Mitten Crab Watch

About

Chinese mitten crabs (Eriocheir sinensis) originate from South East Asia and are known for their furry claws. They are an invasive non-native species found across Europe. First recorded in the river Aller, Germany in 1912 they had spread to the UK by 1935, and established a population in the Thames. Adapted to both fresh and salt water, their range extends far inland with sightings as far inland as Nottingham. They are among the most damaging invasive species in UK waters, causing ecological harm by out-competing native species and burrowing causing destabilisation of river banks. They are also a significant pest to anglers and waterway workers, causing damage to fishing gear and flood defenses, and blocking intake screens. 

The full extent to which these pests have spread in English and Welsh waters is currently unclear. The Mitten Crab Watch project was originally established by the Marine Biological Association (MBA) and the Natural History Museum (NHM) to encourage citizen science efforts focused on enhancing our knowledge of the spread of this species in the UK. The MBA no longer organises citizen science activities directly relating to this project, however you can still help collect data by submitting sightings via the platforms detailed in the following section.

It is important to be aware of UK government guidance relating to non-native species before interacting with Mitten Crabs.

Identification

Mitten Crabs can be easily confused with other species. The following list of features will help you to identify them:

  • Grey-green to dark brown crab
  • Long walking legs
  • Squarish body up to 86 mm across
  • 4+4+4 pattern of teeth around front of shell (Fig 1.)
  • Dense brown 'fur' on the white-tipped claws (Fig 2.)
  • Habitat: rivers, brackish water estuaries, rarely along the marine inshore coast
  • Juveniles may lack 'fur' on claws

You can download the GB non-native species secretariat (NNSS) Mitten Crab ID guide here.

Fig.1
Figure 1: The 4+4+4 spine configuration.
Fig.2
Figure 2: The furry claw of the Mitten Crab.

Submit a Record

If you see a mitten crab, a sighting report requires:

  •  A clear photograph of each individual
  •  The number of crabs seen
  •  The exact location, time and date of the sighting

To help confirm that your crab is a Chinese Mitten Crab, use the identification aid above.

To report 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 iNaturalist 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 through dassh.enquiries@mba.ac.uk with your photograph, location, time and date of the sighting and they will record the sighting for you.

Distribution

Observations of Mitten Crabs including those recorded by citizen scientists are submitted to the National Biodiversity Network (NBN) database. From here data are collated to provide a better understanding of the growing distribution of this invasive non-native species. The map below is produced from data submitted to the NBN. 

Find Out More

Philine zu Ermgassen, Freshwater Ecologist at the University of Cambridge,  talks about the identification, impacts and management of The Chinese Mitten Crab in Great Britain.

Further Information

For further information on Mitten Crabs check out The Natural History Museum - Mitten Crab Research.

In addition, you may find the following resources useful:

  • DEFRA Guidance on Invasive non-native species: rules in England and Wales - Gov.uk.
  • Chinese Mitten Crabs on The Marine Life Information Network (MarLIN) - MarLIN.
  • For more information on UK crab species see the Arthropoda section on MarLIN - MarLIN.
  • The Great Britain Non-Native Species Secretariat (GBNNSIP) - free-to-use images.