Ballast-water invaders pose ecological risk.

They came, they multiplied, they conquered: In the mid-1980s, zebra mussels hitchhiked to the Great Lakes in the ballast tanks of a transoceanic cargo ship, triggering one of the most disastrous ecological invasions in recent U.S. history, But other ballast-water invaders are reaching saltwater ports, inland waterways, and marine estuaries on a vast and largely unnoticed scale, says marine ecologist James T. Carlton of Williams College in Williamstown, Mass.

Carlton and colleague Jonathan B. Geller of the University of North Carolina at Wilmington counted and identified the creatures residing in the ballast water of 159 ships in Coos Bay, Ore., one of the largest exporting ports in the Pacific Northwest. Water from the ships, which hailed from 25 Japanese ports, contained 367 different marine species, including shrimps, sea anemones, jellyfish, snails, clams, fish, flatworms, and a variety of microscopic life forms, Carlton and Geller report in the July 2 SCIENCE.

"The total diversity was a surprise;' Carlton recalls. "We didn't expect to find things like hermit crabs, starfish, or sea squirts."

Since the 1880s, empty and near-empty ships have taken on water as ballast to increase their stability and balance on the open seas. After reaching their destinations and loading cargo, freighters pump the water back out--along with any marine life sucked up into the tanks at the home port.

Dumping ballast water into foreign ports could have dire consequences for native marine creatures and for the people whose livelihoods depend upon them, says Carlton. "All you have to do is insert one new species into a system and the ecological roulette [wheel] is set in motion," he says. But it's very hard to know whether such an introduction will upset the natural balance or prove benign, adds Carlton.

Carlton and Geller emphasize that a significant number of foreign invaders may have established themselves already in U.S. coastal waters. Some will go unnoticed until, like the zebra mussel, they present a major nuisance. Other invaders have been misidentified as native species. "We think the number of invasions is vastly underreported." says Carlton.

Recent events in the Black Sea illustrate the potential hazards of ballast-water dumping. In the early 1980s, the North American comb jellyfish rode a freighter into the Azov Sea, a semi-enclosed body of water in the northern Black Sea. The disruption that followed has virtually wiped out the Azov Sea% anchovy fisheries, causing a "major economic and social disaster," says Carlton.

Closer to home, San Francisco Bay has seen some notable invasions recently. For example, the Asian clam appeared there in 1986, almost certainly transported in the ballast tanks of a freighter, says Peter B. Moyle, a fish biologist at the University of California, Davis.

Today, the bay bottom is covered by 10,000 or more of these creatures per square meter. Moyle fears the clams will out-compete native species for food, retarding the recovery of the bay's declining estuary. And just last year, the European green crab found its way to the bay, Nobody is sure whether this voracious predator will help control the Asian clam invasion or damage the local shellfishing industry, according to Moyle.

"It's a lottery," he says. "Every time one of these ships comes over and dumps water into the system, you never know what's going to make it."

One very important question remains, says marine biologist John W. Chapman of Oregon State University's Hatfield Marine Science Center in Newport: How often do these inadvertently transplanted species actually gain a toehold in foreign harbors and estuaries? "We can speculate," says Chapman, "but there are no data."