Chinese mitten crab (Eriocheir sinensis)

Common Names: Mitten crab, Shanghai hairy crab, big sluice crab

Description:

Chinese mitten crab (Eriocheir sinensis) is a freshwater crab from eastern Asia named for the dense patches of hair ? like furry mittens ? on its claws [1][2]. It is unusual among crabs in that it lives most of its life in rivers and streams but must travel to salty coastal water to breed [2]. MISIN tracks it as an early-detection target because it is one of the world's most damaging invaders ? it erodes stream banks by burrowing, clogs water intakes, and is illegal to move under federal law [1][4].

Identification:

The Chinese mitten crab is a medium-sized crab with a smooth, rounded top shell (carapace) up to about 3 inches (8 cm) across, light brown to olive-green in color [1][3]. Its unmistakable feature is its claws: both claws are roughly equal in size, tipped in white, and covered in dense patches of hair that look like furry mittens [1][2]. Look also for a small notch between the eyes and four sharp spines along each side of the shell [1][3]. The legs are long ? more than twice as wide as the shell ? and the crab can travel surprisingly far over land [1].

It is most likely to be noticed in late summer and fall, when adults migrate downstream toward the coast to breed and turn up in nets, traps, and water-intake screens [2][3]. Because it lives in fresh water, a hairy-clawed crab found in a river, stream, or lake far from the ocean is a strong clue [1][2].

At-a-glance cues:

1. Claws of roughly equal size, white-tipped, and covered in dense furry "mitten" patches of hair [1][2]
2. Smooth, rounded shell up to about 3 inches across, light brown to olive-green, with four spines along each side and a notch between the eyes [1][3]
3. Long legs ? more than twice the width of the shell [1]
4. Found in fresh water (rivers, streams, lakes) far inland, unlike most crabs [1][2]

Habitat:

The Chinese mitten crab is native to eastern Asia, along the coast and rivers from the Yellow and Bohai Seas and Korea south to near Hong Kong [2]. It has been carried far beyond that range and is now established in Europe and in North America, including San Francisco Bay (first found in 1992) and the Hudson River, with more recent records on the West Coast [1][2].

It lives in a wide range of waters and tolerates pollution, a broad span of temperatures, and both fresh and salty conditions [2][4]. As a young crab it lives in fresh water ? rivers, streams, and lakes, sometimes hundreds of miles inland ? and as a breeding adult it moves into brackish and coastal water [1][2]. It digs burrows in the banks of streams and channels, which is part of why it causes so much damage [2][4].

Reproduction:

The Chinese mitten crab has an unusual life cycle that also drives how it spreads. Young crabs travel upstream into fresh water ? sometimes up to 600 miles (1,000 km) from the sea ? and spend two to four years growing to adulthood [2]. In fall, the adults migrate back downstream to brackish, coastal water to mate [2]. A single female carries an enormous number of eggs ? roughly 140,000 to 690,000 ? and the larvae develop in salty water before the new young crabs head upstream; the adults die after breeding once [2].

People move this crab far beyond its own travels. It has spread around the world in the ballast water of ships and through the illegal trade in live crabs for food and aquariums [1][2]. It can also be moved by mistake in bait buckets, nets, and on boats. Because it is a federally injurious species, moving live mitten crabs is illegal ? they should never be released, transported, or kept alive [1].

Impact and Damage:

For people, the Chinese mitten crab causes real, practical problems. Its burrowing into stream banks, levees, and dikes increases erosion and can cause banks to slump or collapse [2][4]. During mass migrations, huge numbers of crabs clog the intake screens and cooling systems of water and power plants, reducing water flow, and they damage fishing nets and steal bait and catch [2][4]. There is also a human-health risk: the crab can carry the Asian lung fluke (a parasite), and people who eat raw or undercooked mitten crab can become infected, so it should never be eaten without thorough cooking [2].

For natural area stewards, the crab is listed among the world's 100 worst invasive species [4]. It preys on and competes with native animals, and its burrowing reshapes stream banks and bottom habitat [2][4]. Where migrations are heavy, it can overwhelm fish-protection facilities ? at California water-salvage facilities, fish losses reached 98 to 99 percent during peak crab migration [2]. Its broad tolerance for pollution and salinity lets it spread through waters that stop many other species [4].

Similar species:

A few crabs are mistaken for the Chinese mitten crab. The surest check is its furry, white-tipped "mitten" claws combined with the four spines along each side of the shell ? and the fact that it turns up in fresh water [1][2].

Japanese mitten crab (Eriocheir japonica) is the closest match ? a very similar Asian relative that also has hairy claws and the same kind of life cycle. The two are hard to tell apart in the field, so any hairy-clawed mitten crab is worth reporting [1].

European green crab (Carcinus maenas) is another invasive crab found in coastal waters. It is easy to separate because its claws are smooth and hairless and it has five spines along each side of the shell rather than four, and it lives in salt water rather than fresh [1].

Blue crab (Callinectes sapidus) is a native crab of East Coast estuaries that shares some habitats with the mitten crab. It has smooth, hairless claws, a bluish color, and flattened, paddle-shaped back legs for swimming, unlike the hairy claws and walking legs of the mitten crab [1].

Monitoring and rapid response:

Signs to watch for:

1. A fist-sized brown to olive crab with furry, white-tipped claws, in fresh or brackish water far from the coast [1][2]
2. Four spines along each side of a smooth, rounded shell, with long legs [1][3]
3. Crabs turning up in nets, traps, or water-intake screens, especially during fall migration [2][3]

What you can do:

1. Do not release it back into the water and do not move it alive ? it is illegal to transport mitten crabs under federal law [1]
2. Take clear photos (including the hairy claws), note the exact location, and keep the crab cold or frozen so it can be confirmed [3]
3. Report the sighting right away to your state's aquatic invasive species hotline or program [3]
4. Never buy, keep, or use live mitten crabs as food, bait, or aquarium pets, and clean, drain, and dry boats, nets, and gear to avoid moving larvae [1][3]
5. For natural areas and agencies, crews monitor and trap at water intakes and along migration routes, and prioritize early detection so new populations can be caught before they establish [4]

When to call a professional:

1. Any suspected sighting ? because this is a federally injurious, "100 worst" invasive species, report it immediately so trained responders can confirm and act [1][4]
2. Established or repeated occurrences in a waterway, which need an agency-led monitoring and control response [4]
3. For chemical control options, contact a licensed applicator or your local cooperative extension office.

Report sightings to MISIN. Help track this species' spread by submitting observations through the MISIN web site or the MISIN mobile app (Android, iOS, Windows, and macOS). Photos at multiple angles paired with a precise GPS location are the most valuable data for confirming new occurrences and triggering rapid-response review.

Credits:

1. U.S. Geological Survey, Nonindigenous Aquatic Species (NAS) Database. Eriocheir sinensis (Chinese mitten crab) species profile. nas.er.usgs.gov
2. Smithsonian Environmental Research Center. Eriocheir sinensis. National Estuarine and Marine Exotic Species Information System (NEMESIS). invasions.si.edu
3. Washington Department of Fish and Wildlife. "Chinese mitten crab (Eriocheir sinensis)." wdfw.wa.gov
4. Dittel AI, Epifanio CE (2009). "Invasion biology of the Chinese mitten crab Eriocheir sinensis: A brief review." Journal of Experimental Marine Biology and Ecology 374(2):79?92. semanticscholar.org

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