Freshwater mussel attracting a fish. Photograph courtesy of the U.S. Geological Survey.

Science

Mussel Mania: Boosting the Freshwater Variety

Wildlife Conservation, February 2007

Since prehistoric times, freshwater mussels have played a sophisticated scam on their neighbors. The con is simple: Set out a lure that looks like a free meal. Then wait for a fish to strike it, releasing thousands of microscopic mussel larvae into the unsuspecting predator's gills. The clam-shaped larvae, called glochidia, clamp their primordial shells onto strands of gill tissue. Once there, they hitch a ride—with free food from the fish's blood—until they develop into juveniles with gills and intestines. At this stage, they drop off, dig into the substrate, and begin to grow into adults. The host swims away, unharmed. Most mussel species use fish to host their glochidia, but some use other organisms such as salamanders.

Their lures can be lifelike reproductions of fish, freshwater snails, insects, worms, or plants. Some species wave the lures from their bodies; others eject the lures onto the riverbed. A few groups release their glochidia directly into the water, where they latch onto the fins or gills of passing fish.

Almost 300 of the world's approximately 900 freshwater mussel species are found in the United States. Their array of shell shapes, sizes, and colors has inspired odd names like purple wartyback, monkeyface, pistolgrip, spike, elephant ear, and spectaclecase. Many U.S. rivers, particularly in the southeastern states, were once full of mussels. But dam-building, development, pollution, and overharvesting have decimated populations. Zebra mussels introduced from Europe and Asia have invaded their territory, suffocating and starving native mussels. At least 35 native freshwater mussel species have gone extinct since 1900. Sixty-two native species have been officially listed as endangered in the United States, and eight are listed as threatened.

In recent years, researchers have released many thousands of juvenile mussels into waterways, attempting to boost mussel populations. Some species, however, are dying out too quickly to be restored.

"It's kind of an exciting time, but just as we're discovering more about mussels, they're disappearing," says Chris Barnhart, a biology professor at Missouri State University. Barnhart's facility at the university produces juvenile mussels to repopulate streams and for use in toxicology testing in laboratories.

Mussel experts are still finding out which chemicals in the water are most harmful to mussels. Of all aquatic organisms, freshwater mussels are among the most sensitive to changes in water quality. According to researchers, small amounts of chlorine and ammonia discharged from a wastewater treatment plant can kill mussels several miles downstream. Likewise, mussels are highly sensitive to zinc and copper, two contaminants that leach into rivers from coal mining waste.

Freshwater mussels are filter feeders; they take in water through their gills and digest the nutrients, spitting out indigestible particles onto the river floor. In a healthy river system, mussels purify the water, ridding it of algae and other microbes as well as sediment. Some of the largest mussels process huge amounts of water. Mature giant floaters, which grow to ten inches in length, can clean five gallons of water per hour.

Richard Neves, a professor at Virginia Polytechnic Institute and State University in Blacksburg, says people often ask him what mussels are good for. "My temptation is to say, 'What good are you?'" Neves says. Instead, he explains that they clean rivers and lakes and are a food source for muskrats, raccoons, otters, and other wildlife. In the past, Native Americans ate freshwater mussels in large numbers. But because toxins in the water accumulate in the mussels' tissues, they are generally no longer considered safe for humans to eat.

The Virginia Tech Freshwater Mollusk Conservation Center, which Neves founded, produces young mussels for agencies in several southern states. It was the first facility in the United States dedicated to mussel propagation. Since 1997, the center has produced more than a million juvenile mussels and has released more than half a million juveniles into Virginia and Tennessee waterways. These efforts are still in their early stages. It is too soon to know for certain whether the tiny mussels, smaller than a child's fingernail, will survive to reproduce in the wild.

There are a few hopeful signs. Two years ago, Virginia Tech researchers found seven living six-month-old oyster mussels at a release site on the Clinch River in Tennessee, proof that at least some of the juveniles from the laboratory were thriving. New techniques for growing mussels to larger sizes are increasing their chances for survival.

In the upper Mississippi River, a consortium of states is working with the federal government to restore the endangered Higgins eye (pearly mussel), which was used for buttons in the late 1800s. The group has produced thousands of juvenile Higgins eye mussels by lowering caged fish infested with their larvae to the river bottom. Elsewhere in the Mississippi, biologists have found an additional reason for optimism: At least 18 species of mussels have recolonized a formerly dead section of river running through the Twin Cities area of Minnesota.

Improvements in water quality made it possible for the mussels to return on their own, explains Bernard Sietman, a malacologist (mussel expert) at the Minnesota Department of Natural Resources. "If steps are taken to [improve water quality], the animals will respond," Sietman says. The challenge is to make those changes before many more species become extinct.