Freshwater mussels may not be cute, but we can’t afford to ignore them

by Sara Shipley Hiles

Mussel, © Annie Griffiths Belt/National Geographic Stock

© Annie Griffiths Belt/National Geographic Stock

The summer sunlight filters down through the maples and tulip trees and glints off the surface of the swirling stream. Monte McGregor stands in the calf-deep water, digging into the sand and gravel with a gloved hand, pulling out what appear at first to be elliptical brown rocks.

Like a magician pulling a rabbit out of a hat, he plunges back in and comes up with more strange creatures: a pink heelsplitter, a monkeyface. Then he grins wide. "There's a fanshell," he says proudly. "This is endangered. This is as rare as anything in Africa."

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McGregor, a malacologist, or mussel scientist, with the Kentucky Department of Fish and Wildlife Resources, counts four endangered fanshell mussels on this trip—but that's nothing. There are hundreds of them here on the Green River in central Kentucky, not far from Mammoth Cave National Park. This particular site has about 25,000 freshwater mussels in an area the size of a football field—a biological treasure chest buried in the sand. Mussels can live for 40 to 50 years, some growing to the size of a dinner plate.

"That section of the river there at Munfordville has historically got about 50-something species. That's pretty remarkable. That's over half of what we have in the state, and about 20 percent of what we have in the country, right there in that one spot," McGregor says.

This part of the Green River is one of the few places in the country where freshwater mussels are thriving. Elsewhere, the spectacular diversity of mussels that once populated North America is disappearing. Damming, dredging, invasive species, toxic runoff—it has all taken a toll on the lowly mussel. Despite protection for at least 70 mussels under the Endangered Species Act, many species have not been seen for years. Another 180 species are identified as imperiled or vulnerable, making freshwater mussels one of the most endangered groups of animals in North America.

With their colorful, pearly shells, mussels were once commercially harvested for the button industry. These mollusks also serve an important function in the ecosystem by filtering bacteria and particles from the water. Mussels can live for years in the same spot, making them very vulnerable to pollution from chemicals, sediment and heat. In fact, mussels are so sensitive that the Environmental Protection Agency (EPA) has proposed more stringent water quality criteria based on mussels.

"Freshwater mussels are indicators of water quality—and we all depend on clean water for life," says Bob Irvin, senior vice president for conservation programs at Defenders of Wildlife. "When we see the serious declines of freshwater mussel species we're experiencing, it's a warning that something's wrong with our rivers and streams. It should be a wakeup call to all of us."

McGregor is one of a small but growing number of scientists working to reverse that trend. He runs a laboratory dedicated to helping mussels create offspring—a difficult feat, given mussels' bizarre reproductive behavior.

Mussels need a host fish to complete their development. To attract these hosts, female mussels have developed a variety of "lures" that resemble live crawfish, worms or even a wriggling fish on a line. When the fish comes near, the mussel spews out its larvae, which attach to the fish's gills and continue to grow. The young later drop off and mature elsewhere in the stream, leaving the fish unharmed.

Males must be upstream of females to fertilize eggs, and the fish must come along at exactly the right time. Add in that each species of mussel uses specific types of host fish, and success at the mating game becomes more like winning "Who Wants to Be a Millionaire?"

See mussel scientist Monte McGregor at work and learn more about endangered mussels.

Looking to increase the odds, McGregor takes pregnant females and hooks them up with host fish at the Center for Mollusk Conservation in Frankfort, Kentucky. The resulting juveniles are then released into the wild. The center, which opened in 2002, holds 60 species of mussels, including eight endangered species.

One of the center's important releases came in 2007, when biologists dropped 1,100 juvenile endangered pink mucket mussels into the Green River. Because the young mussels were so tiny—about the size of a grain of sand—it was not possible to tag and track them.

Now scientists have learned how to raise mussels faster and larger in the lab. It has taken years to learn what baby mussels eat and what host fish each species likes best. McGregor and Kentucky State University researcher Christopher Owen also have learned how to raise mussels without the fish host, using a soupy mix of fish blood and modern cell culture nutrients housed in an incubator. "We do a lot of research on how to raise mussels," McGregor says. "There's not a cookbook."

This year, McGregor's team will release the endangered Cumberland bean mussel and the salamander mussel. These lab-raised juveniles are quarter-sized, big enough for tracking tags.

"We'll be able to check on them next year," McGregor says. "If they survive, it's a pretty major accomplishment."

Mussel-rearing programs are now hitting their stride in terms of raising animals to a size likely to make it in the wild, says Chris Barnhart, a biology professor at Missouri State University in Springfield. His mussel lab, which opened in the mid-1990s, is one of the oldest in the country. Scientists' ability to raise mussels in the lab also has led to an explosion of basic research on mussel biology, Barnhart says.

About three dozen of these mussel studies form the basis of EPA's new draft water-quality criteria for ammonia, a common pollutant found in human and animal waste. The new criteria would be up to four times more stringent in freshwater bodies where mussels are present, says Ephraim King, director of the Office of Science and Technology in EPA's Office of Water. He says the EPA expects to finalize the criteria by the end of the year, and then it's up to states to adopt them.

Biologists say the change would be a positive step—and it may come none too soon for some mussels. Even the relatively healthy Green River is still losing rare species. Researchers have had little luck searching there for the endangered ring pink mussel. "Over the last 10 years, only three live individuals have been found," says Leroy Koch, a biologist with the Kentucky field office of the U.S. Fish and Wildlife Service. "One of those was a female, and she died in captivity while we were looking for males. We found two males, and while we were looking for a female, they both died." As part of its recovery efforts, the agency is funding research on cryopreservation of mussel sperm and eggs as a last-ditch effort to prevent extinction, he says. "The clock is ticking on many of these species," Koch says.

On his trip to the Green River that day, McGregor transplanted about 30 endangered rough pigtoe mussels taken from a lower part of the river to the site near Munfordville. That stretch of the river is one of the last free-flowing sections of the Green. Private landowners, state and federal agencies and groups such as The Nature Conservancy are all working together to preserve and restore the stream.

McGregor planted the mussels foot-first in the sand. "You can see they're all old," he says. "They're not reproducing down there." Whether they will live or create offspring is anyone's guess. "There's so many chances the mussels won't make it," he says. "We're trying to get those chances higher."

Sara Shipley Hiles is a freelance journalist specializing in environmental topics. Any assignment that involves wading in a stream is all right with her.

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