for National Geographic News
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 yearnearly 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 crabso called because its claws are covered in soft bristleswas 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.
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.
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