Image credit: GBNNSS
At a glance
Record Chinese mitten crabs
Type of activity: outdoors in UK
Who can take part? Anyone
When? All year round
Where? Tidal rivers and estuaries around the UK
How long will it take? No set time
How to report your sightings
If you see a mitten crab, a sighting report requires:
- a clear photograph of the individual
- the number of crabs seen
- the exact location, time and date of the sighting
If you have the iNaturalist app (available on iOS and Android), a useful tool to record any wildlife sightings both in the UK and overseas, you can submit the record directly on the app or via the iNaturalistUK website.
You can also upload records to iRecord, which like iNaturalistUK, shares the data with the NBN Atlas, a national biodiversity database.
Alternatively, you can email the Marine Biological Association at dassh.enquiries@mba.ac.uk with your photograph, location, time and date of the sighting and they can record the sighting for you.
Chinese mitten crabs are an invasive species whose spread in the UK requires continuous updating. Everyone can monitor their distribution by reporting sightings.
These crafty crustaceans damage riverbanks, compete with native species, block water outlets and damage fishing gear such as nets with their claws. They live in rivers, canals and estuaries, so are mostly likely to be encountered by anglers, and other users of waterways.
By adding sightings to national databases, we are able to protect endangered native animals and track the spread which in turn helps to inform management activities and protect endangered native species.
Background
Originating in South East Asia, the first mitten crab was recorded in the UK in 1935 in the River Thames at Chelsea. By the 1990s they had become well-established in the River Thames. Subsequently other river catchments including the Humber, Medway, Tyne, Wharfe and Ouse have recorded mitten crab populations.
How to identify a Chinese mitten crab
They are the only crab found in freshwater but may be confused with native species in coastal habitats.
Mitten crabs can be identified by the following features:
- Grey-green to dark brown body
- Dense brown 'fur' (may be absent in juveniles) on the white-tipped claws
Chinese mitten crab legs can span a 25cm dinner plate.
The white-tipped claws of mitten crabs are covered in ‘fur’.
There is more information on how to identify mitten crabs on the Mitten Crab Watch website and an identification sheet (PDF 1.4MB) provided by the Non-Native Species Secretariat (NNSS), or watch their video below:
For more information on this species, see the NNSS website.
Results so far
Over 1,200 records have been submitted so far and these confirm that the mitten crab is extending its range in the UK. Since 2020 the crab has been found in the River Severn and Morecambe Bay area.
An interactive map of Chinese mitten crab records can be viewed on the National Biodiversity Network (NBN) Atlas.
Project team
- Dr Paul Clark, Researcher, Life Sciences
Partner organisations
- Welsh Government
- Tyne Rivers Trust
- Royal Holloway University of London
- Natural Resources Wales
- Newcastle University
- Marine Biological Association
Project sponsors
