Thames alien crab safe to eat

11 February 2009

Chinese mitten crabs that appeared in the River Thames around 75 years ago are safe to eat, scientists report today

This is good news because their commercial exploitation could be a method of controlling these invertebrate pests.

The crabs are causing problems by out-competing and preying on native species, as well as burrowing and eventually collapsing unprotected riverbanks.

Southeast Asian delicacy
NarrowBroad U-shaped abdomen of adult female Chinese mitten crab

Broad U-shaped abdomen of adult female Chinese mitten crab

Natural History Museum scientists became aware that the species was a popular culinary delicacy in Southeast Asia when we where investigating ways of controlling the crabs.

Most Chinese mitten crabs consumed in London are imported from the Netherlands. This new research means setting up a mitten crab fishery in the heart of London is now a possibility.

Testing if the crabs could be eaten

Experts at the Natural History Museum, London Port Health Authority, Cefas, the Food Standards Agency, the Scientific Analysis Laboratory and the Central Science Laboratory, had to assess whether the crabs from the River Thames were suitable for human consumption.

Narrow V-shaped abdomen of adult male Chinese mitten crab

Narrow V-shaped abdomen of adult male Chinese mitten crab

This was important considering the Thames has been heavily polluted in the recent past.

The team had to find out what the potential health risks are to humans. They analysed hundreds crabs to find out what levels of toxic substances they contained.

Toxic substance results

The concentrations of metals and hydrocarbons in the white and brown meat were found to be too low to cause concern.

Levels of orgaonchlorines (PCBs, dioxins and dibenzofurans) in the brown meat of the crab were relatively high. However, the levels were lower than in the crabs analysed from the Netherlands.

Presence of dioxins

Despite the presence of dioxins in the Thames mitten crabs, it is unlikely that any individual would eat enough crabs to be a risk to health. Furthermore, portions are small and mitten crabs are only consumed in the autumn each year.

Too much crab?

The Food Standards Agency already advises that girls and women up to child-bearing age should not eat excessive amounts of mitten crab.

Crabs with 'mittens'
Chinese mitten crab, 'mitten' claw

Chinese mitten crab, 'mitten' claw

The scientific name for the Chinese mitten crab is Eriocheir sinensis. This name is from the Greek meaning ‘wool hand of the Chinese’ in reference to soft bristles covering their large claws.

Spreading fast

Originally from Asia, the Chinese mitten crabs were first introduced to Germany during the early 1900s. They spread rapidly across Northwest Europe and the first reported crab in the River Thames was in 1935.

Their numbers remained extremely low in the Thames until the end of the 1980s when the population dramatically increased. They are also now present in a number of waterways around England.

Great traveller

The crab, which is around the size of a dinner plate, can travel extraordinary distances. In China it migrates up to 1,500km along some rivers. 

However, the crab can be transported much further. If their larvae are sucked into the ballast water of ships, they can then be discharged into a new watershed much later on in the journey. This is how the crab was originally introduced into Germany from China.

UK distribution

In the right conditions, the crab is able to leave the water, cross dry land and enter a new river system. Also, local shipping traffic may be responsible for dispersing the crab around the UK coastline and there are now viable populations reported in the River Ouse, Humber Estuary and Shoreham on Sea, Sussex. 

This distribution is of great concern to scientists in the UK and so any method to successfully control the crabs is greatly needed.

 

This research is published on the Environmental Science & Technology website

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