America's Aching Mussels
Freshwater mussels are among the nation's most endangered animals; scientists say there are good reasons why Americans should care.
Biologist Richard Neves is rummaging through the remains of a muskrat party. Piles of mussel shells line the banks of Pendleton Island, a small, nondescript clump of dirt and brush that bifurcates the Clinch River in the Appalachian Mountains of southwest Virginia. From the look of things, it was a gourmet feast.
"The shiny pigtoe mussel, that's an endangered species," says Neves, wiping the mud off an auburn shell with black striping. "Here's a pimpleback--that's rare in the Clinch River. This birdwing pearly is endangered. So is this fine-rayed pigtoe." The biologist straightens up, drops the shells in the pouch of his waders and shakes his head. "What carnage," he says. "The muskrats really did a number on the endangered ones here."
As devastating as this banquet was to local mussel populations, natural predators are actually the least of the creatures' problems. America's freshwater mussels are in trouble. Once as common throughout eastern U.S. streams and riverbeds as the rocks they often resemble, the tiny animals are being devastated by water pollution, introduced species and poachers, who seek out their distinctive shells for a lucrative trade to the Orient.
In recent years, mussel populations have declined alarmingly in both abundance and diversity. Twelve of the nation's 300 or so species are considered extinct by U.S. officials, and 42 others are federally protected as either threatened or endangered animals. Another 14 may be added to the Endangered Species List this year, and an additional 74 species are candidates for listing. According to researchers for The Nature Conservancy, some 50 U.S. species could become extinct within the next two decades.
"Mussels are far and away the most jeopardized faunal group in the country," says Bill Kittrell, Clinch Valley bioreserve manager for the Conservancy. They also are among the nation's most underappreciated animals. "They're not 'glamorous' like bald eagles," says Kittrell. "Mussels are cold, wet, clammy, out of sight and out of mind."
Their unattractiveness to humans, however, belies their importance. "Mussels are one of our great claims to global biodiversity," says Neves, who points out that Pendleton Island alone is home to more than three times the number of mussel species found throughout all of Europe. "While we're out trying to dictate policy on what Third World countries should be doing to protect their biodiversity, we're losing the most diverse collection of mussels anywhere on Earth."
The United States is home to about a third of the world's freshwater mussel species. With the exception of a few species that live along the Pacific Coast, all of the U.S. mussels are found east of the Mississippi River system, where they occupy a unique niche at the bottom of the freshwater food chain. Mussels are filter feeders, siphoning nutrients out of flowing water and cleaning streams in the process. A dropoff in a local mussel population is the first sign that water quality is deteriorating. It also is a warning that other animal groups may begin to decline, either because they don't have mussels to feed on anymore or because water contamination can eventually kill them too.
"Mussels are an alarm system," says Sue Bruenderman, aquatic biologist for the Virginia Department of Game and Inland Fisheries. "They're our most sensitive indicators of water quality and ecosystem health. Disregarding mussels is like saying your hand isn't important to the rest of your body."
Mussels have been in decline ever since European settlers first explored North American fresh waters. Millions of years of evolution bred an astonishing variety of North American mussels in pristine, free-flowing streams that offered a variety of habitats and a fertile reproductive environment. As the streams have become polluted, dammed or choked with sediment, the mussels-sedentary animals rooted to virtually the same spot their entire 40- to 50-year lifespan-have begun to die.
The southwest corner of Virginia, rural and rich in isolated rivers and caves that escaped the ravages of the glaciers, is one of the last strongholds of not only mussel diversity but endangered fish, bats and a number of rare plants. Yet even that area was clearly in trouble as far back as 1918, when A. E. Ortmann, an expert in vertebrate zoology at the Carnegie Museum in Pittsburgh, surveyed mussel populations in the Clinch and other headwaters of the Tennessee River system. Because of stream pollution, he wrote, "There is great danger that the fauna will largely be destroyed, and that it will be impossible to duplicate this collection."
It was a prophetic statement, written well before the Tennessee Valley Authority began damming river systems in the Southeast and before the U.S. button industry began harvesting many mussel populations into near-extinction to collect the creatures' shells. But even small changes can decimate mussels, which are in a sense victims of their own diversity.
Most mussel species are habitat-specific, requiring a certain depth and flow of water, and a distinct mix of cobble and sand in which to anchor themselves. Scientists frequently have found dramatic differences in mussel species on opposite sides of the same streams. Moreover, mussels need the help of a "host" to incubate their larvae, which only mature when attached to a fish-and mother mussels tend to be very specific about which fish they use. For example, the pocketbook mussel, a species common to the Mississippi River, will attach its larvae to smallmouth bass. If the host fish disappears from a stream, mussels cannot reproduce.
Add the threats posed by natural predators like muskrats, chemical pollutants from coal mines and agricultural runoff, and the silt and sediment that clogs streams when cattle graze along their banks or when farmers clear river edges to plant crops-and scientists are not surprised that mussels are dying off. Neves, a U.S. Fish and Wildlife Service biologist based at Virginia Polytechnic Institute (VPI) in Blacksburg, sees another threat along the Clinch as well. He points it out from a swinging bridge several miles downstream from Pendleton Island.
"This is excellent mussel habitat," he says, indicating a 200-yard stretch of river below. "It used to be one of the better areas to collect specimens. Look at it now." The entire reach has been stripped clean of mussels. How?
"Poaching," Neves speculates, explaining that the Japanese use mussel shells to make cultured pearls. "The price of shell has been high lately," notes the biologist, adding that there is easy access to the river. "Somebody who knows what they're doing could have cleaned this place out in a few days."
Although some states allow the harvest of several species, Virginia has laws prohibiting the commercial collecting of mussels. But policing rivers is difficult, even for groups with a particular interest in mussel welfare. "If the mussels here are going to be saved, it's up to the people living in the area," says The Nature Conservancy's Kittrell. "Especially landowners on the rivers, because they're the ones who have them in their backyards."
The Conservancy, which purchases land in order to conserve it, bought Pendleton Island and turned it into a preserve in the mid-1980s. But the mussels there remain vulnerable to contaminants coming from upstream. With support from the U.S. Fish and Wildlife Service, the Conservancy is now promoting a riparian restoration program to keep riverbanks intact and cows out of southern Virginia rivers, primarily with subsidized fencing.
"I was skeptical of the program at first," says Kevin McGlothlin, a Russell County farmer who raises beef cattle along a scenic stretch of the Little River, a tributary of the Clinch. "There's always been plenty of mussels around here. I didn't see how a little cow manure was doing a whole lot of damage to the river."
At various points where McGlothlin's herd once drank out of the river, the bank is gone entirely, beaten into an erosion nightmare. But now a new wire fence, set 25 feet back from the river, keeps his cows away from the water. "It's a benefit to me to have this fencing for the farming operation," allows the farmer. "So it's a nice compromise-the Conservancy gets protection for the stream, and I get a fence."
This, Kittrell says, is how the free fencing offer has to be packaged to succeed. Not as an attempt to save the mussels, but as a practical benefit to farmers. "You don't initially talk to farmers about mussels, because people can't relate to that," he says. "The idea is to show how the business of farming can accommodate conservation and protection of rare species."
In recent years, the nation's beleaguered mussels have faced yet another threat: the prolific zebra mussel, a species native to the Black and Caspian seas. The zebra probably arrived in North America in the ballast water of ships traveling to the Great Lakes region. The invading mussels first appeared in 1987 in Lake St. Clair, and soon wiped out all of the native mussels in that water body and the western half of Lake Erie. The aggressive zebras overwhelm other mussel species, attaching themselves to virtually anything even other mussels-and then multiplying in dense colonies that starve out the competition. Today, zebras have spread through the Mississippi River as far south as Louisiana, and are rapidly colonizing other river systems that feed into the Great Lakes.
Richard Neves believes it is just a matter of time before the foreign species invades the Clinch River watershed. And so he is employing every strategy he can think of to give endangered mussels a fighting chance. One of the most promising methods is translocationmoving mussels to sites where they once flourished in the hope they can reclaim old habitats.
In 1990, Neves relocated mussels to two such sites along the North Fork of the Holston, where mercury contamination once destroyed all life along an 80-mile stretch of the southern Virginia river. "The fish have come back pretty well, so I think the system is primed for recovery," he says.
The scientist won't know whether the mussels have successfully repopulated the site until 1995. The reason: It takes five years to fully document mussel reproduction. Male mussels release their sperm in late summer. The females siphon it out of the flowing water, then hold on to their fertilized eggs until they develop into mature larvae called glochidia, which are released in early spring. The glochidia attach themselves to a host fish for about three weeks, then drop off. At that point, the fledgling mussels disappear, scientists speculate, into the substratum of the river. The creatures do not reappear until the age of three or four.
Neves has also moved some mussels to experimental farm ponds owned by VPI. If the animals thrive there, such ponds might one day be used to propagate endangered species for return to the wild, or to shelter them until the zebra mussel infestation has run its course.
Research is slow, partly because preservation techniques must be proven successful on common species before they can be tried on endangered species and partly because, until recently, comparatively few studies were conducted on mussels. Scientific and common names for various species were only formalized in 1988. "Of the 300 species in this country, we know the host fish for only 65," says Neves. "That's how limited our knowledge is."
In two years, the biologist will at least know the answer to one mussel puzzle, when he finds out whether his relocation efforts in the North Fork of the Holston have succeeded. Meanwhile, one fact remains clear: "We need to maximize our options now," says Neves, "before it's too late."
Chicago writer Frank Kuznik toured the Clinch River watershed with biologists for this article. Photographer Lynda Richardson lives in the heart of Virginia's freshwater mussel country.