Quagga mussels, an extraordinarily prolific and costly invasive species, jump from the Midwest to Lake Mead. Dealing with them will be anything but a vacation.

On a summer day in 1988, in a shallow, heart-shaped lake near Detroit, two Canadian university students found the future. While searching for native mussels to use in a research project, the students fished up a rock, and on that rock was an unfamiliar striped mussel, no larger than a fingernail.

"We had no idea what it was, except that it was something new and very strange," recalls biologist Paul Hebert, who supervised the project. "So I took a picture of it. I figured it'd be the last and only one I'd ever see."

Experts soon identified the specimens from Lake St. Clair as zebra mussels, a species native to the rivers around the Black and Caspian seas. In a matter of months, the diminutive mussels had conquered the lake, coating hard surfaces with solid layers of sharp shells. They even encrusted the lake's large native mussels, eventually killing them all. "The poor things starved to death," says Hebert.

The zebra mussels hitched rides in boats, bait buckets and river currents, and within five years, they had established themselves in the five Great Lakes and in seven major rivers, including the Mississippi, Tennessee, Hudson and Ohio. Their ecological and economic misdeeds were soon infamous: Zebra mussels can rip apart native food webs, clog water intakes with tons of shells and mussel meat, foster the growth of noxious algae, and turn sugar-sand beaches into treacherous, stinking expanses of jagged shells.

Three years after Hebert snapped his photograph, scientists discovered another species of invasive mussel in Great Lakes, a close relative of the notorious zebra. The quagga mussel, named after an extinct zebra ("It was a jovial moment in the lab," remembers researcher Ellen Marsden), soon proved more than a match for its predecessor. In the Great Lakes, many areas once dominated by the zebra now face quagga rule.

Biologists sometimes compare the movement of invasive species to a series of hubs and spokes. Successful invaders radiate outward until, through feats of endurance or gifts of luck, they leap into new territory, establishing new hubs -- and soon, new spokes.

The quagga mussel made its first major leap in the late 1980s, when scientists believe it stowed away on a Black Sea ship and traveled nonstop to North America. Within the past two or three years -- no one knows exactly when -- the quagga leapt again. This time, the pioneers might have taken refuge in a persistent puddle on a fishing boat, or inside a rubber boot. They rode the freeways across hostile territory, surviving the plains and deserts. Finally, they found a new home in Lake Mead, the Colorado River reservoir just east of Las Vegas.

The mussels settled deep beneath the surface of the reservoir, and did what they do best: They multiplied, quickly and quietly. Lake Mead staff estimate that by the time they were discovered, on Jan. 6, 2007, they numbered in the millions.

Nearly eight years of drought in the Southwest have reduced Lake Mead to its lowest level in more than 40 years, and the reservoir's exposed, dusty shoreline gives the Las Vegas Boat Harbor a makeshift air. But even on this late January weekend, the harbor is busy, and its gently bobbing docks are packed with boats of every description and provenance.

Wen Baldwin retired, more or less, to the Lake Mead area from Colorado 18 years ago, and he's treated as a local here at the harbor; at its café, he greets the waitress and his fellow customers with cheerful teasing -- despite his despondence over recent events.

"I feel like I've been hit by a stick by this thing," he says, shaking his head over a cup of coffee. Baldwin led the crusade to keep invasive mussels out of the Lake Mead National Recreation Area, and it's clearly no fun to be routed by bivalves. "They snuck right in underneath me," he says, in a tone salted with genuine anger.

Baldwin first learned of zebra mussels about five years ago, when he heard an ecologist from Glen Canyon National Recreation Area -- which encompasses Lake Powell, the next reservoir upstream from Lake Mead -- speak about the likelihood of invasive mussel introduction in the Colorado River. "Whoa, it really got my attention," Baldwin says. "Lake Mead was right in line. I could just visualize ... well, I could just visualize it all happening here."

In their native neighborhoods around the Black Sea, zebra and quagga mussels -- both members of the genus Dreissena -- are reined in by natural competitors and predators. But in the early 1800s, new canal systems allowed zebra mussels to invade Western Europe, where they became a widespread pest. Quaggas -- which can tolerate deeper depths, cooler temperatures, and softer surfaces than the zebras and are generally a few hairbreadths bigger, averaging about the size of a pinto bean -- seem to spread more slowly than their cousin mussels. But the quaggas have also made their way upstream from their native range.

Back in 1893, nearly a century before the zebra mussel crossed the Atlantic, English naturalist Harry Kew called Dreissena "one of the most successful molluscan colonists in the world." Little did he know how right he was.

The problems caused by zebra and quagga mussels are intricate and far-flung, but they all start with sex: Though some adult zebra and quagga mussels are as small as grains of rice, each female can produce a million eggs each year. Each mussel that survives to maturity can filter a liter or more of water every day, taking in algae and other edible particles and spewing out water clear enough to use in a swimming pool.

This talent for purification sounds positive; it's anything but. The particulates the mussels strain from their environment feed an array of aquatic invertebrates, which in turn feed fish, which in turn ... you get the idea. Zebra and quagga mussels, by dint of their huge numbers and impressive industry, can disrupt food webs and, therefore, entire ecosystems.

In the Great Lakes, the clearer water produced by zebra and quagga mussels allows sunshine to reach deeper into the lake than it would otherwise, encouraging the bloom of new species of algae; in fact, many local residents met the zebra mussel through a distasteful change in their drinking water. Before water suppliers changed their treatment regimes to deal with the algae growth, "the water had an organic taste, like an infusion of lawn clippings," says Paul Hebert. "Imagine a grass shake."

Steve Yancho, chief of natural resources for Sleeping Bear Dunes National Lakeshore, 35 miles of still-scenic beaches and sand mountains on Lake Michigan, remembers when visitors started complaining about large mats of sewage -- at times bigger than a car -- floating near the beach. The mats smelled revolting, and their texture was even worse. "We went out to grab some samples of the stuff with a boat hook," says Yancho, "and it was like we were passing the hook through warm petroleum jelly."

When park staff could bear to get closer, they realized the gunk had come not from a passing ship but from the bottom of the lake. It was the remnants of algae beds -- fostered by the extra light and fertilized by mussel waste -- that had died and decayed below the surface.

Scientists are still trying to understand the extent of the biological mischief caused by zebra and quagga mussels. In the Great Lakes, the mussels changed the migration patterns of ducks, some of which now snack on mussels all winter rather than fly south. (Zebra and quagga mussels have no North American predators capable of controlling their spread, but ducks and some species of fish, including the invasive round goby, do eat them.) Die-offs of loons and other birds -- including 180 loon casualties at the Sleeping Bear lakeshore last summer -- are blamed on the rare type E botulism, probably nurtured in the oxygen-poor conditions caused by rotting algae and mussel waste. The mussels' filtering prowess can then concentrate the botulism toxin -- and countless other contaminants -- in their tissues and wastes, likely opening a new route for contamination in the Great Lakes food web.

The human costs are direct and enduring. "Anyone with a pipe in the water has reason to worry," says Charles Ramcharan, an aquatic ecologist at Laurentian University in Ontario. In late 1989 and early 1990, the water supply for the Michigan town of Monroe was interrupted three times -- once for more than two days -- when mussels, drawn to the municipal water intakes by the flow of water and nutrients, mobbed the structures inside and out. Throughout the Great Lakes, power plants have fought off infestations of their cooling water systems, and in 1991, besieged Monroe hosed and vacuumed 30 tons of mussels out of a single water-intake structure.

Municipalities and industries along the Great Lakes have tried to defend their equipment with tactics ranging from electric currents to anti-mussel paint, but most now protect their intakes by installing feed lines for chlorine or potassium permanganate, which kill larvae and settling adult mussels. The horrifying initial costs have abated somewhat, but ongoing maintenance is required. Charles O'Neill Jr., an invasive-species specialist with New York Sea Grant, a university-based research and education program, estimates that zebra and quagga mussels have cost the region between $1 billion and $1.5 billion.

Baldwin has never fished in Lake Mead -- he prefers to fly-fish the mountain streams of Colorado -- but he loves the lake nevertheless, and often makes the 10-mile trip to the Las Vegas Boat Harbor from his home in Henderson.

"I go out in the middle of the lake, shut my boat off, and sit back and relax," he says. "Or maybe I'll get out on the beach, go climb in the hills. That's how I get away from the rest of the world, let it go on by."

The prospect of the lake's degradation -- and the associated risk to the city of Las Vegas, which gets about 90 percent of its drinking water from the lake -- kept Baldwin interested in zebra mussels. His attention is notable, considering that his career has included stints as a rancher, hunting guide, design engineer, and, most recently, slots floorman at the Rio in Las Vegas. "Every time it gets routine, it's time to change," he says. "This never gets routine."

Though Baldwin likes to insist on his scientific illiteracy, he learned enough mussel biology to deliver a forceful stump speech about the species and took his message into state and federal offices. In some quarters, his cause was a difficult sell: With plenty of immediate crises to worry about, public agencies had little time or money to spare for a theoretical problem, even one as reality-based as the zebra mussel. The Bureau of Reclamation, for example, funded an extensive zebra mussel monitoring program at its Western reservoirs for several years in the early 1990s, only to abandon the effort when the threat failed to materialize.

But Baldwin found allies in Lake Mead staff and other members of the 100th Meridian Initiative, a publicly funded consortium of state and federal agencies dedicated to keeping the zebra mussel -- and other aquatic invasives -- on the far side of the 100th meridian, the historic border of the West. They surveyed recreational boaters, the most likely cross-continental mussel carrier, and mounted educational campaigns to keep infested boats out of Western waters.

Baldwin was soon volunteering 10 to 15 hours a week on zebra mussel prevention. He talked to groups of boaters, agency staffers, and others about effective boat cleaning and inspection. "The best way to get their attention is to get in their knickers," he says. "Tell them how it's going to cost them personally." He learned to build zebra mussel samplers out of PVC pipe and lengths of nylon cord and installed them at six Lake Mead marinas. He checked them monthly for evidence and reported his results to a mussel data clearinghouse at Portland State University in Oregon.

Whenever he could, he talked to marina workers about zebra mussels, describing what to look for as they went about their business. "Wen just kept after us," says Gail Kaiser, whose family runs the Las Vegas Boat Harbor.

But what is one person's persistence, compared with the powers of zebra and quagga mussels?