IUCN threat status:

Least Concern (LC)

Brief Summary

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Species Abstract

The Signal Crayfish is an aggressive species indigenous to the northwest USA, but which has been introduced into much of Europe and parts of Asia to the detriment of extensive aquatic ecosystems in those regimes.


P. leniusculus has a native range from British Columbia to central California and eastward to the Rocky Mountains. The introduced range includes Britain, much of western Europe including Sweden, most of eastern Europe, part of western Russia and some of the Japanese islands. The introduction of Signal Crayfish to these vast areas was motivated by commercial interests in creating a robust industry for harvesting this species; however, the outcome has produced an ecological nightmare by driving extinct a number of native crayfishes and producing ecological imbalance from this voracious competitor.


Length of males measures to 16 cm from end of telson to rostrum tip, with females about three quarters of this size. Body mass varies between 55 and 115 grams. Exterior color varies between brownish blue, reddish brown, and less commonly a plain brown. There are smooth rostrum flanks; the acumen is very quite sharply pointed and manifests significant shoulders. A median carina runs entire length. Claws are massive and smooth, the underside being red; with a lone inner tubercle (relative to the fixed finger).A distinctive white patch decorates the top of the join of fixed and movable fingers.


P. leniusculus exploits a gamut of habitats including streams, major rivers and sub-alpine lakes; (Lewis) it is able to adapt to warmer waters, pH above six and even slightly saline water bodies. This aggressive species can colonize streams at the rate of one kilometer per annum. (Stanton) Burrows can occur at very high organism densities, up to 14 per square meter; moreover, this dense clustering of burrows can compromise streambank integrity, in some cases causing severe bank collapse and subsequent erosion. This phenomenon has been particularly observed in Europe, where there are differing bank forms sometimes lacking in the natural burrow crevices of the species native range; therefore, in Europe, much more extensive burrowing has been noted compared to use of streambed armor in the Western USA. (Sibley) Colonization of new area is facilitated by P. leniusculus' ability to creep overland and around terrestrial barriers.

Although polytrophic in diet, faunal intake is preferred by this crayfish, which has caused major attrition to certain macro-invertebrates, benthic fish and aquatic vegetation. (Nyström) For example, P.leniusculus has been shown to decimate Atlantic salmon populations.. Pacifastacus nigrescens, endemic to the western USA, went extinct partially due to interspecific competition with the Signal Crayfish, which humans introduced into its range. Signal Crayfish has caused a contraction in the range of the western USA narrowly endemic P. fortis. (Taylor).


There is no concern regarding the viability of this species; rather, there are significant issues regarding Signal Crayfish as an invasive species, threatening the existence of numerous other taxa. Introduced into Japan as early as 1926, this crayfish has been cultivated in Asia and Europe for its substantial catches, exceeding 400 tons per year in Europe. There are literally no management techniques known for the successful management of P. leniusculus. Trapping is size selective, with the result that smaller individuals elude Preventing the further introduction of Signal Crayfish into new bodies of water is the most important single element of future decision-making regarding this species. Public awareness of the ecological risks this species pose and identifying new populations are key components to arrest the spread of this species. Current research is examining the use of pheromones to attract male P. leniusculus into traps. Tough legislation has been invoked to P. leniusculus in Britain, which labels it a pest and bans the keeping of it in Scotland, Wales and much of England. Nevertheless, P. leniusculus colonizes new waters and threatens the extinction of the indigenous UK crayfish population (Hiley).


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© C. Michael Hogan

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