Scientists have found Chinese mitten crabs mating and producing eggs in the Chesapeake and Delaware bays - evidence that the alien species is likely breeding and established on the East Coast.
Two hairy-clawed invaders were caught recently by watermen, one south of Kent Island and one in the upper Delaware Bay, raising to 10 the number confirmed in the region over the past three years.
More significantly, the two were the first females found on the East Coast. Both showed evidence of recent mating, and the Delaware crab was carrying eggs, said Greg Ruiz, senior scientist at the Smithsonian Environmental Research Center in Edgewater.
"It suggests they are certainly reproducing, mating and producing eggs, but we don't know if it's a self-sustaining population," said Ruiz. "It's a concern that we continue to find more crabs."
Authorities in Maryland and other Eastern states have broadcast alerts for Chinese mitten crabs because the creatures often stow away in the ballast water of ships, then reproduce in excessive numbers in their new environment.
Their burrowing has caused erosion in a few of their adoptive homes, such as San Francisco and England. They can also clog water pipes and fight for food with native species. One concern is they could compete with Chesapeake blue crabs.
Mitten crabs are brownish-green, with skinny spider-like legs and tufts of thick black hair surrounding their white claws. Their bodies are generally smaller than a Chesapeake blue crab. They live most of their lives in freshwater streams, where they dig burrows in riverbanks. But they migrate to salt water in the spring to lay their eggs.
The female mitten crab caught in a waterman's trap was found June 23 just south of Kent Point in the Chesapeake Bay. The fisherman reported it to the state; researchers dissected it and found sperm in an organ the crab uses for storing the male cells.
"That indicates the crab has mated," Ruiz said. "Perhaps she had already reproduced, but we simply don't know."
The crab caught July 10 above Port Mahon in Delaware Bay, near the Simons River, not only had mated but was carrying eggs under its abdomen. Upon dissecting the crab, researchers found it was in the process of making more eggs.
"It's a concern, and we should be on the lookout for more crabs," Ruiz said.
Mitten crabs also have been found recently in fishing nets in the Hudson River, Lake Erie, the St. Lawrence Seaway and along the Louisiana coast of the Gulf of Mexico.
Jonathan McKnight, associate wildlife director at the Maryland Department of Natural Resources, said the crab found off Kent Island wasn't in water salty enough for successful reproduction.
The salinity of the Chesapeake Bay increases from the northern tip of the estuary, fed by the fresh water pouring out of the Susquehanna River, to the southern end, which opens into the salty Atlantic Ocean.
A mitten crab would likely have to be as far south as Cape Charles, Va., to have enough salt for its eggs to survive, said McKnight. Perhaps the crab caught near Kent Island was heading south, but it wasn't in an ideal breeding habitat, he said.
No mitten crabs have yet been confirmed in Virginia waters. Two have been found near Baltimore and a third off Calvert County in Southern Maryland.
Even if mitten crabs are breeding in the Chesapeake Bay, they could be devoured by the native blue crab, said McKnight. Another exotic species from Asia, the green crab, was introduced more than a century ago along the East Coast but has been kept out of the bay by the blue crab's appetite, McKnight said.
Maryland has in the past launched campaigns to kill other invasive species. For example, the state worked with the federal government to trap more than 10,000 nutria, a South American rodent that has devoured marsh grass in the Blackwater National Wildlife Refuge.
The state Department of Natural Resources over the past two years has also killed about 2,500 mute swans, native to Europe, because the large, aggressive birds have been crowding out smaller, native species such as the threatened black skimmer, McKnight said.
In 2002, the state poisoned all the fish in a Crofton pond in an attempt to prevent the spread of Asian northern snakeheads. The toothy predator might have been dumped in Maryland by a pet owner or someone in the seafood businesses.
But the chemical warfare failed, and the snakehead now multiplies in tributaries to the Potomac River.
No such aggressive attack is being considered for the mitten crabs, McKnight said, in part because it would be impossible to poison the whole bay.
Instead, the state is monitoring crabs by setting nets and asking watermen to turn them in. McKnight said he's hopeful the mittens will disappear on their own because they'll find the bay inhospitable.
"There is always the happy possibility that those snakeheads will eat the mitten crabs," said McKnight. "It's a fight to the finish."