Individually, the quagga mussel (Dreissena rostriformis bugensis) doesn't look like much. Barely the size of a human thumbnail, with a non-descript shell, most people probably wouldn't give a quagga mussel a second glance if they saw one in a lake or river.

Unfortunately, quagga mussels don't appear individually, or in pairs, or in tens, but in tens of thousands. This invasive species, native to the Ukraine, has found a home in the United States, causing ecological damage wherever it spreads, and according to experts, it isn't going away any time soon.

The worst damage so far is in the Great Lakes, specifically Lake Michigan, where the quagga mussel has disrupted the food chain, with disastrous results. Over the last decade, populations of tiny shrimplike burrowing crustacean called diporeia have dropped by an astounding 96 percent, says Thomas Nalepa, a biologist at Great Lakes Environmental Research Laboratory, run by the National Oceanic & Atmospheric Administration.

Nalepa says quagga mussels eat the same algae that diporeia feed upon. The mussel is a much more efficient algae consumer—"It's a machine, really," says Nalepa—and since it sits on the surface of lake beds, it gets first digs at algae, unlike diporeia, which burrows beneath the sediment and feed upon algae that settle to the bottom of the lake.

The drop in diporeia numbers has had a dangerous effect on other species in Lake Michigan, which used to feed upon them. Whitefish, for example, are bottom feeders, and without the diporeia to eat, the fish are getting smaller. And the quagga mussels can't pick up the slack: Few fish are adapted to efficiently eat the mussels, whose shells are indigestible to fish, says Nalepa. Mussels also lack the caloric content that make diporeia such a healthy food for fish.

Still, Nalepa says, scientists haven't conclusively proven the cause-and-effect between quagga increase and loss of diporeia. Remaining diporeia aren't underweight, says Nalepa, so they still seem to be getting food. And while diporeia have completely disappeared from some areas of Lake Michigan, Nalepa says the two species have reached more of an equilibrium in other places, such as New York's Finger Lakes.

Like their more well known cousins, the zebra mussel (Dreissena polymorpha), quagga mussels are now spreading well beyond the Great Lakes, hitching rides with commercial and recreational boats as they are ridden or moved into new water systems. They recently muscled into Nevada's Lake Mead, where they breed faster thanks to the area's warmer waters. Quagga mussels have been found clogging pipes in Hoover Dam, and it could cost millions of dollars and huge amounts of manpower to keep systems there mussel-free.

Can the invasion of the quagga mussel be stopped or reversed? "I don't see that happening," says Nalepa. "With quagga mussels, we're still in the expansion stage," especially in deeper waters.

But some aren't taking this threat sitting down. Several organizations, such as the Great Lakes Coalition and the Natural Resources Defense Council are pushing for stronger ship ballast water regulations, which could help keep other invasive species from entering the U.S., and improved infrastructures between bodies of water to keep quagga and zebra mussels from further migrating. But even if regulations pass, the quagga "is here to stay," says Nalepa.

Image: U.S. Geological Survey