North American signal crayfish

Pacifastacus leniusculus


Photo credit: ©GBNNSS
Other Potentially Invasive Crayfish Species:
  • Marble Crayfish (Procambarus sp),
  • Louisiana Crayfish (Procambarus clarkii),
  • Spiny cheeked crayfish (Orconectes limosus),
  • Noble Crayfish (Astacus astacus).

  • It inhabits wide range of habitats from small streams to large rivers.

  • Its claws are robust and smooth on both surfaces, the underside is red in colour; with a single tubercle on the inner side of the fixed finger; and a white-turquoise patch on top of the junction of fixed and moveable fingers.
  • Adult males are massive either lengthways or in width. Males are up to 16cm in length, females up to 12cm; much larger individuals have been recorded, i.e. 95mm carapace length. The weight is typically 60 and 110g at 50 and 70mm carapace length.
  • Its colour bluish-brown to reddish-brown, occasionally light- to dark-brown.

Download N.I.E.A. ID guide

Origin and Distribution:
  • Signal crayfish are now widespread across many parts of Europe, but are not present in Ireland.
  • The nearest neighbour populations to Ireland are present in England, Wales and parts of southern Scotland.
  • Due to trade and travel links, Britain is considered the most likely source of non-native crayfish, but they are present across Europe, so there are a number of pathways that would bring non-native crayfish into Ireland.

  • Invasive non native crayfish are known to have a detrimental effect on populations of white clawed crayfish. American crayfish species can be carriers of the so-called 'crayfish plague' - a disease caused by a fungus (Aphanomyces astaci). The 'plague' does the American crayfish little apparent harm but is lethal to European species such as the white-clawed crayfish ( Crayfish plague carried by signal crayfish is one of the main reasons for the collapse and extinction of native white-clawed crayfish across Europe, including in Britain.
  • Some species of invasive crayfish can cause additional damage to river systems by constructing burrows in the banks leaving them prone to collapse. This presents a hazard to human health and safety by weakening walkways and of the river side and impacts on the flood defence of the areas affected.

How might it get here?
  • Intentional introduction or escape from aquaria is considered the most likely route into Ireland for each of these species.

Is it found in Northern Ireland?
  • Not present in Northern Ireland.

Prevent Spread:
  • To maximise the effectiveness of any control measures, and thereby protect the endangered white-clawed crayfish and other aquatic organisms, early detection of any introduced populations is crucial. Everyone is encouraged to report all potential sightings of the species of crayfish.
  • If you see some of these animals on sale in your local areas, or suspect some have escaped into a river system near you, please report your sighting and include a photograph if you can.
  • Remember the native white-clawed crayfish is an endangered species so please try not to disturb this species.

You can help by reporting any sightings: @ the Centre for Environmental Data & Recording (CEDaR) - Or via the iRecord
Current Legislative Position (Listed on 03 August 2016)
  • This species must not intentionally be brought into the Union; kept; bred; transported to, from or within the United Kingdom, unless for the transportation to facilities in the context of eradication; placed on the market; used or exchanged; permitted to reproduce, grown or cultivated; or released into the environment.
For further queries, you can contact the Invasive Non Native Species (INNS) Team in the Northern Ireland Environment Agency on 028 9056 9558 or Email:

NIEA Invasive Species Alert Poster - North American Signal CrayfishDownload

Species Related Files:

Invasive Species Northern Ireland

Invasive Species Northern Ireland