Spiny water fleas, furry mitten crabs, northern snakeheads, dead man's fingers-they all sound like something out of a horror movie. But unfortunately, the story of the invaders that took over the nation's seas is all too real.

These marauders enter our waterways, either introduced accidentally or on purpose, and within a few short years, many establish breeding populations. They gobble up native fish and native habitats. With no natural predators, there's no stopping their growth. They breed like rabbits-or, as the case may be, nutria.

With nature unable to control them, wildlife managers try their best-but often, they're simply too late and the results are devastating.

Invaders take several paths into the waterways. Some are brought in for a specific reason, and then things go terribly wrong. MSX, one of two diseases that have devastated native oyster populations, was accidentally brought to the East Coast with foreign oysters imported for research.

In December, the Mid-Atlantic Panel of Aquatic Nuisance Species met in Baltimore to discuss the various vectors for bringing in the invaders and how to better manage them. The panel, which was organized by Maryland Sea Grant, was established in 2003.

It took the place of the Bay Program's Invasive Species Workgroup and, based on lessons learned here and elsewhere, is aimed at preventing new invasions when possible, and containing them when prevention doesn't work.

"Some of them hang out in a bay and stay for 50 years, and don't spread-until they do," said James Carlton, director of the Mystic Seaport Maritime Studies Program at Williams College in Massachusetts.

The case of the Asian carp and the Great Lakes is an example of the threat an invasive species can pose, and the millions of dollars in effort it takes to combat it.

The U.S. Fish and Wildlife Service brought the Asian carp to the United States, seeking a natural weed killer. In the 1970s, catfish farmers in the Southeast began importing them as a natural pond cleaner.

But floods in the Mississippi caused the ponds to overflow, and the carp swam into the great river. In some parts of the Mississippi, the voracious carp is the dominant species. They can weigh up to 100 pounds and can consume 40 percent of their body weight daily. They have no natural predators, and are so bony that U.S. consumers don't want to eat them.

The carp was recently discovered in the Chicago Sanitary and Ship Canal, a man-made body of water connecting the Mississippi River to the Great Lakes. The Army Corps of Engineers has an electronic barrier at the canal to stop the fish from entering the Great Lakes. But Michigan authorities are complaining that the electronic barrier-which costs $40,000 a month to power-is not enough to keep the carp from the Great Lakes. They want the Corps to close the canal and protect Michigan's $7 billion tourism/recreational fishing industry. But, closing the canal would disrupt a huge amount of interstate commerce between the Mississippi and the Great Lakes, and the Corps is planning to use federal stimulus money to erect another barrier to further insulate the Great Lakes.

So, in 40 years, the Asian carp has gone from a helpful pond cleaner to a multimillion-dollar nuisance.

The northern snakehead hasn't proven to be the nuisance that Asian Carp is, but it's still worrisome that scientists have discovered hundreds of them in the Potomac River and several of its Northern Virginia tributaries. Scientists believe the snakeheads got to the Potomac sometime around 2002, when a male and female were dumped into Dogue Creek. So far, the bass in the river are tolerating the voracious Chinese fish, but scientists worry the peaceful co-existence won't last long, given snakeheads' copious breeding practices.

Live bait is also an excellent vector for invasive species. Fishermen should never release unused bait into the water or leave it on shore. They should save it, give it to another angler or put it in their freezer.

In Montana, where recreational boaters have unwittingly spread invasive mussels that hitched a ride on their boat bottoms, managers are turning to social marketing to get the word out. Robert Wiltshire, founder and director of the Center for Aquatic Nuisance Species, said that it's not enough to tell people to clean their boats. It has to be socially important for them to do so. Managers can appeal to the environmental sensibilities of a fly fisherman in a canoe, but Wiltshire said, that same message wouldn't go over well with a jet skier.

"We can't do this through regulation. The real action is through peer-to-peer sharing," Wiltshire said. "You need your fishing buddy to tell you to clean your boat, not the Game Board." One way to do that, he said, is to reach out to professional athletes in competitions such as the X-Games.

The biggest conduit for aquatic invasive species, though, is ballast water used to balance ships that travel the world.

Carlton, of the Mystic maritime program, said the nation's scientists and port managers must work together to reduce the surprises. For decades, environmentalists have pushed for stricter federal standards. And when they didn't materialize, many states took matters into their own hands. In 2000, Washington state required ships to exchange their ballast water 50 miles from shore. Oregon and California soon followed suit.

In 2004-eight years after Congress passed a voluntary program to regulate ballast water, the Coast Guard required ships to flush ballast water from their tanks and replace it with ocean water when they were at least 200 miles from shore. But most of the ships couldn't comply with that standard.

Gregory Ruiz, who studies invasives and ballast water at the Smithsonian Environmental Research Center in Edgewater, MD, said the exchanges, while "better than nothing" are an imperfect answer. The exchange does not get rid of all organisms. And it can be dangerous for a ship to destabilize itself in the middle of the ocean.

Over the last several years, the shipping industry and marine scientists have agreed that onboard treatment systems using chemicals to kill all of the invasive species are a far better option. The Coast Guard is now proposing that all ships have a treatment system on board by 2016.

The Chesapeake Bay is now home to 170 invasive species, from the invasive reed, phragmites, to nutria, a foreign muskrat, on Maryland's Eastern Shore to the shellfish-eating rapa whelk in Virginia and zebra mussels in the Susquehanna.

Bay policy makers didn't really start to study the problem of invasive species until the mid-1990s, when they were already clearly a problem in San Francisco Bay and the Great Lakes.

But now, the Chesapeake is one of two places in the country where new ballast water treatment systems are being tested. The Maritime Environmental Resource Center, which does its research aboard the Cape Washington near Baltimore's Inner Harbor, is testing ballast water treatment options. Part of a partnership between the University of Maryland Center for Environmental Science's Chesapeake Biological Laboratory and the Maryland Port Administration, the center is also looking at ways to limit hull fouling from invasive species and to rein in air emissions from ships.