|Chinese Mitten Crab
( Eriocheir sinensis )
DESCRIPTION: Adult Chinese mitten crabs are easy to identify by the extensive mitten-like covering on the claws. The Shape of the carapace is different from other brachyuran crabs in North America (it is markedly convex and very uneven, with four sharply edged epigastric lobes). Carapace width 30-100 mm; markedly convex and uneven with four lateral carapace spines (fourth spine is small). Legs are approximately twice the length of the carapace width. Chinese mitten crabs have a notch between the eyes, with two small spines located on either side. Claws, normally equal in size are "hairy", i.e., densely covered with brown setae and tend to be fuller on males. Sex is determined by the structure of the abdomen: females have a wide abdominal flap that extends to the edge of the abdomen when fully mature, whereas males have a narrow abdominal flap. Chinese mitten crabs are brownish-orange to greenish-brown in color.
PATHWAYS/HISTORY: The Chinese mitten crab is native to Southeast Asia from southern China to the Korean Peninsula was and was first reported in San Francisco Estuary in 1992 . This species is established in only in California, although it has been reported in the Detroit River, Mississippi River, Great Lakes, and Columbia River near Portland, Oregon. Its presence in North America is of particular concern since its prior introduction to and spread throughout Europe had significant adverse impacts. The probable means of introduction was by transport of larvae and small crabs in ship ballast water, adult crabs clinging to ship hulls and barges (very unlikely means of introduction into California), or by deliberate release to establish a fishery or local food resource.
RISKS/IMPACTS: Females are capable of producing 100,000-1,000,000 eggs per brood. Chinese mitten crabs are omnivorous and opportunistic. They consume a wide variety of plant and animal material, including algae, macrophytes, terrestrially derived detritus, invertebrates (both hard and soft-shelled) and will scavenge fish carcasses; also steal a wide range of bait from fishermen. Predation on small invertebrates increases with size. Chinese mitten crabs interfere with operations at water facilities and pumping stations. Entrainment during fish salvage operations results in large losses of fish. Burrowing activities cause damage to dykes, coastal protection systems, harbor installations, and soft sediment banks when populations reach high abundance. Damage to soft sediment banks has the potential to affect flooding events, thus increasing erosion and repair expenses. Feeding on fish in nets has the potential to reduce harvest of fishing industry. The spiny carapace and legs of crabs damage fish catch. Crabs get entangled in gear, increasing handling time and cause damage to fishing nets. Chinese mitten crabs are intermediate hosts of the mammalian lung fluke of the genus Paragonimus and they have the potential to damage rice crops by consuming the young rice shoots.
MANAGEMENT: In Germany, traps placed on the upstream side of dams captured juvenile crabs as they migrated upstream. Installation of traveling screens and trash racks, in conjunction with collection and disposal efforts, has the potential to drastically reduce a large portion of the mitten crab population during migration events; this is most effective if done at a place where the river is entirely contained by a regulatory structure (e.g., dam, fish facility). A National Management Plan, created in 2002, reviews strategies and methods for population control and management: The Chinese Mitten Crab Invasion of California: <A HREF="mittencrabplan.pdf">A Draft Management Plan for the Genus Eriocheir</a>. One the positive side, Chinese mitten crabs are a potential food source for predatory fishes (e.g., pike, eel, brown trout, white sturgeon, striped bass, largemouth bass, large sunfish), bullfrogs, loons, egrets and other wading birds, river otters, and raccoons.
WHAT YOU CAN DO: Do not intentionally spread these crabs. Clean all bilge and bait wells. Clean fishing nets and other gear before moving to new destinations.
PROFILE CREDIT: Dani Crosier and Dan Malloy - IMAGE CREDIT: U.S. Fish & Wildlife Service