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.
DISTRIBUTION
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.
MORPHOLOGY
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.
HABITAT AND ECOLOGY
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).
CONSERVATION ISSUES
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).
The signal crayfish (Pacifastacus leniusculus) is a North American species of crayfish. It was introduced to Europe in the 1960s to supplement the North European Astacus astacus fisheries, which were being damaged by crayfish plague, but the imports turned out to be a carrier of that disease. The signal crayfish is now considered an invasive species across Europe, Japan, and California, ousting native species there.
Members of this species are typically 6–9 cm (2.4–3.5 in) long, although sizes up to 16–20 cm (6.3–7.9 in) are possible.[3] They are bluish-brown to reddish-brown in colour, with robust, large, smooth claws. They have a white to pale blue-green patch near the claw hinge,[4] like the white flags that signalmen used for directing trains—hence the name.
The lifecycle of the signal crayfish is typical for the family Astacidae. Around 200–400 eggs are laid after mating in the autumn, and are carried under the female's tail until they are ready to hatch the following spring.[3] The eggs hatch into juveniles, which pass through three stages (two moults) before leaving their mother. Sexual maturity is reached after 2–3 years, and the lifespan can be up to 20 years.[3]
The signal crayfish is an omnivore, with most of its dietary intake being detritus.[5]
The signal crayfish is native to North America west of the Rocky Mountains, including the Canadian province of British Columbia, and the U.S. states of Washington, Oregon, and Idaho.[6] It was introduced to California in 1912 into the San Lorenzo River watershed and from there rapidly spread throughout the state.[7] The only native crayfish remaining in California (aside from Pacifastacus leniusculus klamathensis, a subspecies of signal crayfish believed to be native to the Klamath River in Northern California) is the Shasta crayfish, of Shasta County, California (Pacifastacus fortis), where efforts are being made to create a barrier to signal crayfish invasion.[8] Within North America, it has also been introduced to Nevada, and the populations in Utah may be the result of introductions.[6] It has also been found in Alaska, specifically Kodiak Island, in the Buskin River and Buskin Lake. It is listed as a species of least concern on the IUCN Red List.[1]
From 1907, crayfish plague, an infectious disease caused by the water mould Aphanomyces astaci, damaged stocks of the native European crayfish Astacus astacus. Since the signal crayfish occupied a similar ecological niche in its native range, it was imported in the 1960s to Sweden and Finland to allow recreational and commercial crayfish capture.[3] At the time, the signal crayfish was not recognized as a carrier of the crayfish plague.[3] All American species carry the infection, but it is only lethal to individuals that are already stressed; to European species, the infection is rapidly fatal.[9] The signal crayfish is now the most widespread alien crayfish in Europe, occurring in 25 countries, from Finland to Great Britain and from Spain to Greece.[3][10] It was first introduced to Great Britain in 1976,[11][12] and is now widespread across the British mainland as far north as the Moray Firth. It has also been observed on the Isle of Man, but not in Ireland,[9] the last European country to have no alien crayfish.
In both Sweden and Finland (where crayfish are eaten), the catch of signal crayfish exceeds that of A. astacus (European/noble crayfish). The former is sold at roughly half the price compared to the latter.[13]
In Europe, the signal crayfish is included since 2016 in the list of Invasive Alien Species of Union concern (the Union list).[14] This implies that this species cannot be imported, bred, transported, commercialized, or intentionally released into the environment in the whole of the European Union.[15] The signal crayfish is often considered a nuisance species amongst anglers in Europe.[16]
Multiple studies have been published to find out how the damage caused by the settlement — and subsequent overpopulation — of invasive signal crayfish in Europe can be mitigated, including studies regarding effective upstream barriers against signal crayfish that don’t negatively impact the migration of fish,[17][18] as well as other, aggressive but more efficient approaches which may harm an existing ecosystem further, such as eradication (by means of drainage or destruction of waterways, and biocides) and suppression (by means of extensive trapping, electrocution of waterways, and introduction of predatory fish), with eradication being most successful.[18]
{{cite journal}}: Cite journal requires |journal= (help) {{cite web}}: CS1 maint: url-status (link) The signal crayfish (Pacifastacus leniusculus) is a North American species of crayfish. It was introduced to Europe in the 1960s to supplement the North European Astacus astacus fisheries, which were being damaged by crayfish plague, but the imports turned out to be a carrier of that disease. The signal crayfish is now considered an invasive species across Europe, Japan, and California, ousting native species there.
