Bay Journal

Chinese mitten crab may be latest alien invader in the Chesapeake

Fall survey will look for evidence of a breeding population

  • By Karl Blankenship on September 01, 2006
The mitten crab’s long legs make it easier for the creature to walk.  (Maryland Department of Natural Resources)

Biologists around the Bay are on the lookout for large, spiderlike crabs that have the potential to be the latest foreign species to invade the Chesapeake.

A Chinese mitten crab, which has shown the ability to spread rapidly after being introduced in Europe and San Francisco Bay, was reported for the first time in the Chesapeake late this spring when one turned up in a waterman’s crab pot in the Patapsco River.

Biologists learned about it after seeing photos in the Waterman’s Gazette taken by the waterman, Capt. John Delp of Pasadena, seeking to identify what he described as “an imported crab.”

Lynn Fegley, a biologist with the Maryland Department of Natural Resources who spotted the photo, contacted experts from the Smithsonian Environmental Research Center, who confirmed it was a mitten crab.

At least one more confirmed crab and two likely sightings have followed, raising worries that a breeding population could exist.

In response, Fegley said biologists are designing a survey this fall—when the crabs would normally migrate from fresh to salt water to spawn—to look for evidence of a breeding population. The effort will involve scientists from the DNR, SERC the U.S. Fish and Wildlife Service and the National Oceanic and Atmospheric Administration.

The Chinese mitten crab, Eriocheir sinensis, is a concern because it is a prolific invader, and females can produce between 250,000 and 1 million eggs each.

Scientists at SERC’s Marine Invasions Research Lab consider the mitten crab as a “potentially harmful invasive species.” It was accidentally introduced in Germany in the early 1900s and over the next few decades its population spread into many other northern European rivers and estuaries. The expansion continues today. Most recently, a population explosion of mitten crabs was seen on the River Thames in England.

On the U.S. West Coast, the crabs were first discovered by shrimp trawlers in southern San Francisco Bay in 1992. Since then, the crab has spread rapidly in that bay and moved upstream.

A rapid expansion of mitten crabs in the Chesapeake, scientists say, has the potential to displace other species, and it is also a likely predator of some fish eggs. Also, because it burrows into banks to hide from predators, large numbers could increase shoreline erosion. In other areas, they have caused economic damage by clogging water intakes and damaging commercial fishing nets.

Because of its potential to harm ecosystems, it is illegal to import, export or sell under the federal Lacey Act without a permit.

But Fegley said she’s skeptical that a substantial breeding population exists in the Bay at this point. Since the discovery of the first crab was reported, only one other confirmed mitten crab has shown up—one from a ranger at North Point State Park who had received it from a waterman on the Patapsco at least a year earlier. He kept it in an aquarium until it died, then froze it.

Through late August, two possible sightings came from watermen, one near Solomons and one between Solomons and Annapolis. The crabs were not kept so the reports could not be confirmed, but Fegley said they are considered probable sightings because of descriptions provided by watermen.

“Given the range of habitats these animals need to cover in order to complete their life cycle, if this crab were present in any numbers, I think we would have heard about it,” she said.

It’s a hard crab to overlook. The adult mitten crab has a carapace (shell) about 4 inches across, which is smaller than an adult blue crab. Unlike the blue crab, which is a swimmer, the mitten crab is a walker, and is even known to walk out of water to get around obstructions.

It has eight long legs—typically twice as long as the width of the carapace—to make it easier to walk. “It’s very spidery in appearance,” Fegley said. Its name comes from black hair on its claws that make it appear they’re wearing mittens.

The mitten crab is catadromous, meaning it lives most of its life in fresh water but breeds in salt or brackish water. Young crabs spend two to five years in freshwater tributaries and can be found more than 50 miles inland. Mature males and females migrate downstream in huge numbers in late summer or early fall to mate and spawn, after which they die.

Given the crab’s life history in other places, Fegley said it was unusual for an adult male to be found in brackish water in early summer. “It shouldn’t be there,” she said. “They should be dead.”

But the multiple observations raise concerns that even if the crabs aren’t breeding here now, that could change in the future, Fegley said.

If not born here, it’s possible the crabs made it into the Patapsco, home to the Port of Baltimore, in ship ballast water. Ballast is routinely taken on board at ports to stabilize ships during ocean voyages, then released at their destinations. Another possibility is they were illegally imported; the mitten crab is considered a delicacy in Asia and live crabs are known to have been illegally imported to markets on the West Coast.

“I don’t think it’s unlikely that this would become a species that would be here,” Fegley said. “There is definitely a doorway for it to get here.”

According to Greg Ruiz, a marine invasive species specialist at SERC, more than 150 nonnative species are established in tidal waters of the Bay. “The rate of invasions appears to have increased in the past few decades due to human-mediated transfer,” Ruiz said.

People who see suspected mitten crabs are asked to contact the DNR or SERC. For information visit www.dnr.state.md.us/dnrnews/infocus/mitten_crab.asp.

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About Karl Blankenship

Karl Blankenship is editor of the Bay Journal and Executive Director of Chesapeake Media Service. He has served as editor of the Bay Journal since its inception in 1991. .(JavaScript must be enabled to view this email address).

Read more articles by Karl Blankenship

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