The geographical history of hairy crabs
Hairy crab ( Eriocheir sinensis) is a cave-dwelling crab native to coastal rivers and Yellow Sea estuaries in Korea and China. Although it is not yet present in the Pacific Northwest, scientists predict that it, like the European green crab (Carcinus maenas), may eventually reach Oregon and Washington through larval dispersal or intentional release.
Hairy crabs are native to Northeast and Southeast Asia and have also spread to Europe, the Middle East and North America. In China, the main natural habitats are located in the Yangtze River, Yellow River and Liaohe River basins, while the dominant cultural areas are located in the Yangtze River Delta region. Due to its unique flavor and high nutritional content, as well as its good balance of essential amino acids, fatty acids and minerals, Chinese stickleback is one of the most important aquaculture species in China's aquatic food industry, with a production volume of 812,103 tons in 2016. In order to improve the social and economic benefits of agricultural products, the Chinese government is promoting the "Protection of Geographical Indication Products". Currently, Dendrobium sinensis from 24 well-known origins has been registered with the General Administration of Quality Supervision, Inspection and Quarantine (AQSIQ). Due to its high commercial value (especially PGI products), the rapidly developing Chinese crab industry continues to be plagued by mislabeled, counterfeit and shoddy crabs, which seriously damages the rights and interests of consumers and has an adverse impact on the market. For example, the famous PGI Chinese hairy crab, native to Yangcheng Lake, has long been the target of crab adulteration (such as the so-called "bath crab"). Attempts have been made to differentiate Chinese crab populations originating from the same or different river systems based on crab morphological characteristics and microsatellite markers.
What does a crab look like and where does it live?
The main identifying feature of hairy crabs is the dense hairs on the white pointed claws of the larger juvenile and adult crabs. The claws are equal in size, have four spines on each side of the carapace, and are about 3 inches (80 mm) wide. The leg length of an adult crab is generally more than twice the width of the carapace. As a precipitation species, adults migrate downstream to breed in the salty waters of estuaries. The female carries 250,000 to 1 million eggs before hatching, and both sexes die shortly after breeding. After a planktonic larval stage of 1 to 2 months, juvenile crabs settle in salt or brackish water in late spring and then migrate often long distances to freshwater for feeding. Hairy crabs are reportedly found in China's Yangtze River basin, 800 miles upstream from the Yellow Sea.
Hairy crabs are omnivores, eating both plants and animals. Young fish mainly eat plants. As they mature, crabs increasingly prey on animals, especially small invertebrates, including worms and clams. Adult crabs have become a nuisance for anglers in California, eating everything from ghost shrimp to shad. Predatory fish, including sturgeon, striped bass, and channel cattails, as well as bullfrogs, raccoons, otters, and wading birds may prey on crabs.
Eriocheir crabs are land walkers and if they are blocked by dams, weirs or other obstacles during their migration, they can easily jump over river banks or embankments to get around them. In Germany, large numbers of hairy crabs leave the water at night when they encounter obstacles, occasionally wandering the streets and entering houses. In California, hairy crabs have been found on roads, airport runways, parking lots, yards and swimming pools.
In 1998, a male Japanese mitten crab (Eriocheir japonica) was caught in Oregon's Columbia River. This species is very similar to the Chinese mitten crab currently found in California, and its presence is likely the result of attempts to introduce it into the watershed.
successful intruder
Hairy crabs have a long history as invaders. Crabs were accidentally introduced to Germany in the early 1900s. In the 1920s and 1930s, crab populations exploded, and the crab's range rapidly expanded to include many Nordic rivers and estuaries. There has been a recent surge in crab populations in Britain's Thames River.
In 1992, commercial shrimp trawlers in southern San Francisco Bay caught the first mitten crabs on the West Coast. Since then, the crabs have spread rapidly, colonizing San Francisco Bay and spreading to river areas upstream of the Delta. The most likely mechanisms of introduction into estuaries are intentional releases to establish fisheries (hairy crabs are a delicacy in Asia and the crabs are illegally imported into markets in Los Angeles and San Francisco) or accidental releases through ballast water.
Attempts have been made to control hairy crab populations, but little information is available on the results. European hairy crab populations declined in the late 1940s.
Why should we worry?
Hairy crabs pose a potential threat to human health. It is the intermediate host of Oriental paragonimiasis and mammals, including humans, can become infected by eating raw or undercooked hairy crabs. However, neither paragonimiasis nor freshwater snails, the primary intermediate hosts of Asian flukes, have been found in the Pacific Northwest or California.
Expanding hairy crab populations pose multiple threats to ecology, economics, and human health. Hairy crabs may have profound impacts on biological communities through predation and competition, and may alter the structure of freshwater and saltwater benthic invertebrate communities in areas they invade. Also of concern is the possible predation of salmon and sturgeon eggs and juveniles. In tidal areas, hairy crabs burrow into river banks to protect themselves from predators and avoid dehydration at low tide. This burrowing activity may increase erosion and instability of embankments and river banks. Hairy crabs are hosts for Oriental paragonimiasis, a human health problem. Additionally, hairy crabs often inhabit areas with higher levels of pollutants. Bioaccumulation of contaminants may be transferred to predators, including humans.
In Europe, the most widely reported economic impact of hairy crabs is damage to commercial fishing nets and catches caused by hairy crab fisheries. In San Francisco Bay, removing crabs from their nets is time-consuming and expensive for shrimp trawlers (one trawler reportedly caught more than 200 crabs in a single trawl on multiple occasions), resulting in damaged catches or death. Another significant issue in California is the impact on translocation and fish salvage facilities. Hairy crabs clog water pumps, filters and water intakes, and damage and kill fish in salvage facilities associated with water diversions. As salmon and trout populations decline, any further disturbance or damage to fish passage is a major concern.
