Typically dark red, Procambarus clarkii is capable of reaching sizes in excess of 50g in 3-5 months (NatureServe, 2003; Henttonen and Huner, 1999). Adults reach about 5.5 to 12cms (2.2 to 4.7 inches) in length. Its rostrum is cuminate with cervical spines present, and its areola is linear to obliterate. The palm and the mesial margin of the cheliped bare rows of tubercles. Their chela are elongate. There are hooks on the ischia of male at the 3rd and 4th pereiopods. A male's first pleopod terminates in four elements, and the cephalic process is strongly lobate with a sharp angle on the caudodistal margin that is lacking subapical setae. The setae have strong angular shoulders on their cephalic margin that are proximal to the terminal elements. The right pleopod is wrapped around the margin to appear reduced or absent (Washington Department of Fish and Wildlife, 2003). Additionally, it possesses a strong spur at the inner side of the carpopodite. The propodite is armed with strong spines on its inner side as well as conspicuous knots on its dorsal face. The branchiocardiac grooves of the carapace converge dorsally. Lateral spines or tubercles in front of and behind the cervical groove are absent or reduced. The rostrum is devoid of a median keel and has an obvious triangular shape, the sides tapering anteriorly. The head itself is elongated and narrowing towards the front. Juvenile are not red and appear very similar to other Procambarus species (Boets et al, 2009).

Procambarus clarkii exhibits a cyclic dimorphism of sexually active and inactive periods alternating during the lifecycle. After the young hatch, metamorphosis takes place, followed by two to three weeks of voracious eating. After this they molt and again assume their immature appearance (Hunter and Barr, 1994, in Ackefors, 1999). Egg production can be completed within six weeks, incubation and maternal attachment within three weeks and maturation within eight weeks. Optimal temperatures are 21-27 degrees and growth inhibition occurs at temperatures below 12 °C (Ackefors, 1999). P. clarkii shows two patterns of activity, a wandering phase, without any daily periodicity, characterized by short peaks of high speed of locomotion, and a longer stationary phase, during which crayfish hide in the burrows by day, emerging only at dusk to forage. Other behaviors, such as fighting or mating, take place at nighttime. During the wandering phase, breeding males move up to 17 km in four days and cover a wide area. This intensive activity helps dispersion in this species (Gherardi & Barbaresi, 2000).

Procambarus clarkii is an extremely widespread and common food source. Its ability to grow and mature rapidly and to adapt to seasonal waters enabled widespread commercial establishment of it and made it the dominant freshwater crayfish in the world (Henttonen and Huner, 1999). It accounts for 85–90% of the world’s annual crayfish consumption (Huner, 1997 in Kerby et al, 2005). In Louisiana, USA P. clarkii has created a multi-million dollar industry, with more than 50,000 ha under cultivation (Guierrez-Yurrita et al, 1999). In Europe, the introductions especially benefited Spain, creating a flourishing crayfish industry and revitalizing the local economy in certain districts (Ackefors, 1999). The commercial success of P. clarkii in Europe is partly due to its ability to colonize disturbed habitats and resist the crayfish fungus plague, Aphanomyces astaci to which native European crayfish are susceptible (Lindqvist and Huner, 1999). Crayfishing for P. clarkii has become a significant source of income for many people throughout its range (Alcorlo et al, 2008).
In Kenya, P. clarkii has been introduced as a biological control for human schistosomes (Schistosoma haematobium) and (S. mansoni) because it preys on the parasite’s intermediary snail vector Bulinus and Biomphalaria spp. (Lodge et al, 2005; Mkoji et al, 1992 in Foster & Harper, 2007). Under certain conditions P. clarkii has significantly reduced the spread of schistosomiasis in some locations of Kenya (Mkoji et al, 1999a in Foster & Harper, 2007).

Procambarus clarkii may inhabit a wide variety of freshwater habitats including rivers, lakes, ponds, streams, canals, and seasonally flooded swamps and marshes. It is very tolerant and adaptable to a wide range of aquatic conditions including moderate salinity, low oxygen levels, extreme temperatures, and pollution (Cruz & Rebelo, 2007; Gherardi & Panov, 2006; NatureServe, 2003). P. clarkia thrives in warm, shallow wetland ecosystems of natural and agricultural lands as in the case of south and central Europe where it has established (Henttonen & Huner, 1999). In the cooler regions of Europe, it prefers small, permanent ponds because it is unable to survive predation by fishes in large water bodies (Troschel and Dehus, 1993; Roqueplo et al, 1995; Demastro and Laurent, 1997, Huner, pers. obs., in Henttonen and Huner, 1999). P. clarkii also frequently inhabits disturbed environments such as rice fields and irrigation channels and reservoirs (Oliveira & Fabião, 1998). Populations have been negatively correlated with high elevation and flow velocity (Cruz & Rebelo, 2007).

Procambarus clarkii employs an r-strategy, exhibiting a short life cycle and high fecundity. It matures when it reaches a size of between 6 and 12.5 cm. A 10 cm female may produce up to 500 eggs, while smaller females may produce around a 100 eggs. The eggs are 0.4 mm, notably smaller than those produced by European native members of the family Astacidae. Newly hatched crayfish remain with their mother in the burrow for up to eight weeks and undergo two moults before they can fend for themselves (Ackefors, 1999). Unlike the European native Astacus and Austropotamobius species, populations of P. clarkii contain individuals that are incubating eggs or carrying young throughout the year (Huner and Barr, 1994, in Lindqvist and Huner, 1999). This allows P. clarkii to reproduce at the first available opportunity, which contributes to its colonization success (Huner, 1992, 1995, in Gutierrez-Yurrita and Montes, 1999). In places with a long flooding period, greater than 6 months, there may be at least two reproductive periods in autumn and spring. The spring period is longer and more prolific and persists until the drying of the marsh. For large females to reproduce it is necessary to have hormonal induction induced by the photoperiod, a hydroperiod longer than four months, a temperature above 18 °C, and a pH between 7 and 8 (Gutierrez-Yurrita, 1997). If females have only a short period to prepare themselves for reproduction they must prematurely their burrow to feed; in such circumstances many females will die of dehydration, bringing about a depression in the population (Huner, 1995; Gutierrez- Yurrita, 1997, in Gutierrez-Yurrita and Montes, 1999).

Procambarus clarkii is considered an opportunistic omnivore with a primarily plant based diet (Rodreguez et al, 2005). The results of one study showed that P. clarkii is selective when offered fresh plants, consuming a relatively larger biomass of green algae (Urtica sp.) in spring, and Polygonum sp. in summer and autumn. P. clarkii did not exhibit preference for any animal and preferred Urtica sp. over earthworms (Gherardi & Barbaresi, 2007).

In Kenya attempts have been made to use P. clarkii as a biological control agent to reduce the numbers of snails that act as intermediate hosts for the disease-causing organism that causes schistosomiasis (Bilharzia) (Hofkin et al.,Procambarus clarkii can spread to new areas by anglers using them as bait (Aquatic Non-native Species Update, 2000). Popular as a bait species for largemouth bass, this is believed to have been the most likely cause for their introduction into Washington (The Washington Department of Fish and Wildlife, 2003).The crayfish that now occur in African freshwaters are thought to have been introduced without the knowledge and permission of the relevant authorities (Mikkola, 1996, in Holdich, 1999).Procambarus clarkii is a popular dining delicacy, accounting for the vast majority of crayfish commercially produced in the United States (Washington Department of Fish and Wildlife, 2003). It was the most dominant freshwater crayfish in the world during the 20th century and its commercial success led to intentional introductions throughout Spain, France and Italy during the 1970s and 1980s (Henttonen and Huner, 1999).The habit of selling Procambarus clarkii alive as an aquarium or garden pond pet may have accelerated the spread of the species through natural waterways in Europe (Henttonen and Huner, 1999).Commerce in live crayfish from neighbouring Spain and more distant countries including the Far East, the USA and Kenya have been responsible for some of the introductions of P. clarkii into England, the Netherlands, France, Germany and Switzerland (Henttonen and Huner, 1999).

Procambarus clarkii is a successful colonizer which may quickly become established and eventually become a keystone species, a primary contributor to the ecosystem it inhabits. Its introduction may cause dramatic changes in native plant and animal communities (Schleifstein, 2003). P. clarkii may severely impact native crayfish through competition and transition of the crayfish plague, reduce macrophyte assemblages and diversity, alter water quality and sediment characteristics, accumulate heavy metals, interact with additional invasive species, damage agricultural irrigation systems, impact fishing industry, and reduce populations of invertebrates, mollusks, and amphibians through predation and competition.
P. clarkii has contributed to the dramatic decline of the European native crayfish in the Astacidae family through its transmission of the crayfish plague (Aphanomyces astaci) and direct competition. Specifically threatened species include the endangered white clawed crayfish Austropotamobius pallipes), the “vulnerable” noble crayfish (Astacus astacus), and the stone crayfish Austropotamobius torrentium (Garcia-Arberas et al, 2009; Dehus et al, 1999; Gherardi, 2006, Gil-Sanchez & Alba-Tercedor, 2006). P. clarkii is also known to compete with native crayfish in Japan (Kawai & Kobayashi, 2005).
Intense herbivory by P. clarkii often causes the reduction of macrophyte mass and biodiversity and has been recorded in the Lake Chozas, Spain (Rodriguez et al, 2003); Lake Naivasha, Kenya (Smart et al, 2002); Lake Massaciuccoli, Italy (Gherardi et al, 1999); Lake Doccia, Italy (Gherardi & Acquistapace, 2007); Mediterranean wetlands (Geiger et al, 2005); and the Iberian peninsula (Rodriguez et al, 2003 in Cruz & Rebelo, 2007). Affected species include Nymphoides peltata, Potamopeton crispus, Ultricularia australis, Potamogeton spp. (Gherardi & Acquistapace, 2007; Gherardi et al, 1999).
Another effect of the feeding, as well as burrowing, behavior of P. clarkii is altered water quality, increased bioturbation, and increased nutrient release from sediment (Angeler et al, 2001). These changes in water characteristics alter aquatic ecosystems and are believed to induce cyanobacterial blooms (Yamamoto, 2010). These effects have been recorded in Las Tablas de Daimiel National Park, Spain (Angeler et al, 2001); Alentejo, Portugal (Geiger et al, 2005); and Japan (Yamamoto, 2010).
P. clarkii is known to compete with, prey on, and reduce populations of a wide variety of aquatic species including amphibians, mollusks, macroinvertebrates, and fish. Competitive pressure and predation on native amphibians have been recorded from the Iberian Peninsula (Cruz & Rebelo, 2005), Sweden (Nystrom et al, 2002 in Ilheu et al, 2007), Europe (Gherardi, 2006). More specific reports include effects on Rana sp., Bufo bufo, and Triturus vulgaris in Italy (Gherardi et al, 2001; Renai & Gherardi, 2004 in Ilheu, 2007) and the Natterjack Toad (Bufo calamita in Donana Natural Park, Spain (Cruz et al, 2006b), and the California newt, Taricha torosa, in California (Gamradt & Kats, 1996 in Nystrom, 1999). Predation and competition pressure on mollusks include native snails in Doccia Lake, Italy (Gherardi & Acquistapace, 2007) and in the Iberian Peninsula (Cruz & Rebelo, 2007). P. clarkii preys on fish eggs and young as well and was found to consume lake trout (Salvelinus namaycush), gila chub (Gila intermedia), suckers (Catostomusspp.), and speckled dace (Rhinichthys osculus) in the laboratory (Mueller et al, 2006). It may also reduce macroinvertebrate populations and diversity (Correia et al, 2008).

Effects on agriculture and fisheries have been recorded from many locations. The burrowing and behavior of P. clarkii is often problematic to levees, dykes, and irrigation systems which can result in water loss and damage to fields (Holdrich, 1999; Yue et al, 2010a). This has been reported from China (Yue et al, 2010a), Japan (Sako, 1987 in Kawai & Kobayashi, 2005), Egypt (Hartnoll pers. Comm., in Holdich, 1999), Kenya (Picard, 1991 in Arrignon et al, 1999), Italy (Gherardi et al, 2000), Spain (Holdrich, 1999), and the United States (Chang & Lange 1967 in Holdich, 1999). P. clarkii frequently becomes a dominant species in disturbed habitats such as rice fields. If present in irrigation structures including reservoirs, channels of rice fields, P. clarkii may cause significant economic loss due to its burrowing activity, which alters soil hydrology and causes water leakage, and its feeding, which damages to rice plants (Correia, 1993; Ilhéu and Bernardo 1993a, b, in Oliveira and Fabião, 1998). Additionally, this damage may lead farmers to use aggressive pesticides such as organophosphorous to control P. clarkii (Ganhão, Germano and Grilo, 1991, in Oliveira and Fabião, 1998; MacKenzie, 1986, in Holdich, 1999).

P. clarkii interferes with commercial fishing by damaging nets, preying on fish eggs, competing for food with tilapia, reducing the number of submerged macrophytes, and disturbing nesting grounds of Tilapia zilli (Holdich, 1999; Gieger et al, 2005).

Additional impacts associated with P. clarkii include its accumulation of toxins and heavy metals, acting as an intermediary host for trematodes, and serving as a primary food source for other introduced species. It is known to accumulate heavy metals and toxins produced by cyanobacteria such as Microcystis aeruginosa and may transfer them up the food chain and to humans (Gherardi & Panov, 2006). P. clarkii also serves as an intermediary host to trematodes of the genus Paragonimus which are potential human pathogens if the crayfish are undercooked and consumed (Gherardi & Panov, 2006). P. clarkii has been found to promote other invasive species populations including largemouth black bass (Micropterus salmoides and pike by serving as a primary food source (Hickley et al, 1996 in Holdich, 1999; Elvira et al, 1996 in Holdich, 1999).

Finally, direct impacts of P. clarkii may cause additional indirect impacts and cascading ecological changes. Dramatic reduction of aquatic vegetation results in many indirect effects as it serves habitat for invertebrates, amphibians, and fry; as a substrate for epiphytic algae; source of refuge for prey, and a primary food source for birds and other species (Nystrom, 1999; France, 1996 in Nystrom, 1999; Steinman, 1996 in Nystrom, 1999). For example, the introduction on P. clarkii to Lake Chozas, Spain caused a reduction of macrophyte plant coverage by 99% which in turn caused a 71% loss in macroinvertebrate genera, 83% reduction in amphibian species, a 75 loss in duck species, and a 52% reduction in waterfowl (Rodriguez et al, 2005).

Possible management options for Procambarus clarkii include the elimination or reduction of populations via mechanical, physical, chemical or biological methods; the restocking of native crayfish populations threatened by the crayfish plague fungus and interspecific competition with alien species; the development of plague-resistant strains of native crayfish; and the use of legislation to prohibit the transport and release of alien crayfish.

Preventative measures: Legislation designed to prevent the spread of crayfish has proven difficult to enforce due to the presence of conflicting social motivations such as the desire to propagate the species for recreational or commercial purposes. Political barriers, particularly in Europe, may also hinder conservation goals. For example the free trade policy backed by the European Union has hindered the attempts of European countries to prohibit the importation of live crayfish from other countries within the EU (Holdich et al, 1999).

Physical: Reduction of P. clarkii populations may be possible through physical control methods. However, eradication is unlikely unless the population is particularly restricted in range and size. All physical methods have environmental costs, which should be weighed up against the environmental benefits of employing them. Mechanical methods to control crayfish include the use of traps, fyke and seine nets and electro-fishing. Continued trapping is preferable to short-term intensive trapping, which may provoke feedback responses in the population such as stimulating a younger maturation age and a greater egg production. Bait, such as roach, bream, bleak or white bream, may increase the number of crayfish caught in traps, although freshwater fish should be avoided to prevent spread of the crayfish plague fungus, which may be transmitted on their scales (Gherardi & Panov, 2006; Westman, 1991; Alderman et al, undated in Holdich, Gydemo and Rogers, 1999; Kerby et al, 2005). A fair population reduction of P. clarkii by removal was achieved in Lake Naivasha, Kenya using traps and removal from floating vegetation in attempts to promote recovery of native macrophytes (Smart et al, 2002). Further control methods include the drainage of ponds, the diversion of rivers, or the construction of physical or electrical barriers to limit its spread (Kerby et al, 2005).

Chemical: Chemicals that can be used to control crayfish include biocides such as organophosphate, organochlorine, and pyrethroid insecticides; individual crayfish are differentially affected depending on their size, with smaller individuals being more susceptible (Gherardi & Panov, 2006). Furadan 5G, active ingredient carbofuran, has also been found fatal to P. clarkii in Kenya (Rosenthal et al, 2005). Since no biocides are crayfish-specific other invertebrates, such as arthropods, may be eliminated along with crayfish, and may subsequently have to be re-introduced. There is cause for concern about toxin bioaccumulation and biomagnification in the food chain, although this is less of a problem with pyrethroids. Another chemical solution lies in the potential to use crayfish-specific, or even species-specific, pheromones to trap P. clarkii (Gherardi & Panov, 2006).

Biological control: Possible biological control methods include the use of fish predators, disease-causing organisms, and use of microbes that produce toxins, for example, the bacterium Bacillus thuringiensis var. israeliensis (Pedigo, 1989, in Holdich et al, 1999). Only the use of predaceous fish has been used successfully; eels, burbot, perch and pike are predators are all partial to crayfish (Westman, 1991, in Holdich et al, 1999). Pike are being reintroduced into Massaciuccoli Lake, Italy to help control P.clarkii (Schleifstein & Fedeli, 2003). The amount of cover, type of fish predator used and location are all important variables in determining the success of such an approach, and in general reduced coverage is correlated with increased predation rates.

Integrated management: The application of 20 Gy x-rays ionizing radiation to males has been found to reduce the size of testes and alter spermatogenesis. Reproductive success decreased and hatchlings were reduced by 43% in a test study (Alquiloni et al, 2000).