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 last 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.