A native of the Black and Caspian seas, zebra mussels and quagga mussels invaded the Great Lakes in the late 1980s, and then 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. Their ecological and economic misdeeds were soon infamous: zebra and quagga 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.

Getting rid of the problem is more a little complicated.

Invasion biologists like to talk about "propagule pressure," or the number of times an invasive species enters an unfamiliar environment. The more numerous the propagules, the more likely the species will take hold. The propagule pressure of zebra and quagga mussels on Lake Mead was, and is, formidable.

The lake hosts some 8 million visitors each year, and on a summer weekend, as many as 5,000 boats launch on Lake Mead and its neighboring reservoir, Lake Mohave. While a small survey of boaters at the lakes in 2002 suggested that little more than 1 percent come from mussel-infested states, any one of those boats could carry enough live mussels or mussel larvae -- vanishingly small bits of life known as veligers -- to seed a new population.

Spot inspections at Lake Mead since May 2004 caught at least nine boats infected with invasive mussels, almost certainly a small percentage of the contaminated boats that entered the park. Officials for various agencies have also nabbed mussels in transit in Washington, California and elsewhere, and assume that many more have slipped by unseen. In the West, mussel propagules are present, persistent and resistant to detection; to settle in, all they need is a bit of good fortune.

The quaggas also had the advantage of anonymity. Wen Baldwin, who has taken up the cause to protect Lake Mead, and the biologists he talked to expected zebra mussels, which are now established in the Mississippi River and range as far west as Kansas and Oklahoma, to lead any invasion of Lake Mead. While the scientists were aware of quagga mussels, the species has only a small outpost on the Mississippi. The closest substantial population is in Lake Michigan, some 1,000 miles away from Lake Mead. Baldwin, playing the odds, designed his samplers for the shallower waters preferred by zebra mussels.

In February of last year, Baldwin dragged a plankton net through the Las Vegas harbor and turned up a microscopic globe of jelly that looked worryingly like a zebra mussel larva. Before taxonomists decreed it was benign, Park Service divers inspected the cables and mooring blocks at the harbor. They found nothing.

On Jan. 6 of this year, Baldwin got the call he dreaded. Eric Virgin, son-in-law to Gail Kaiser -- whose family runs the Las Vegas Boat harbor -- and a member of the Las Vegas harbor staff, had found something suspicious while repairing an underwater cable. "It just seemed wrong," he says now.

Baldwin drove to the harbor, where the marina workers showed him one of the "Zap the Zebra" educational pamphlets they'd been handing out to visitors. Taped to it was the mussel Virgin had plucked off the steel cable.

Later that day, divers found more mussels in the harbor, and at the nearby Lake Mead Marina. Over the next week, they found mussels at other locations throughout Boulder Basin, the western lobe of Lake Mead. The Lake Mead fish hatchery, a Nevada Department of Wildlife operation that stocks the lake with trout, was quarantined on Jan. 11 after workers discovered quagga mussels on the edges of the boards damming the fishponds.

Robert McMahon, a mussel expert at the University of Texas-Arlington, soon confirmed that the invaders weren't zebras, but quaggas. Size measurements suggested that the population was at least two years old, though some researchers speculate that the lake's warmer waters could lead to fast growth; the timing of the invasion remains uncertain.

While the evidence of infestation proliferated, so did its implications. The Southern Nevada Water Authority found quaggas on the outside of one of its Lake Mead intakes. On Jan. 17, downstream from Lake Mohave in Lake Havasu, divers for the Metropolitan Water District of southern California found mussels attached to the outside of an intake for the Colorado River Aqueduct, the pipeline that waters Los Angeles. The following week, divers found mussels at the Central Arizona Project intakes in Lake Havasu. Later surveys turned up about a dozen mussels on an intake structure for Davis Dam, the plug for Lake Mohave, and another handful on the spillway gates for Parker Dam below Havasu. And in mid-February, an estimated 2,000 mussels were found on a "trash rack" protecting one of the intakes for Hoover Dam.

Park Service divers spent most of January scouring lakes Mead and Mohave. During the weekend of Jan. 20, they found more quaggas, this time stuck to docks and houseboats at the south end of Lake Mohave.

Lake Mead resource management chief Kent Turner says that given the number of mussel finds in Boulder Basin, and the density of mussels found at each location, it's reasonable to assume that the quaggas in the reservoir already number in the millions.

At the Las Vegas Boat Harbor, more than three weeks after the initial discovery, Baldwin is still distressed. "I feel like I've been hit by a stick."

He crosses the floating dock platforms, heading for the quiet boat slips on the northern edge of the marina. There, he unrolls his newest mussel sampler from a white plastic bucket. Redesigned to pick up quaggas, he dunks its concrete anchor into the clear green water, and watches the nylon cord and its sampling tubes spin into the depths.

The quagga mussel discoveries in January set off a flurry of calls and meetings among park staff, state game officials and scientists of all sorts, but definitive answers were hard to find. There are plenty of experts on invasive mussels in the Great Lakes; there are experts on the inner workings of Lake Mead. But the invasion of quagga mussels into the Colorado River Basin began an unintentional experiment, and no one is sure of the results.

"I think we have about a year before we're going to really find out," says Jim LaBounty, a lake ecologist for the Southern Nevada Water Authority and a longtime student of Lake Mead. "But we've got an ideal set of conditions." Lake Mead isn't nearly as rich in algae as the Great Lakes. But reservoirs are, of course, rivers trapped within lakes, and LaBounty surmises that the constant flow of water in Lake Mead will carry plenty of nutrients to the reservoir's newest filter feeders.

No one could call Lake Mead pristine. Held in place by Hoover Dam, the reservoir is dominated by introduced sport fish, such as striped and largemouth bass. But it and Lake Mohave still harbor a handful of endangered razorback suckers, and Mohave is also home to a few endangered bonytail chub. Exactly how the mussels will affect them and the rest of the food web in the Colorado Basin remains largely unknown, especially since most research on invasive mussels has focused on zebras. Paul Hebert, the biologist whose students discovered zebra mussels in Lake St. Clair in the Great Lakes area, supplies a rule of thumb: "You should prepare for broader consequences than you expect."

Though zebra and quagga mussels can be controlled at water intakes with chemical treatments, and driven out of very limited areas with poisons, high-pressure hoses, and other shock-and-awe tactics, eradication on any scale is expensive: Last winter, state officials in Virginia spent more than $400,000 on potassium chloride treatments for a 12-acre quarry.

There is still some talk of eradication at Lake Mead, despite its enormous size and complex topography. "I don't think we know enough to say it's impossible," says Andrew Cohen, a senior scientist at the San Francisco Estuary Institute and a member of the lake's quagga mussel advisory team. While the park will never eliminate every mussel, he says, a well-funded effort could bury, overheat or poison enough mussels to interfere with the species' famous fecundity. Considering the potential future costs of the mussel's spread, such an undertaking could even make economic sense. But most agree that the lake's primary concern is containment.

"Some lakes, because they receive so much boat traffic, can become what epidemiologists call super-spreaders," says David Lodge, a researcher at the University of Notre Dame who works with the 100th Meridian Initiative. "Lake Mead will become such a lake." The intiative is 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.

Lake Powell, which many expected would be first in line for a mussel invasion, now requires boats fresh from mussel-infested states to get a free hot-water wash, courtesy of park concessionaires. But these thorough scrubdowns won't be practical this summer, when visitors begin hauling their boats from one Southwestern reservoir to the next. "Last year, we washed 45 high-risk boats. This year, we could easily be dealing with 4,500," says Glen Canyon National Recreation Area aquatic ecologist Mark Anderson.

Quaggas already could be headed overland from Lake Mead to the West Coast. Two houseboat-rental operations on lakes Mead and Mohave also rent boats on lakes Trinity and Shasta in Northern California, and workers do move boats between the Nevada and California reservoirs. That means the quaggas might already have access to salmon watersheds, a prospect that worries many in the Pacific Northwest. The Columbia River could easily host a mussel infestation, and researchers foresee a raft of problems for the region's salmon, ranging from disruption of the food web to encrustation of the ladders designed to allow salmon to migrate around dams.

In San Francisco Bay, which Cohen and his colleague James Carlton have called "the most invaded aquatic ecosystem in North America," the precipitous recent decline of several fish species has been blamed on another invasive bivalve, a type of Asian clam that dominates the northern reaches of the bay. With the industrious clam consuming much of the algae and other plant matter in the bay itself, many species rely on an infusion of nutrients from the Sacramento-San Joaquin river delta. But if the quagga mussel were to invade the delta's fresh waters, it could vacuum up the bay's remaining food supply.

"The thought of another massively filter-feeding bivalve upstream, in the delta, is causing great concern," says Cohen. "We might end up with a system that's good for a couple species of clams, and things that feed on clams, but not much else."

The 100th Meridian Initiative's Colorado Basin Team meeting in late January draws about 80 people -- a record crowd -- to Las Vegas from all over the region. Wen Baldwin displays an extra-large jug of quagga mussels, and Lake Mead staff and state agency representatives report their discoveries and progress.

California has assembled a three-person "incident command" team, the same authority structure it uses for earthquakes and oil spills. Emergency funding is keeping three of its state agricultural inspection stations near the Colorado River open around the clock, an effort -- some say a quixotic one -- to resist the continuing propagule pressure from Lake Mead.

With only two findings of quagga mussels within its borders so far, and no quagga carriers nabbed at its inspection stations, California officials remain optimistic that its mussels will get out and stay out. The Metropolitan Water District, whose board recently approved $180,000 for quagga decontamination and sampling equipment, hopes that a previously scheduled draining of the Colorado River Aqueduct in March will dry up any resident mussels. But quagga monitoring in California has already taken a tragic turn: On Feb. 7, two Department of Water Resources divers drowned while searching for quagga mussels at a pumping plant along the California Aqueduct. State officials are investigating the deaths, and as of Feb. 22, diver surveys at the department's facilities remained suspended.

Throughout the Colorado Basin, most of the early response to the mussel invasion is underfunded and overtaxed. "We're all dealing with very small pots of money," says Tina Proctor, an aquatic nuisance species coordinator for the Fish and Wildlife Service. Press coverage has been spotty, and political attention is inconsistent: Democrat Gov. Janet Napolitano of Arizona is receiving regular briefings on the issue, but a spokesman for Republican Utah Gov. Jon Huntsman indicates that it isn't high on his agenda, pointing out that "Lake Mead isn't actually in Utah."

So while "there's an immediate need to do something," as one audience member at the 100th Meridian meeting puts it, well-provisioned, strongly coordinated action is scarce.

Yet quagga mussels aren't likely to choke off Southern California's water supply, or otherwise reprise the worst deeds of zebra mussels in the Great Lakes. Western managers can draw on the painful experience of their Midwestern counterparts to deal with mussels earlier and more aggressively. And some Western waters are too shallow, too warm, or otherwise unfriendly to quagga and zebra mussels.

The fact remains, however, that mussels and infrastructure don't mix. Mussel defense is an expensive, eternal, and -- as California has already shown -- occasionally dangerous business. On the Great Lakes, chemical feed lines for water intakes have cost anywhere from $50,000 for a small facility to upwards of $1 million. The only way to keep the price down is to contain, and keep containing, quagga and zebra mussels.

Researcher David Lodge expects the Western quagga response will come to resemble a more rigorous version of the 100th Meridian Initiative, with a concerted campaign of public education and a prioritized system for boat inspections. These humble tactics are credited with slowing the spread of zebra mussels in the Great Lakes, and so far, they're the best tools on hand in the Colorado Basin.

"If you break the invasion down into components -- if you look at the most likely ways for the mussel to move -- then you can start to do something about it," Lodge says. "Boaters, by and large, are people who care about the environment. You just have to give them an incentive to practice better boat hygiene."

Westerners "could be forgiven for thinking that they don't need to care about invasions in the Great Lakes," says Lodge, "but it's clear now that the desert is no barrier, and the Great Plains are no barrier, at least not an insurmountable one. The invasions from the Great Lakes are not going to stop with quagga mussels, unless effective policies are put in place."

Julie Lockwood, a researcher who studies biological homogenization at Rutgers University in New Jersey, says that humans have certainly ratcheted up the rate of invasions and extinctions, but points out that not all invasive species have equal homogenizing powers. While some resemble global fast-food chains, others are more like regional franchises, still dominating, but on a smaller scale. "If we make an effort to contain the species that spread a long way, like the zebra and quagga mussels, nature does better than we might think," she says.

Lockwood and her colleagues found that in California's freshwater fisheries, for example, some remnants of regional diversity have survived a long history of intentional introductions and losses of native species. "We haven't McDonaldized the whole place," she says. "Some things do hang on in the face of invasions, partly because we want them to." Habitat protection, pollution control, and other strategies, she notes, can still help fend off the problems.

At the close of the 100th Meridian meeting, the attendees set out for their various parks and states throughout the Colorado River Basin and beyond, newly charged with both containing the quagga and keeping its brethren at bay. They disperse, like so many propagules, into the currents of rush-hour traffic. As evening falls on the northbound freeway, a loaded boat trailer with Colorado plates climbs out of the glittering Las Vegas Valley with them, carrying its cargo upstream.

Michelle Nijhuis is High Country News contributing editor.

This article was made possible with support from the William C. Kenney Watershed Protection Foundation and the Jay Kenney Foundation.

This story appeared in The Daily Herald on page A1.