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.

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 Ireland or Northern Ireland?:
  • Not present in Ireland or 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 to CEDaR. 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.

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 Non Native Invasive Species Team in the Northern Ireland Environment Agency on 028 9056 9558.
Invasive Species Ireland

Invasive Species Ireland