July 19, 2007

Invasive Crab Crawls Lowly Toward Maine


Mainers are being asked to keep an eye out for a strange crab that has furry claws and can live in both fresh and salt water.

The Chinese mitten crab appears to be spreading up the East Coast, and so is its reputation for displacing native animals, fouling fishing nets and destabilizing riverbanks.

Scientists think the crabs may be breeding in Chesapeake and Delaware bays. In May, Chinese mitten crabs were found in the waters near New York's Tappan Zee Bridge.

"They're as close as the Hudson River in New York, and vigilance is really the first line of defense," said Paul Gregory, an invasive- species expert for the Maine Department of Environmental Protection.

The crab is named for the fur on its pincer claws. It spends much of its life in freshwater rivers, returning to coastal estuaries to spawn. It's regarded as an aggressive invasive species because it can take over habitats in estuaries and rivers, clog commercial fishing nets and water intake pipes and accelerate erosion by burrowing into shorelines and embankments.

The crab has slowly made its way from Asia to Europe to the United States, first causing problems on the West Coast and only recently appearing on the East Coast.

"The leading theory is that it's moved around by ship ballasts," Gregory said.

Large cargo ships fill ballast tanks with sea water in one port and then empty the water in another port. Critters that survive the move may find a new home with plentiful food and no natural predators.

The mitten crab made news this week in Maryland when scientists reported another troubling find: two females with eggs.

"They have definitely mated," said Lynn Fegley, fisheries biologist with the Maryland Department of Natural Resources. "The next question is, has the reproduction been successful? We don't know because we have not yet found larvae or juveniles."

Officials don't know yet how much of a threat the mitten crab may pose to the Chesapeake's prized blue crab population.

"The concern is we don't know what to be concerned about," Fegley said. "Any time something arrives in your ecosystem from somewhere else, you have no way to determine what the impacts will be. And the impacts may be immediate or they may be a decade out."

If, or when, the crab does make it to Maine, it should at least be easy to spot. An adult has a shell that's 4 inches across. Legs included, it's roughly dinner-plate size. It's the only crab that you'll find in fresh water well upriver from the ocean. And, of course, there are the fur-covered claws.

"It would catch my attention," Gregory said.

Maine officials want anyone who finds one to hold onto it, alive or frozen, and call (207) 633-9539. Quickly figuring out how an invasive species gets here can help slow or stop an invasion, Gregory said.

"The history of invasive species here in Maine," he said, "is that they've been first detected by citizens."

Staff Writer John Richardson can be contacted at 791-6324 or at:

[email protected]



APPEARANCE: Body as wide as to 4 inches, with a light-brown shell, hairy claws that are white at the tips and legs more than twice the length of the carapace.

RANGE: Native to Fukien province in China. The crab's introduced range includes Europe, Russia and North America, specifically Hawaii, California, the Great Lakes and Maryland.

HABITAT: Lakes, rivers, wetlands and estuaries

INTRODUCTION: The crab can travel over dry land but is probably introduced to non-native habitats through ship ballast water, the live food trade and an effective reproductive system that employs plankton-like larvae.

LIFE CYCLE: A female crab carries as many as 1 million eggs. The crabs travel to salt water to breed, then move back into fresh water when they are several months old, spending two to five years in water that is often far from the ocean.

IMPACT: Young crabs eat vegetation. Mature crabs eat invertebrates, competing with native species for food. The crabs burrow into the banks of streams, undermining them and causing erosion. They affect commercial and recreational fishing by stealing bait and breaking gear. They can block water intake pipes and systems meant to safeguard fish from dam turbines.

Source: Global Invasive Species Database