Eat the Invading Alien Crabs, Urge U.K. Scientists

James Owen
for National Geographic News
November 13, 2003
In Asia it's considered a culinary delicacy. But in Britain, the Chinese
mitten crab is seen as a dangerous alien that could cause lasting harm
to the environment.

Scientists studying the furry-clawed crustacean, which is thought to have arrived from China as larvae in ballast water, now want people to eat the crabs into retreat.

The suggestion comes from zoologists at the Natural History Museum in London, England. Writing in the October Institute of Biology journal, the Biologist, Philip Rainbow and his team have called on commercial fishermen to target the species and send it back to where it came from.

Rainbow, the museum's keeper of zoology, said: "The Chinese love them, especially when they're full of gonads during the breeding season. The carapace of a large one measures eight centimeters (about three inches) across—that's a decent-sized meal."

Rainbow says the export of British-caught mitten crabs would help take the pressure off native plants and animals at risk from the advancing invaders. Mitten crabs are ravenous omnivores and the zoologists fear they could both eat and out-compete vulnerable freshwater species.

"It's quite a big crab and capable of disturbing the environment for other organisms," said Rainbow. "Because there isn't already a freshwater crab in Britain, species like the native crayfish, which is already in decline, are likely to be affected."

Studies show the crabs can also cause serious damage by burrowing into banks and earthworks along rivers. While sport fishermen in the London area say the crabs often hijack proceedings.

"You talk to any angling club on the tributaries of the River Thames and you find anglers really don't like them," Rainbow said. "The main problem is the general disturbance the crabs cause, such as worrying their bait—and now and again they'll reel one in that's hanging by its claws."

Ballast Water

First recorded in the River Thames in the 1930s, scientists believe the Chinese mitten crab (Eriocheir sinensis) arrived in Britain as tiny larvae in ballast water in ships from the Far East. Native to China and Korea, the species took a long time to become established, possibly because of the river's polluted state. Since the early 1990s, however, numbers have mushroomed. The crab, which can travel over dry land, has now spread to many other English rivers.

Rainbow said: "The population started to climb about ten years ago, then there was this snowballing effect, with more reproduction and more larvae."

He says numbers will probably level out eventually, but added, "There's a lot more available habitat before we reach that stage."

The crab is now established throughout much of Europe and the U.S., giving rise to similar concerns over its environmental impact.

In San Francisco Bay, California, they proved so successful that in 1998 they threatened to shut down the state's water supplies. The crabs, which migrate downstream to estuaries to breed, collected in such vast numbers that they blocked screens covering the water intake facility in the Sacramento-San Joaquin delta. To prevent southern California running out of freshwater some 20,000 crabs had to be removed from the screens each day.

Other problems include damage to riverbanks, bait-stealing from sport anglers and clogging of water cooling systems at power plants, according to Kathy Hieb, biologist with the California Department of Fish and Game. She says the mitten crab's wider ecological impact has yet to be established.

Although the population fluctuates enormously from year to year, Hieb suspects the mitten crab has become a permanent resident in San Francisco Bay.

"They are here to stay," she said. "Although numbers are now low, we predict they will increase again and be cyclic. We do not believe the crab can be eradicated as it has pelagic [free floating] larvae and is widespread in the bay and its watershed. A single female can have up to a million eggs, so many young crabs can result from a few adults if physical conditions are beneficial to their survival."

Prohibited Species

The mitten crab is a prohibited species in California, in order to prevent it spreading to other rivers. This makes it illegal to transport the crustacean while alive.

Despite this, the species does have commercial possibilities, according to Richard Tullis, biology professor at California State University, Hayward, who has studied the crab.

"I think the idea of eating the crabs into retreat is a marvelous idea," he said. "There is a thriving black market in the Chinatowns of Oakland and San Francisco. If open to commercial fishing the population would dwindle in the wild, like it has in China."

Fears about the crab's ability to colonize new areas are underlined by the staggering distances it can travel inland. In China, the crab has been reported 1,300 kilometers (800 miles) from the sea, while in Europe it has already reached Prague, capital of the Czech Republic.

In Britain, Rainbow believes there could be home demand, as well as an export market, for commercially-fished mitten crabs, particularly within London's Chinese community.

"We did give some we'd caught to a Chinese restaurant who cooked them and said they were delicious," he said.

Tullis agrees, adding, "Fixed Asian style, stir-fried with garlic, soy and ginger, it will also turn on non-Asians."

The potential for British mitten crabs is enhanced by the fact they are free of lung flukes which infest Chinese populations. This parasitic flatworm requires three different hosts to complete its life cycle: freshwater snails, mitten crabs, and mammals. In the Far East, humans who eat fluke-infected mitten crabs are vulnerable to a tuberculosis-like disease which can be fatal.

Rainbow says the snails that harbor lung flukes need warm waters and are absent from northern Europe. "We don't think the British crabs are infected by the parasites," he added.

This leads Rainbow and his research team to conclude: "The culinary route may represent our best culling strategy if we are to limit its potentially damaging environmental effects."

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