raithel
Photographer - Jim Rathert
Photographer - Cliff White
Staff Writer - Jim Low
Staff Writer - Joan McKee
Composition - Libby Bode Block
Circulation - Bertha Bainer