Invasive Species Alert for Chinese mitten crab

Chinese mitten crab. Photo: Alan Cullagh – IFI

Species Alert for: Chinese mitten crab (Eriocheir sinensis).

Issued by: the National Biodiversity Data Centre on behalf of the National Parks and Wildlife Service (NPWS)

Reason for issue: to notify of the reappearance of Chinese mitten crab in Ireland, a regulated Invasive Alien Species of Union concern.

What was seen and when: One berried female with eggs showing signs of development to Zoea stage, captured by net on 25th January 2021. Another female was photographed 25th December 2020. No other individuals detected during monitoring at this time (up-date May 2021).

Summary of potential impacts:

  • Chinese mitten crabs form burrows in riverbanks. In high densities, the species can damage soft sediment banks through burrowing causing soil erosion and may lead to flooding areas becoming more vulnerable.
  • Likely to have widespread impacts on native fish and invertebrate species as there are no native freshwater crabs in Ireland.
  • May compete with the native Green shore crab (Carcinus maenas) in estuaries by excluding them from shelters.
  • The species may impact on the endangered and protected white clawed crayfish (Austropotamobius pallipes) as it has been shown to act as a host for Crayfish plague (Aphanomyces astaci) under lab conditions.
  • Possible economic impact – estimated cost in Germany approximately 80 million euro.
  • Potential hazard for human health as the species is an intermediate host for a mammalian lung fluke (Paragonimus ringer) which can infect humans if eaten. However, this human health risk would only be an issue if the crabs were infected by the fluke which would require it to also be present in its host in the invaded area. Under EU law, this species is banned from trade, transport and reproduction in captivity plus other restrictions so this species should not be available as live food for consumption.

Identification features:

  • The main feature that distinguishes this crab from others is the mitten-like ‘fur’ on its two front claws.
  • The edges of the long legs are hairy.

    Click image to download the Chinese Mitten Crab ID Guide by the NIEA
  • It is a small crab with a carapace (main body shell) of about 5-7 cm across.
  • Upper-side of carapace is brownish to greenish in colour with 4 spines on either side and a notch between the eyes.
  • Underside is off-white colour.
  • There are no freshwater species of crab found in Ireland so it cannot be confused with any crab in inland waters.
  • In the marine environment, it is distinguishable from other marine crabs by the presence of dense, brown hairs on the claws.

Introduction status: Localized. It has been recorded on three occasions in Ireland from Waterford harbour. One individual was recorded in 2006, 16 crabs in 2009 and 2 crabs in 2021. One of the Chinese mitten crabs caught in 2021 was of a berried (reproducing) female. It is not known to be present in Northern Ireland. See Biodiversity Maps.

Is there a reference specimen? Yes. The berried female crab taken from Waterford harbour has been retained for further examination and will be kept as a reference specimen.

Pathway of introduction:

It is unknown how they were introduced into Ireland. Most likely source of introduction is as larvae in ballast water or as adults on ship’s hulls, though it is possible they could be intentionally brought to Ireland as an aquarium or food species and accidentally or intentionally released.

Circumstances of the discovery:

The berried female crab was detected by a private individual netting for codling and reported under the ‘Targeted monitoring and survey for the presence of Chinese Mitten Crab, an invasive alien species’ survey.

Management actions taken to date for this 2021 sighting:

  • The berried female netted in January 2021, was verified by visual examination and DNA analysis.
  • The National Biodiversity Data Centre was alerted of the initial find in January and informed the relevant authorities.
  • Investigation into potential for eradication is underway.
  • Formal notification to the European Commission of presence of Chinese mitten crab issued May 4th 2021 under the EU Regulation on Invasive Alien Species (1143/2014)
  • Species alert issued May, 2021.
  • National Biodiversity Data Centre maintains an online reporting function: https://records.biodiversityireland.ie/record/invasives

Life cycle: 

The Chinese mitten crab life cycle includes phases spent in the marine, brackish and freshwater environments. Most of its life cycle is spent in rivers as an adult but it must migrate to deep, open saltwater locations to reproduce. Once the crabs have mated the males are thought to die, leaving the females to brood the eggs. In the spring, the eggs hatch into larvae and after about six to seven weeks these metamorphose into juvenile crabs, which then migrate back up the river into freshwater to complete the life cycle.

Where might I see it? 

Adults usually live in burrows in muddy riverbanks where field signs include small burrows on soft sediment river banks. See NIEA ID guide. Aquatic vegetation and marshes may provide an alternative habitat. Dead crabs might be seen washed ashore on river banks and shore areas. Chinese mitten crab are also able to migrate across land unlike all other crab species in Ireland.

What should I do?

  • Report any suspected sightings with a photograph if possible.online https://records.biodiversityireland.ie
  • If you own one, do not allow it to reproduce, escape or release it to the wild.
  • Do not introduce to new sites.
  • Clean all equipment before moving between waterbodies. Follow Check, Clean, Dry, Disinfect protocol.
  • Please circulate this species alert web page and especially to those who may fish in estuary waters and upriver of estuaries.

Invasive status: Assessed as having the potential to be a high impact invasive species in Ireland. It is in the Top 10 2017 Horizon scan list of species most likely to arrive, establish and have impact between 2017 and 2027. Chinese mitten crab is one of 66 invasive alien species of Union concern under EU Regulation (1143/2014). It is listed on the Third Schedule Part 2 of the European Communities (Birds and Natural Habitats) Regulations 2011 in Ireland. This species is listed as among 100 of the worst invasive species in Europe.

Additional Resources:

For queries or information relating to invasive alien species:

email the NPWS ias@housing.gov.ie and see National Parks & Wildlife Service (npws.ie)

or

email invasives@biodiversityireland and see www.biodiversityireland.ie/projects/invasive-species/