The cephalothorax is smooth with two pairs of post-orbital ridges, the anterior pair with an apical spine; and no spines on shoulders of the carapace behind cervical groove; the areola between branchiocardiac grooves is obvious. The rostrum sides are smooth and more or less parallel until the apex; the acumen is very pointed with prominent shoulders; and a simple median carina down whole length. 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 from tip of rostrum to end of telson, 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 (David Holdich., pers. comm., 2005).

Pacifastacus leniusculus has a typical life cycle of a member of the crayfish family Astacidae, and which is therefore very similar to that of indigenous European crayfish. The eggs hatch into miniature crayfish that stay with the mother for three stages, the third stage gradually becoming more and more independent of the mother. Juveniles undergo as many as 11 moults during their first year, but by age 3 this is reduced to two moults per year, and by age 4 onwards to one moult per year (Lewis, 2002).

Commercially harvested in the western USA, mainly in Washington and Oregon States, although a larger harvest is obtained from the introduced population in the Sacramento River (Lewis, 2002). It was originally hoped that stocking P. leniusculus into European waters would revive catches of crayfish to their pre-plague levels, particularly in Sweden and Finland (Skurdal et al. 1999), this has not proved to be the case. In Sweden the catch in 1996 was 265 tonnes (compared to 52 for A. astacus) and that cultured amounted to 42 tonnes (compared to 12 for A. astacus). The catch of P. leniusculus in Finland in 2001 was 22 tonnes (compared to 57.5 for A. astacus). However, the Finnish catch of P. leniusculus is increasing and is estimated to double every 1-2 years. In 2004 it exceeded 50% of the catch (Erkamo et al. 2004). P. leniusculus fetches approximately half the price as A. astacus in Finland and Sweden. The introduced species has done better in southern Sweden than in the north and in Finland, and this may be a consequence of the cool climatic conditions in the latter two regions (Henttonen & Huner, 1999). In Europe as a whole in 1994 a total of 355 tonnes of P. leniusculus originated from capture fisheries and 51 tonnes from culture. This represents only 9% of European capture fisheries and 32.5% of culture fisheries (Ackefors, 1998, 1999).

Pacifastacus leniusculus occupies a wide range of habitats from small streams to large rivers (e.g. Columbia River) and natural lakes, including sub-alpine lakes, such as Lakes Tahoe and Donner (Lowery & Holdich, 1988; Lewis, 2002). However, it also grows well in culture ponds. It is tolerant of brackish water and high temperatures. It does not occur in waters with a pH lower than 6.0. P. leniusculus is very active and migrates up and down rivers, as well as moving overland around obstacles. However, their rate of colonisation is relatively slow and may only be about 1 km yr-1. In one stream in England it took 17 years for them to spread 12 km downstream (Stanton, 2004). Their burrows can reach high densities, i.e.14 m-1, and they can have a serious impact on bank morphology, causing them to collapse. It was considered to be a non-burrowing species, but in Europe in constructs burrows under rocks or in river and lake banks (Guan, 1994; Sibley, 2000).

The breeding cycle is typical of a cool temperate zone species, although P. leniusculus grows faster and reaches a greater size than its counterparts. Size at maturity is usually 6-9cm TL at an age of 2-3 years, although maturity can occur as early as 1 year. Mating and egg laying occurs during October in the vast majority of populations. Egg incubation time ranges from 166 to 280 days. In natural populations hatching occurs from late March to the end of July depending on latitude and temperature. Egg numbers usually range from 200 to 400, although some individuals of 66mm CL have been reported as having over 500 eggs. Based on the use of the lipofuschin technique it has been estimated that some individuals can live 16 years, and other estimates state that it may be as long as 20 years. Some individuals may grow to a large size, i.e. 95mm CL, but this may not represent a great age, but that of a fast-growing newly introduced population that encounters little competition. Estimates of survivorship to age 2 vary from 10-52%, being dependent on both abiotic and biotic factors. Competition and cannibalism can greatly affect survival in dense populations. Stebbing et al. (2003) demonstrated for the first time the presence of a sex pheromone, released during the breeding season by mature females, that stimulates courtship and mating behaviour in male P. leniusculus.

As an opportunistic polytrophic feeder, P. leniusculus will eat anything that is available, including other crayfish. The diet was found to shift from aquatic insects in juveniles, to more plant material in adults in some American populations (Lewis, 2002). However, Guan & Wiles (1997) found that cannibalism increased with size and that more animal than plant material was consumed by adults in a British river.

P. leniusculus was first introduced into Japan from North America for use as food in 1928 (Kawai et al. 2002b).

Pacifastacus leniusculus displays opportunistic polytrophic feeding habits, although more animal than plant material may be consumed if available. It can have a considerable impact on populations of macro-invertebrates, benthic fish, and aquatic plants (Guan & Wiles 1997; Nyström, 1999; Lewis, 2002), it also has been used to clear weed from ponds on fish farms. Griffiths et al. (2004) found that the presence of P. leniusculus significantly reduced the number of Atlantic salmon using shelters in artificial test arenas. Sooty crayfish (see Pacifastacus nigrescens in IUCN Red List of Threatened Species), a native to the western USA, has become extinct partly due to interspecific competition with P. leniusculus, which was introduced into its range. P. leniusculus has also been implicated in causing a reduction in the range of the already narrowly endemic shasta crayfish ( see Pacifastacus fortis in IUCN Red List of Threatened Species) in the western America (Taylor, 2002).

P. leniusculus was introduced into Japan from Portland, Oregon five times during 1926 to 1930, where it has reduced the range of the indigenous Cambaroides japonicus on the island of Hokkaido (Hiruta, 1996; Kawai & Hiruta, 1999). It has also been found in some lakes on Honshu (Hiruta, S., 2005, pers. Comm.). In Europe, it has extirpated populations of the indigenous crayfish species, particularly the white-clawed crayfish (see Austropotamobius pallipes in IUCN Red List of Threatened Species in England (Holdich, 1999; Hiley, 2003). However, in Finland it coexisted with the noble crayfish, (see Astacus astacus in IUCN Red List of Threatened Species), in a lake for 30 years, before reproductive interference led to the demise of the latter species (Westman et al. 2002). Its main impact has been as a vector of the crayfish plague fungus, Aphanomyces astaci, which has caused large-scale mortalities amongst indigenous European crayfish populations, particularly in England (Alderman, 1996). The disease has recently been confirmed in P. leniusculus from western Hungary, which could have serious implications for indigenous crayfish in the Danube catchment (Kiszely, 2004).

There are no documented control agents for the successful management of P. leniusculus available at this time (Holdich et al. 1999). Trapping is size selective and the smaller individuals remaining take advantage of the lack of competition to grow rapidly (Sibley, 2000). Preventing the further introduction of this species into new bodies of water is one of the few options available. Educating the public to the environmental risks this species pose and identifying new populations are key elements to stopping the spread of this species where it is not wanted. Stebbing et al. (2003, 2004) have researched into the possibilities of using pheromones to attract male P.leniusculus into traps. Stringent legislation has been applied to P. leniusculus in Britain, which effectively makes it a ‘pest’ and bans the keeping of it in Scotland and Wales and much of England (Holdich et al. 2004). Despite this P. leniusculus continues to spread and may well cause the extinction of the single indigenous crayfish species within 30 years (Hiley, 2003; Sibley, 2003). Work is in progress in the UK to assess the use of natural pyrethrum again nuisance populations of P. leniusculus in enclosed waterbodies (Peay, 2005).