The Chinese mitten crab, Eriocheir sinensis (also known as the big sluice crab (Chinese: 大閘蟹; pinyin: dà zhá xiè) and Shanghai hairy crab, Chinese: 上海毛蟹; pinyin: shànghǎi máo xiè) is a medium-sized burrowing crab that is native in the coastal estuaries of eastern Asia from Korea in the north to the Fujian province of China in the south. It has also been introduced to Europe and North America where it is considered an invasive species.

Formerly in the family Grapsidae, it is now placed in the Varunidae.

Description and ecologyEdit

This species' distinguishing features are the dense patches of dark hair on its claws. The crab's body is the size of a human palm. The carapace width is 30–100 mm and the legs are about twice as long as the carapace is wide.

Mitten crabs spend most of their life in fresh water, but they must return to the sea to breed. During their fourth or fifth year in late summer, the crustaceans migrate downstream, and attain sexual maturity in the tidal estuaries. After mating, the females continue seaward, overwintering in deeper waters. They return to brackish water in the spring to hatch their eggs. After development as larvae, the juvenile crabs gradually move upstream into fresh water, thus completing the life cycle.

As invasive speciesEdit

This species is very invasive and has been spread to North America and Europe, raising concerns that it competes with local species, and its burrowing nature damages embankments and clogs drainage systems. The crabs can make significant inland migrations. It was reported in the London Evening Standard in 1995 that the residents of Greenwich, UK, saw the Chinese mitten crabs coming out of the River Thames and moving towards the High Street, and other reports indicate that the crabs have been known to take up residence in swimming pools. In some places the crabs have been found hundreds of miles from the sea. There is especial concern in areas with a substantial native crab fishery, such as the Chesapeake Bay in Maryland and the Hudson River in New York (both locations where the crabs were first spotted in 2005), as the impact of the invasion by this species on the native population is unknown.

It is generally illegal to import, transport, or possess live Chinese mitten crabs in the United States, as accidental release or escape risks spreading these crabs to uninfested waters. In addition, some states may have their own restrictions on possession of mitten crabs. California allows fishing for mitten crabs with some restrictions.

The Chinese mitten crab has been introduced into the Great Lakes several times, but have not yet been able to establish a permanent population.