A third Chinese mitten crab was found in Maryland’s portion of the Chesapeake on May 18, renewing concerns that the exotic species may take up residence in the Bay.

The first two confirmed specimens, caught in the Patapsco River near Baltimore Harbor, were identified last July by scientists from the Smithsonian Institution.

One of the crabs had been captured a year earlier in 2005 and kept frozen until the Maryland Department of Natural Resources and the Smithsonian announced that they were on the lookout for the foreign crab.

The latest mitten crab was captured off Chesapeake Beach, 40 miles south of the location where the first two confirmed crabs were found. All three crabs were mature males.

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 the Smithsonian Environmental Research Center’s Marine Invasions Research Lab consider the mitten crab a “potentially harmful invasive species.”

Scientists do not know whether the crabs are reproducing in the Bay or are being repeatedly introduced, potentially in ballast water from commercial ships.

“We can’t say whether they are established [in the Bay],” said Greg Ruiz, a marine biologist who runs the Invasive Species laboratory at the Smithsonian Environmental Research Center, “but we can say that they are arriving to the system, and our earlier work has shown that the Chesapeake Bay is a place they could establish.”

Elsewhere, young mitten crabs spend two to five years in freshwater tributaries and can extend more than 50 miles inland. Mature male and female crabs migrate downstream to mate and spawn in saltwater estuaries.

The crabs have not been easy to find in the Bay region. Despite intense publicity last summer after the first crab was discovered, only one additional confirmed mitten crab was produced. Surveys by the Maryland DNR and other agencies last fall failed to turn up any mitten crabs.

With two large claws covered with hair that give the crab the appearance of wearing mittens, they are easily distinguished from other crabs found in the Bay.

Mitten crabs invading Europe and on the U.S. West Coast have caused economic and ecological damage by clogging equipment and burrowing into embankments.

On the West Coast, the crabs were discovered by shrimp trawlers in southern San Francisco Bay in 1992. Since then, they have spread rapidly in that bay and moved upstream.

Ruiz said that mitten crabs are also being found in the Great Lakes, indicating that the crabs are finding their way to the Eastern United States, possibly in the ballast water of commercial ships coming from Europe, where the invader is already present.

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.

The public is asked to be on alert for Chinese mitten crabs in the Chesapeake Bay watershed. Identifying photos can be seen on SERC’s website: www.serc.si.edu/labs/marine_invasions/

Anyone finding a mitten crab is asked to note where it was caught, keep the crab on ice, and e-mail SERC scientists at SERCMittenCrab@si.edu.