Chinese Mitten Crabs have come to the East Coast of the United States
We are seeking reports of crab sightings and collections
The Chinese Mitten Crab (Eriocheir sinensis) is native to East Asia and is an established invader in Europe and the West Coast of the United States. It was discovered in the US in 1962 in the Great Lakes and has subsequently been reported along the Gulf Coast (1978), San Francisco Bay (1992), Chesapeake Bay (2005), the Hudson River (2007) and other East Coast areas from 2007-2009. This crab is known to have negative ecological and ecological impacts and is listed as Injurious Wildlife under the Federal Lacey Act, which makes it illegal in the United States to import, export, or conduct interstate commerce without a permit.
Mitten crabs occur in both freshwater and saltwater environments. Adults migrate from freshwater to estuaries to breed. Their larvae develop in marine environments until they become juvenile crabs, at which time they migrate into freshwater tributaries. Adults spend between two and five years in the freshwater tributaries before returning to the estuaries to breed.
The Smithsonian Environmental Research Center and the Mitten Crab Network are requesting your help locating the Chinese Mitten Crab along the East Coast of the US to determine the status, abundance, and distribution of this species. Please report any mitten crab sightings along with details (date, specific location, size) and a close-up photograph or specimen if possible. Your help in reporting new sightings is vital to preventing their spread. Please call 1-443-482-2222 to report a crab sighting.
If you catch a mitten crab
- Do not throw it back alive!
- Take a close-up photo.
- Note the precise location where the animal was found.
- Freeze the animal, keep on ice, or as a last resort preserve it in rubbing alcohol.
- Mitten crab specimens may be needed to confirm sightings, contact the Mitten Crab Hotline at 443-482-2222 or email your photo to SERCMittenCrab@si.edu to determine if your specimen is needed and for shipping directions.
Mitten crabs hatch from winter through early summer in brackish to marine water. Larval crabs (zoea) develop in primarily marine conditions until the last larval stage (megalope), when they settle in brackish near-shore waters. Juvenile crabs migrate from estuaries to freshwater rivers, lakes and marshes in the late summer and fall. Upstream migration up to 1500 km can occur in areas with high densities of crabs. Adult crabs stay in freshwater environments from two to five years before migrating back to the estuaries in late winter or early spring to reproduce.
Mitten crabs got through 7 larval stages, a prezoea stage, 5 zoeal stages, and a megalope stage, before developing into juvenile crabs. Larval mitten crabs hatch and complete development in a wide range of temperatures and salinities that varies with larval stage. The salinity optimum shifts from brackish to seawater during zoea development and back to brackish by the megalope stage corresponding to larval dispersal patterns.
Adult mitten crabs are easy to identify due to their distinct furry claws for which they are named. The main identifying characteristics are:
- The only crab found in freshwater in North America
- Adults and juveniles (>1 inch) have furry white-tipped claws that are equal in size. Juveniles with a carapace length less than one inch may not have hair on their claws but those over one inch will have hair.
- Light brown to olive green body (carapace) that is 3 to 4 inches wide in adults. The carapace is smooth and round and has a 4 lateral spins (the forth may be small).
- There is a U-shaped notch in the carapace between the crab’s eyes.
- This crab has eight sharp-tipped walking legs but no swimming legs. The legs are over twice the length of the carapace.
The introduction of the Chinese Mitten Crab San Francisco Bay and its tributaries in the mid- 1990’s caused problems throughout the Bay particularly to fisheries (shrimpers and others) and in years when the crab was most abundant they hindered fish salvage operations in the Bay. In Tracy, CA a mass of migrating mitten crabs killed thousands of threadfin shad and other fish when the crabs got into a facility designed to help the fish migrate. Nearly all the fish died. In addition to the ecological costs, the incident caused more than a million dollars in damage.
Chinese Mitten Crabs have been problematic in some areas because they burrow into banks and levees along estuaries reducing the structural integrity of the banks. In one California study, the crabs removed 0.8 to 5.7% of the stream bank sediment.
In Asia where they are native, the mitten crab is host to the Oriental lung fluke, which, if eaten, can cause tuberculosis-like and influenza-like symptoms in humans. This parasite hasn’t been reported in San Francisco Bay or other introduced areas; however, this may be due, in part, to that fact that mitten crabs are not a common food item in their introduced range.
The impact that these crabs will have on the East Coast is unknown because they have only recently been discovered and are not abundant. At this time it unknown if these crabs have established breeding populations in the east, though the presence of ovigerous (egg carrying) females increases the likelihood. We need your help to collect these data.
The means of introduction into East Coast estuaries is uncertain; however possible pathways might be accidental or intentional release of live seafood. Mitten crabs are a popular food in many Asian countries, so even though it is illegal to import these species, there is a demand for these crabs as food. Shipping is another possible pathway because the larval mitten crabs are marine and thus available to be picked up and transported in ballast water.