The aliens are here. And they’re wearing mittens
The Chinese mitten crab (Eriocheir sinensis) is a poorly known yet highly invasive species in the UK. Originating in southeast Asia, the mitten crab was accidentally introduced into Germany in the early 20th century through the transport of larval crabs in ballast water. Populations have since become established in the waters of many other European countries including Holland, Belgium and France. Further introduction into North America has enabled this species to adopt a wide global distribution spanning three continents. In England, these crabs have been reported from many river catchments including the River Thames, where the population appears to be increasing in size and northward progression; in June 2014 a specimen was found as far north as the River Clyde in Scotland. This advance is extremely concerning to UK conservationists and this species now lies within the top one hundred invasive species as classified by the Global Invasive Species Database (GISD).
Whilst these crabs, characterised by a fluffy mat of hair resembling mittens (hence the common name) on their chelae, may appear charismatic and sound rather harmless, they are in fact extremely threatening to our native wildlife. Not only are they responsible for habitat destruction through their burrowing activity, mitten crabs directly threaten native fauna and flora through predation and competition. In addition, they can act as vectors for disease; recent research has confirmed the mitten crab transmits the crayfish plague pathogen Aphanomyces astaci, which is thought to have played a key role in the decline of the endangered European Cray fish (Astacus astacus).
Most recently, scientists have expressed concern about the potential for mitten crabs to consume fish eggs. As these crabs are catadromous, spending much of their lifecycle in freshwater but returning to the sea (where they are born) to breed, both marine and freshwater fish species could be at risk. A recent (unpublished) research project carried out by Royal Holloway University and experts at the Natural History Museum suggested that these crabs do have the ability to consume a range of marine and freshwater fish eggs and could therefore contribute to the decline in economically and ecologically important fish stocks. This study built upon a previous study conducted in California, which also suggested that mitten crabs have the potential to ingest fish eggs. This is of particular significance in light of the recent discovery of a crab in Scotland, where important salmon and trout fisheries are located.
All of these risks are enhanced by the fact that controlling this species has proved difficult and largely unsuccessful. Without an effective method for preventing the spread of crabs, it is likely that the negative ecological impacts associated with this species residing in non-native environments will only continue and probably increase. It has been suggested by scientists that we could harvest mitten crabs for food as a means of limiting the UK population, but this has not been authorised. This is certainly not the case in China, where the Chinese mitten crab is a delicacy and you can even buy them from vending machines (Literally– no joke).
For more information on the Chinese mitten crab and to record any live sightings please visit http://mittencrabs.org.uk/
Culver C.S. (2005) Assessing the potential for Chinese mitten crab predation on eggs and Salmonid larvae. Report for U.S. Department of Interior Fish and Wildlife Service, Marine Science Institute. University of California, Santa Barbara.
Invasive Chinese mitten crab (Eriocheir sinensis) transmits crayfish plague
pathogen (Aphanomyces astaci):
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