Signal crayfish (Pacifastacus leniusculus)

Signal crayfish, close up
IUCN Red List species status – Least Concern LEAST

Top facts

  • The signal crayfish is a voracious predator, feeding on a variety of fish, frogs and invertebrates, as well as plants, and even eating individuals of its own species.
  • The signal crayfish is highly invasive, and through both deliberate introductions and accidental release, this species has expanded its original North American range to include 25 European countries.
  • In the UK, the signal crayfish is known to be able to expand its range downstream by about 1.2 km per year.
  • As a carrier of crayfish plague, the signal crayfish is a major threat to other crayfish species in Europe.
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Signal crayfish fact file

Signal crayfish description


A lobster-like crustacean (3), the signal crayfish (Pacifastacus leniusculus) is named for the conspicuous white, oval patch at the base of each claw where the two parts of the pincers meet (4) (5). This marking, which is usually edged with blue, is a key characteristic used to identify the signal crayfish (4).

The signal crayfish can grow up to a maximum length of about 18 centimetres (2) (3), and has rather robust, broad claws (3) (4). The upper surface of this species is usually brown to greenish-brown, while the lower surface is often a contrasting bright orange or red. The signal crayfish has a distinctly smooth ridge running along the middle of the rostrum (4).

Young signal crayfish resemble the adults (5).

There are three known subspecies of the signal crayfish: Pacifastacus leniusculus leniusculus, Pacifastacus leniusculus trowbridgii and Pacifastacus leniusculus klamathensis (2).

Also known as
American signal crayfish, Californian crayfish, Simontorp crayfish.
Astacus leniusculus, Potamobius leniusculus.
Maximum length: 18 cm (2) (3)

Signal crayfish biology

A highly aggressive and voracious predator (3), the signal crayfish is known to grow large enough to predate on some fish species (9), including the bullhead (Cottus gobio) and stone loach (Barbatula barbatula) in Britain (3) (10). In addition, the signal crayfish feeds on frogs and invertebrates, and has even been recorded eating individuals of its own species (3) or occasionally feeding on nematode worms (7). Although the signal crayfish tends to show a preference for consuming animals (8), this species is omnivorous, and so also eats vegetation (2) (3).

The signal crayfish is a nocturnal species (2), and is known to take shelter under rocks and boulders, within tree roots or in burrows and cavities within banks (6). In the winter, adult signal crayfish shelter in burrows and enter a state of torpor. These burrows are formed of many inter-connecting tunnels, and can be up to two metres deep (5).

The signal crayfish is an extremely successful competitor, particularly within its introduced habitats. A large and hardy species (10), the signal crayfish is more active and aggressive than other crayfish, and often has larger pincers. In addition, this species is thought to be able to tolerate relatively high temperatures, and may also be less vulnerable to winter conditions than native crayfish species (2).

The signal crayfish has a relatively high reproductive potential (2), with the female of this species laying between 200 and 400 eggs in the autumn (3) (5). The female then carries the eggs around under her tail through the winter until the spring (3) (5), when the eggs hatch (5). Once hatched, the young remain attached to the female’s tail until they are released in May or June (5). Signal crayfish reach sexual maturity at about 2 or 3 years old (5), and can live for up to 20 years (3) (5).

The signal crayfish is known to grow faster than any of the native European crayfish species (2) (4). It was this quality, in addition to its resistance to crayfish plague, which made the signal crayfish an attractive commercial species and a popular replacement for disease-ravaged native crayfish populations in Europe (4) (10). Unfortunately, the signal crayfish is a carrier of crayfish plague, and is thought to be responsible for passing the deadly disease to the white-clawed crayfish (Austropotamobius pallipes) in the UK (3).

In the UK, the signal crayfish is known to be predated by otters (Lutra lutra), American mink (Mustela vison) and predatory fish such as Atlantic salmon (Salmo salar) and European eels (Anguilla anguilla) (5).


Signal crayfish range

The signal crayfish is native to North America (2) (4) (5) (6) (7), where it occurs in the cool, temperate regions of the north-western United States and south-western Canada (2). This natural range includes the southern part of British Columbia in Canada, and parts of Idaho, Washington, Oregon, Utah and Montana in the United States (7) (8).

However, the signal crayfish has been widely introduced outside its natural range (6). Since being introduced to Sweden and Finland in the 1960s for aquacultural purposes when native crayfish species were struck down by crayfish plague (2) (3) (6), this species has spread. Through escapes and deliberate introductions it has become established in the wild in most northern European countries (6), including Great Britain, France and Spain (3) (4). The signal crayfish was also introduced to Japan several times between 1926 and 1930 (8).


Signal crayfish habitat

The signal crayfish occurs both in still and slow-flowing freshwater environments (2) (4) (5) (7), including rivers, streams, lakes, reservoirs and canals (2) (3) (4) (5). This species is also known to be able to tolerate slightly salty water (4) (5).

The signal crayfish is usually found in water along the shoreline of the various aquatic areas it inhabits, often over stony bottoms or other substrates which can provide hiding places (2)


Signal crayfish status

The signal crayfish is classified as Least Concern (LC) on the IUCN Red List (1).

IUCN Red List species status – Least Concern


Signal crayfish threats

The signal crayfish is not currently considered to be a threatened species in its native range. Where it has been introduced, either deliberately or accidentally, this species is considered to be highly invasive (6).

Although the introduction of the signal crayfish to Europe bolstered the continent’s commercial crayfish industry, it unfortunately also brought with it a host of problems. The most severely damaging effect has been through the signal crayfish’s role as a vector for crayfish plague, which has now been transported across Europe, most likely through the aquaculture trade and the transport of infected fishing gear (10). The signal crayfish is itself immune to crayfish plague, but as a carrier of the disease (3) it has contributed to the dramatic decline of populations of highly susceptible native European crayfish (3) (6), including the British white-clawed crayfish (3).

The signal crayfish has also had a negative impact on the white-clawed crayfish and many other European crayfish populations as a result of its highly aggressive and competitive nature (10). The signal crayfish is capable of displacing entire populations of native crayfish, as well as many fish species, through its ability to outcompete them for food and shelter (3) (6) (8) (10). A voracious predator, the signal crayfish also preys upon local and endemic species in its adopted aquatic habitats (3) (5) (9), including the bullhead and stone loach in Britain (3) (10), negatively affecting food webs (5). Also feeding on aquatic plants, the invasive crustacean can cause severe damage to vegetation (5).

Signal crayfish burrows are known to cause significant damage to stream and canal banks, which increases flooding risks and displaces threatened riverside species such as Britain’s water vole (Arvicola amphibius). In addition, the presence of signal crayfish burrows can destabilise structures built near the edges of rivers (3) (5) (9).

In the UK, some water regions have reported a worrying increase of 43 percent in the occurrence of this species between 1997 and 2001 (5). It has become well established in England and Wales, particularly in south-eastern regions (5), and is known to be capable of spreading downstream at a rate of 1.2 kilometres per year (6). Commercial fisheries in the UK have already been affected by the signal crayfish feeding on fish eggs and competing with salmonids for shelter (5).


Signal crayfish conservation

No specific conservation measures are currently known to be in place for the signal crayfish in its native range. However, as a highly invasive species there are steps being taken to prevent its further spread in its non-native range (3) (11).

Legislation was passed in Britain in the mid-1990s to forbid people from keeping exotic crayfish species, but this was not effective in preventing the further spread of this aggressive invasive crustacean (3). The British Government is now encouraging anglers to kill any signal crayfish they catch (3), and there are suggestions that, as an edible species, the signal crayfish should be caught to be eaten (9). The release of the signal crayfish into the wild is now illegal (5).

Control of the signal crayfish is considered to be difficult, particularly once the species has fully established itself in an area. In such instances, it is thought that trapping the animals may be the only control option, although in the UK a licence is currently required to carry this out (9).

English Nature and the Environment Agency are currently working together on a research and development programme to investigate the best methods of eradicating non-native crayfish from the UK. Part of this work has involved the production of a crayfish identification leaflet, and future work will include searching for non-native crayfish populations and keeping records of finds, so that an effective eradication programme can be designed and implemented (11).


Find out more

Find out more about the signal crayfish:



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Diverse group of animals with jointed limbs and a hard external skeleton, characterised by the possession of two pairs of antennae, one pair of mandibles (mouthparts used for handling and processing food) and two pairs of maxillae (appendages used in eating, which are located behind the mandibles). Includes crabs, lobsters, shrimps, woodlice and barnacles.
A species or taxonomic group that is only found in one particular country or geographic area.
Animals with no backbone, such as insects, crustaceans, worms, molluscs, spiders, cnidarians (jellyfish, corals, sea anemones) and echinoderms.
Active at night.
Feeding on both plants and animals.
A forward-projecting and occasionally long spine situated between the eyes of crustaceans.
A population usually restricted to a geographical area that differs from other populations of the same species, but not to the extent of being classified as a separate species.
A sleep-like state in which the body processes slow to a fraction of their normal rate.


  1. IUCN Red List (September, 2013)
  2. Leppäkoski, E., Gollasch, S. and Olenin, S. (Eds.) (2002) Invasive Aquatic Species of Europe. Distribution, Impacts and Management. Springer, Berlin.
  3. Williams, G. (2011) 100 Alien Invaders. Bradt Travel Guides, Buckinghamshire, England.
  4. Ingle, R.W. (1997) Crayfishes, Lobsters and Crabs of Europe: An Illustrated Guide to Common and Traded Species. Springer, Berlin.
  5. GB Non-native Species Secretariat (NNSS): Factsheet - Signal crayfish (September, 2013)
  6. Bubb, D.H., Lucas, M.C. and Thom, T.J. (2002) Winter movements and activity of signal crayfish Pacifastacus leniusculus in an upland river, determined by radio telemetry. Hydrobiologia, 483(1-3): 111-119.
  7. Thorp, J.H. and Covich, A.P. (2010) Ecology and Classification of North American Freshwater Invertebrates. Academic Press, Massachusetts.
  8. Johnson, L., Gonzalez, J., Alvarez, C., Takada, M., Himes, A., Showalter, S. and Savarese, J. (2007) Managing Hull-borne Invasive Species and Coastal Water Quality for California and Baja California Boats Kept in Saltwater. University of California Agriculture and Natural Resources, Richmond, California.
  9. Wellby, I., Girdler, A. and Welcomme, R. (2010) Fisheries Management: A Manual for Still-water Coarse Fisheries. John Wiley & Sons, Hoboken, New Jersey.
  10. Board on Agriculture and Natural Resources, Division on Earth and Life Studies, National Research Council (2000) Incorporating Science, Economics, and Sociology in Developing Sanitary and Phytosanitary Standards in International Trade: Proceedings of a Conference. National Academies Press, Washington, D.C.
  11. Birmingham and Black Country Biodiversity Action Plan: Species Action Plan - White-clawed crayfish (Austropotamobius pallipes) (September, 2013)

Image credit

Signal crayfish, close up  
Signal crayfish, close up

© Malcolm Schuyl /

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