Chinese Crabs Rapidly Invading U.K., Scientists Warn

James Owen
for National Geographic News
February 15, 2006
A furry-clawed crustacean native to China is quickly scuttling its way
across Britain, according to a new study.

The freshwater crab could cause major environmental problems unless its numbers are brought under control, researchers warn.

The study shows the Chinese mitten crab has begun to invade Britain's waterways, after having been confined to a handful of estuaries since it first appeared in the United Kingdom (map) some 70 years ago.

Researchers from the University of Newcastle in northeast England say the crab, which can walk for miles over land, has advanced rapidly from the Thames and Medway estuaries in southeast England to waterways along the country's eastern and southern coasts.

The team found that by 1999 the mitten crab was colonizing 278 miles (448 kilometers) of coastline a year—nearly six times the annual rate of spread since the 1970s. Rivers are being invaded three times faster than before.

Originating from China, the first mitten crab—so called because its claws are covered in soft bristles—was reported in London's River Thames in 1935.

Elsewhere the species has spread across mainland Europe from Portugal to Sweden. Populations are also established in North America, including California's San Francisco Bay area.

Mitten crabs are thought to have arrived in Europe as stowaways in the ballast water of commercial ships. A single female can carry from 250,000 to a million eggs.

Researchers say cleaner rivers resulting from pollution control efforts are a likely factor behind the crab's onward march since the 1990s.

Native Species

Experts fear the pesky crustacean, which lives in rivers but breeds at sea, could have a serious impact on native freshwater species such as the protected white-clawed crayfish.

The crab also preys on fish eggs and fry, including those of salmon.

The crustacean has been known to migrate more than 900 miles (1,450 kilometers) upriver in China, and it can weaken dikes and riverbanks with its networks of muddy burrows.

Marine biologist Matt Bentley, lead author of the study, compares the crab's threat to that of another invader, the gray squirrel from North America. The squirrel is blamed for pushing Britain's indigenous red squirrel from much of its former range.

"With most invasive species, such as the gray squirrel, the problem is not recognized until it is too late, and you can't eliminate it without taking drastic environmental measures," he said.

Bentley urges action to halt the crab before its numbers spiral out of control.

"Low-cost options could include a public awareness campaign where anglers and other users of rivers and the coastline are encouraged to report sightings," he added.

Writing in the journal Biological Invasions, the Univeristy of Newcastle team also recommends a nationwide trapping program.

In addition, the Natural History Museum in London has launched a one-year study into the possibility of harvesting London's mitten crabs for the city's Chinese restaurants.

"We're looking into the feasibility of fishing or trapping as a potential control method to feed into the restaurant trade," said Philip Rainbow, the museum's keeper of zoology.

Tasty Gonads

In Asia the mitten crab is considered a delicacy. The gonads, which ripen during the crabs' seaward breeding migration, are especially prized.

The Natural History Museum project will assess whether crabs in the Thames are safe to eat and whether they could be caught on a commercial scale for export back to China.

"We know they are suitable for food, but they need to have a clean bill of health from the authorities," Rainbow said.

In Asia the species is known to host parasitic lung flukes, which can infect humans if the crabs are eaten undercooked. These flukes aren't thought to occur in Europe, however.

Rainbow says the best time to catch the creatures is probably during their migration, when their numbers are most concentrated.

"This is also the best time to eat the crabs, because they are full of gonads—that's when the Chinese love them," he added.

But he suspects that baited traps, a common crab-catching method, wouldn't be as effective during migrating season.

"When they're going downstream food isn't important," he said. "Sex and getting to the estuary is more on their minds."

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