Invasive Species Compendium

Detailed coverage of invasive species threatening livelihoods and the environment worldwide


Orconectes virilis
(virile crayfish)



Orconectes virilis (virile crayfish)


  • Last modified
  • 20 November 2018
  • Datasheet Type(s)
  • Invasive Species
  • Host Animal
  • Preferred Scientific Name
  • Orconectes virilis
  • Preferred Common Name
  • virile crayfish
  • Taxonomic Tree
  • Domain: Eukaryota
  •   Kingdom: Metazoa
  •     Phylum: Arthropoda
  •       Subphylum: Crustacea
  •         Class: Malacostraca
  • Summary of Invasiveness
  • O. virilis is thought to be one of most widely invasive crayfish species in the USA (Larson and Olden, 2011), and...

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Orconectes virilis (virile crayfish); large adult, caught in River Lee, UK.
CaptionOrconectes virilis (virile crayfish); large adult, caught in River Lee, UK.
Copyright©Environment Agency
Orconectes virilis (virile crayfish); large adult, caught in River Lee, UK.
MaleOrconectes virilis (virile crayfish); large adult, caught in River Lee, UK.©Environment Agency
Orconectes virilis (virile crayfish); adult female, with radio-tag attached for monitoring.
CaptionOrconectes virilis (virile crayfish); adult female, with radio-tag attached for monitoring.
Copyright©Environment Agency
Orconectes virilis (virile crayfish); adult female, with radio-tag attached for monitoring.
FemaleOrconectes virilis (virile crayfish); adult female, with radio-tag attached for monitoring.©Environment Agency
Orconectes virilis (virile crayfish); juvenile.
CaptionOrconectes virilis (virile crayfish); juvenile.
Copyright©Environment Agency
Orconectes virilis (virile crayfish); juvenile.
JuvenileOrconectes virilis (virile crayfish); juvenile.©Environment Agency


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Preferred Scientific Name

  • Orconectes virilis Hagen, 1870

Preferred Common Name

  • virile crayfish

Local Common Names

  • Netherlands: geknobbelde Amerikaanse rivierkreeft
  • USA: fantail crayfish; northern crayfish

Summary of Invasiveness

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O. virilis is thought to be one of most widely invasive crayfish species in the USA (Larson and Olden, 2011), and has been translocated to a large number of states outside of its natural range. Populations have also been found in Mexico and Europe (UK and the Netherlands). Many translocations within the USA and Canada have been attributed to anglers using the species as bait. The two populations in Europe are thought to be linked to aquarium escapees (Ahern et al., 2008). O. virilis is highly mobile, fecund and tolerant of a wide range of environmental variables making the species very successful invaders. They are however locally threatened within parts of their native range (Hamr, 2002).

Taxonomic Tree

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  • Domain: Eukaryota
  •     Kingdom: Metazoa
  •         Phylum: Arthropoda
  •             Subphylum: Crustacea
  •                 Class: Malacostraca
  •                     Subclass: Eumalacostraca
  •                         Order: Decapoda
  •                             Suborder: Reptantia
  •                                 Unknown: Astacoidea
  •                                     Family: Cambaridae
  •                                         Genus: Orconectes
  •                                             Species: Orconectes virilis

Notes on Taxonomy and Nomenclature

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Part of the North American genus Orconectes, classified as Orconectes virilis by Hagen, 1870. Previous names include: Cambarus virilis, Cambarus debllis, Cambarus wisconsinensis, Cambarus couesi, Faxonius virilis (Hobbs, 1974). The species recieved their name from the long white hair like structures that can be found on the copulatory stylets of sexually active males. O. virilis are sometimes known as ‘Northern Crayfish’, as they are naturally found at higher latitudes than any other crayfish species.


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Diagnostic Characteristics taken from ‘Identifying native and alien crayfish species in Europe’, a key produced as part of the European CRAYNET project (Pöckl et al., 2006):

Rostrum: Smooth, borders more or less parallel until shoulder region; acumen prominent, shoulders with prominent spine; median carina absent.

Body: Carapace smooth, one pair of post-orbital ridges; areola very narrow; row of tubercles on shoulders behind cervical groove, one with prominent spine; hepatic spines absent; areola narrow. Females with annulus ventralis located between bases of posterior walking legs, cornified in sexually mature individuals. Hooks on each side of the second abdominal segment absent. Colour typically chestnut or chocolate, posterior end of carapace with a bowl shaped or wine glass shaped light brown pattern, which is not usually as clear as in Orconectes immunis. Abdomen without longitudinal or transverse bands.

Appendages: Chelae broad, flattened, tuberculate: moveable finger with straight margin; prominent yellow tubercles typically arranged in two rows along the inferior margin of the propodus and dactylus; tips of the fingers yellow in colour, cutting edges with yellow teeth; upper surface same colour as body, under-side dirty white in colour. Prominent spur on inferior margin of cheliped carpus. Form I males have larger chelipeds than form II and have a distinct grasping hook on the ischium of the second pair of walking legs that are used during mating; form I gonopods become hardened.

Size: Within their introduced range in England, mean total length of O. virilis in established populations is 88mm (44mm carapace length), although in the build up phase of the population individuals were found up to 135mm total length (67mm carapace length (CL)). Within the northern part of their native range maximum carapace length (CL) is around 55mm (Weagle and Ozburn, 1972), however they have been reported up to 69mm CL in more southerly regions (Hazlett and Rittschof, 1985; Page, 1985).


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In some areas within the USA where historic data is not present there appears to be differing opinions of the native/introduced status of virile crayfish. The native range of the species covers a large part of North America, east of the continental divide (Larson and Olden, 2011). In many areas O. virilis co-exists with other species, however it has shown to be outcompeted by invasive Orconectes rusticus (Hayes et al., 2009).

In Europe, O. virilis is only known to be present in the Netherlands and the UK. It is believed to have been introduced to the Netherlands via the aquarium trade and has only recently become established and widespread (Soes, 2007; Ahern et al., 2008). In the UK, a population of the species was detected in the River Lee system of North London in 2004. The extent of its impacts and establishment remain unknown (Ahern et al., 2008).  

Distribution Table

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The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

Continent/Country/RegionDistributionLast ReportedOriginFirst ReportedInvasiveReferenceNotes



North America

CanadaPresentPresent based on regional distribution.
-AlbertaPresentNative Not invasive Aiken, 1969; Williams et al., 2011
-ManitobaPresentNative Not invasive Sebastian and Reinhart, 1989; Huner, 1990
-New BrunswickPresentIntroduced2005 Invasive McAlpine et al., 2006
-OntarioPresentNative Not invasive Berrill, 1978
-QuebecPresentHuner, 1990
-SaskatchewanPresentHuner, 1990; Williams et al., 2011
MexicoPresentIntroduced1982Campos-Gonzales, 1985A small number of crayfish found in Chihuahua State in 1982
USAPresentPresent based on regional distribution.
-AlabamaPresentIntroduced Invasive Schuster et al., 2008
-ArizonaPresentIntroduced Invasive Huner, 1990; Davidson et al., 2010Introduced in early 1970s, no native crayfish species present. Causing significant ecological damage
-ArkansasPresentNativeTaylor et al., 2007
-CaliforniaPresentIntroduced Invasive Riegel, 1959
-ColoradoPresentIntroduced Invasive Olden et al., 2011; Martinez, 2012
-ConnecticutWidespreadIntroducedBeauchene, 2011
-IdahoPresentIntroduced Invasive Larson and Olden, 2011
-IllinoisPresentNative Not invasive Brown et al., 1995
-IndianaPresentNative Not invasive Siewert and Buck, 1991
-IowaPresentNative Not invasive Caldwell and Bovbjerg, 1969
-KansasPresentIntroducedTaylor et al., 2007
-KentuckyPresentNative Not invasive Bouchard, 1975
-MainePresentNative Not invasive Martin, 1997It is unclear whether this species in native or introduced within Maine
-MarylandPresentIntroduced1963 Invasive Odell and Grimm, 1966
-MassachusettsPresentIntroducedHobbs et al., 1989
-MichiganPresentNative Not invasive Dye and Jones, 1974
-MinnesotaPresentNative Not invasive Huner, 1990
-MissouriPresentNative Not invasive Bovbjerg, 1953
-MontanaPresentIntroduced Invasive Larson and Olden, 2011
-NebraskaPresentNativeHuner, 1990
-New HampshirePresentIntroduced Not invasive Aiken, 1965
-New MexicoPresentIntroducedHuner, 1990
-New YorkPresentNativeHobbs et al., 1989Thought to be native
-North DakotaPresentNativeHuner, 1990; Williams et al., 2011Thought to be native
-OhioPresentNativeHuner, 1990; Loughman and Simon, 2011
-OklahomaPresentNative Not invasive Peck, 1985; Varza and Covich, 1995
-PennsylvaniaPresentIntroducedTaylor et al., 2007
-Rhode IslandPresentIntroducedTaylor et al., 2007
-South DakotaPresentNativeHuner, 1990
-TennesseePresentIntroducedHuner, 1990
-TexasPresentNativeTaylor et al., 2007
-UtahPresent Invasive Johnson, 1986; Larson and Olden, 2011Unclear from the literature whether the species is considered native or not
-VermontPresentIntroducedHobbs et al., 1989
-VirginiaPresentIntroducedTaylor et al., 2007
-WashingtonPresentIntroduced2006 Invasive Larson et al., 2010Introduced to Columbia River Basin
-West VirginiaPresentIntroducedTaylor et al., 2007
-WisconsinPresentNative Not invasive Threinen, 1958; Olden et al., 2006Under threat from introduced O.rusticus
-WyomingPresentIntroduced Invasive Hubert, 1988; Larson and Olden, 2011Replaced native P.gambelii


NetherlandsWidespreadIntroduced2004 Invasive Soes, 2007
UKLocalisedIntroduced2004 Invasive Ahern et al., 2008Currently confined to the River Lee in South-East England

History of Introduction and Spread

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The range extension of O. virilis across the USA and Canada appears to be largely linked to anglers using the species as fishing bait, and consequently moving crayfish between catchments and even states. Although it occurs naturally in many regions in the USA and Canada it has been introduced to some regions such as New Brunswick, Canada. It has also been introduced to Chihuahua in Mexico.

Within Europe it is thought introductions are linked to accidental or deliberately released aquarium specimens. It was deliberately introduced into France in 1897 and Sweden in 1960 but both attempts were unsuccessful. Currently in Europe, O. virilis is only known to be present in the Netherlands and the UK. It was introduced to both countries in 2004, and is likely to be as a result of the aquarium trade. In the Netherlands the species has only recently become widespread (Ahern et al., 2008).

Risk of Introduction

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The use of crayfish as live bait by anglers is attributed as a major factor in the spread of non-native crayfish species within the USA (Lodge et al., 2000; Distefano et al., 2009). The use of live baits is regulated within some states, with certain species prohibited. Risk of introductions can be limited by the use of live baits caught within the same water body, although this is open to abuse. Within Europe, England has banned the use of any species of crayfish as live bait allowing for more realistic regulation.

The two introduced O. virilis populations within Europe are thought to be linked to released aquarium specimens. The species is not commonly kept in aquariums, and in England the keeping of any crayfish species other than Cherax quadricarinatus is prohibited. As it is impossible to know what is kept in private aquariums, further releases of O. virilis within Europe may be inevitable.

In areas where crayfish are harvested either recreationally or commercially there is concern that the movement of live crayfish will result in either deliberate seeding of waterways or accidental introductions to new watercourses. Therefore, the moving of live crayfish should be kept to a minimum to reduce the risk of future introductions.


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O. virilis is a generalist when it comes to habitat, and can be found in a range of flowing and still waters within its natural and introduced ranges. It can be found in streams, rivers, canals, ponds and lakes. In Iowa, it is suggested that O. virilis are more prominent in rock-bottomed rivers and streams than in ponds and lakes, where other species dominant (Caldwell and Bovbjerg, 1969). In England, the species appears to be doing equally well in small gravel substrate rivers, heavily modified canals and water filled gravel pits. Where present, the species will use excavations under rocks and cobbles for cover, and if suitable substrate is present some populations have been known to construct burrows.

Experimental studies have shown that acidification can affect the moulting process in O. virilis. Low pH values effected crayfish in post-moult stages and a slower progression of the moult process was observed at pH6 and below (Malley, 1980). Crayfish have been shown to survive for up to 10 days in pHs as low as 4, when not moulting. France (1984) suggested that an average annual pH below pH5.5 could result in eventual population extinction in lentic systems. Newly hatched crayfish and juveniles are more susceptible to low pH than adults, possibly because of the higher number of moults in juveniles (France, 1984). In a 10 week study 25% of O. virilis died at pH5, 70% at pH4 and 95% at pH3 (Siewert and Buck, 1991).

Habitat List

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Irrigation channels Principal habitat Natural
Lakes Principal habitat Natural
Reservoirs Principal habitat Natural
Rivers / streams Principal habitat Natural
Ponds Principal habitat Natural

Biology and Ecology

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Work has been carried out to compare genetics of European introduced O. virilis to that of the species in its wide USA range. Both populations within Europe were genetically similar, but formed a separate clade from any found in North America. In addition a population sampled from Iowa (USA) also represented a new clade (Filipova et al., 2010). This work suggests that lineage variation within O. virilis is high.

Reproductive Biology

Female Form Alternation

Form alternation in female O. virilis, similar to that in conspecific males, has been suggested by Wetzel (2002). In this study a number of orconectid females were classed as form I when observed with either swollen white glair glands, dependant offspring (embryos to 3rd instar juveniles) or egg stalks with remnants of eggs attached to pleopods. Only form I females were observed mating with form I males, therefore it was proposed that form I in females denotes sexually active individuals. Morphological differences were tested, carapace length to pleonite 2 width ratio (CL/P). Form I females had a wider abdomen than similar size form II females. Form II females were most prevalent during summer growing seasons, form I females appeared in the autumn mating season and were the dominant form until the spring spawning season (Wetzel, 2002). For O. virilis in particular, form I females appeared only in September/October, which was deemed as the mating season. Initial results from work in the UK suggest that female O. virilis alternate between FI and FII although work is still ongoing.


O. virilis reproduce once a year, with mating tending to occur during autumn and the young hatching in the following spring. In Michigan lakes (Momot and Gowing, 1975) and in both lentic and lotic systems in Iowa (Caldwell and Bovbjerg, 1969) reproduction takes place at the end of the second growing season. Two year old females are thought to produce most of the eggs in a population (92.5%)(Momot, 1967).

In a Michigan lake, pairs of crayfish were observed copulating from mid-August to September. Females with eggs were first observed mid-May and on average 94 eggs were found attached to the abdomen. This study found that the number of ovarian eggs increased linearly with the size of the parent, although carapace length (CL) and number of eggs on the abdomen were not linear. Females continued to feed while carrying eggs (Momot, 1967).

Studies in Ontario have found the mean number of external eggs per female to be from 139 to 214 with a maximum egg count of 310 (Weagle and Ozburn, 1972; Corey, 1987). Weagle and Ozburn (1972) observed egg laying occurring from 28th May to 3rd June in the McIntyre River, which coincided with a 5°C rise in water temperature. There did not appear to be a linear relationship between the number of eggs on the carapace and the carapace length (CL).

In the Beaver-Amisk river system  in Alberta mature females have been observed to move to winter hibernacula by the 1st October, secluded in darkness at low temperatures (0-4°C) for approximately seven months (Aiken, 1969). Mesocosm experiments showed four and a half months of darkness at 4°C were required for optimum ovarian growth. With less than four months darkness the number of eggs layed after a ‘spring’ stimuli was greatly reduced. Egg laying was shown to be stimulated by a water temperature of 10-11°C, and not by photo period. However, ovarian maturation was stimulated by both temperature and photoperiod (Aiken, 1969). In Ontario, females have been found to carry eggs at temperatures of 8°C in the spring (Berill, 1978).

During breeding season, when offered a choice between female crayfish pheromones and food stimuli males showed a significant preference for the pheromones, even when starved of food (Pecor and Hazlett, 2008).

O. virilis in Iowa were found to copulate at any time when males were in FI form, females free of eggs and young, and temperatures high enough to permit activity, early July to mid-April in this case, with the exception of the winter months (Caldwell and Bovbjerg, 1969).

Egg Laying/Hatching

Egg laying generally tends to occur in spring from April to May, although the exact time varies geographically. For instance, surveys in Wisconsin have found berried females as early as the beginning of April up until the third week in June and in Ontario oviposition has been observed as late as the end of May (Threinen, 1958). It is thought that the process is closely timed between females within a population for example, one study found 26 of 27 females collected carried eggs of a similar developmental stage that were thought to have been laid from mid-April. The majority of females are free of eggs and start to moult by mid-June, however in Ontario and some lentic areas females have still been found carrying young at this time, with hatching occurring in mid-July (Weagle and Ozburn 1972; Caldwell and Bovbjerg, 1969).

In Michigan, in a cool low productivity lake, newly hatched young of year had a CL of 4.5mm, when they leave the female in the spring the young are 6mm long (Momot, 1967). Juveniles undertake five moultsmoult in the first summer, one while still attached to the mother (Weagle and Ozburn, 1972). By September males have an average CL of 15.2mm and females are 14.1mm in Michigan (Momot, 1967). In the UK, females are found with external eggs between April and the end of May, and by mid August juveniles reach a mean carapace length of 18mm.

Sexual Maturity/Growth

In Michigan O. virilis males become sexually active (FI) after the summer moult in July at age 1 (Momot, 1967). In Ontario, 40% of year 1 males reached maturity on their third moult and an additional 10% reached maturity on their fourth moult. In Ontario, the smallest mature (FI) male was found to be 24.9mm (CL) by Weagle and Ozburn (1972) and 26mm (CL) by Berill (1978). The smallest female found with mature ovaries in autumn was 23.9mm (CL) and the smallest female found bearing eggs in spring was 25.4mm (CL) (Weagle and Ozburn, 1972). In Michigan, berried females of less than 35mm were rarely found in the population (Hazlett and Rittschof, 1985), however this may be attributed to faster growth rates in this population. In Ontario, 64% of year 1 females reached a mature size, however examination of ovaries in spring suggested that few year 1 females had mates, this was attributed to dominance for larger females in attracting males (Weagle and Ozburn, 1972).

Yearling males in a lentic environment in Michigan have a mean CL of 31.2mm and age 2 males have a mean CL of 36.5mm. Yearling females are 29.5mm (CL) and age 2 females 36.4mm (CL). However, variation in growth rate during the first growing season can be significant. Yearling males collected in May ranged from 16-25mm, females from 14-24mm. By August, males ranged from 24-33mm, females from 20-30mm (Momot, 1967). Another study in Michigan, this time with a lotic population extrapolated known growth in larger crayfish to estimate size/year classes for the population as:  0 (6-20mm), 1 (20-40mm), 2 (40-62mm), 3 (62-69mm) (Hazlett and Rittschof, 1985). In a cool, low productivity Michigan lake males had faster growth rate than females and after their first year, mortality was greater in females than in males. Both sexes matured after a moult in July at age 1, mating followed and eggs were laid the following spring. Overwintering mortality of all ages and both sexes was severe, very few males but no females lived through the fourth winter (Momot, 1967).

In a Michigan stream O. virilis of 58-62mm (CL) were common, with the largest male 64mm and the largest female 69mm (Hazlett et al., 1974). It is suggested that these larger sizes are caused by increased growth rates rather than increased survivorship or longevity (Hazlett and Rittschof, 1985). In this population it is proposed that adult females moult twice in the summer and some males moult three times, however sexual form was not recorded. One study in Lake Winnipesaukee, New Hampshire found that most males completed their summer moult within a week (Aiken, 1965). The late summer moult (FII-FI) takes place during the last week in August, and the spring moult (FI-FII) takes place during the final week of June. In Aiken’s study (1965) over 90% of males passed through winter as FI and following spring moult 35-40% of males were FI, 7% of which moulted from FII to FI in the spring. The remainder were large males that didn’t moult to FII. 

Physiology and Phenology

O. virilis inhabit shallow river systems in their northern most range in Ontario, Canada. These rivers freeze in winter but O. virilis do not appear to have any physiological adaptation to allow them to survive freezing (Aiken, 1967). Nor are they thought to burrow to escape freezing temperatures but instead this species tends to hide in crevices under or between rocks, which may become silted over. Furthermore, both males and females move into deeper areas of the river as temperatures drop. Distribution of young of the year (YOY) and yearlings, however, appeared random, utilising both deep and shallow areas (Aiken, 1967).


In most cases O. virilis was thought to live to 3 or 3.5 years (Momot and Gowing, 1975; Hazlett et al., 1979). Threinen (1958) however, found the life span of males to be from two growing seasons to two years, for females two years, with occasional specimens living to three years old. Due to the difficulty in ageing wild crayfish it can be difficult to get accurate estimates of population structures. In Europe, it is possible that O. virilis may live for 4 or 5 years.

Activity Patterns

Crayfish activity is linked to water temperature, and O. virilis activity decreases at very low temperatures. They have been shown to migrate between different water depths in relation to water temperature (Aiken, 1967; Momot and Gowing, 1972).

Population Size and Density

It is difficult to get quantifiable density data for wild crayfish populations, especially in larger water bodies. In an Oklahoma spring O. virilis density varied from 1-9/m2, with peaks in May, June and July (Varza and Covich, 1995). Within its introduced range the species has been found in densities of up to  9.2/m2 (Martinez, 2012).


Many crayfish species are poly-trophic omnivores, and will take advantage of whatever food source is locally abundant. O. virilis has been shown to actively graze on aquatic plants, with a preference for small, short bottom dwelling plants such as Chara and Lemna (Chambers et al., 1991; Dorn and Wojdak, 2004). It has been suggested that males may graze more heavily on some species of aquatic plants than females (Chambers et al., 1990b).

O. virilis was found to feed on Lake Trout eggs in Canada, the effect was more pronounced with larger substrate (Savino and Miller, 1991). A further study to see if crayfish could be used as a biological control for the invasive zebra mussel (Dreissena polymorpha) found that O. virilis would predate on zebra mussels, although trout eggs were preferred if available (Love and Savino, 1993). It was concluded that O. virilis preferred prey that provided the greater net benefit.

Macro-invertebrates can play a significant part in the diet of O. virilis, a study in Canada found abundance of snails was greatly reduced in the presence of O. virilis (Hanson et al., 1990), the same study found the crayfish would show preference for macro-invertebrate food even when plant material was present. Within the River Lee (UK) the diet of O. virilis was found to consist of Molluscs and Crustaceans (Pisidium sp. (14%) andGammarus Pulex (14%)) with detritus making up approximately 25% of their diet. Initial finding suggest O. virilis may have a lower degree of diet flexibility than the co-existing P. leniusculus (Jackson et al., Unpublished).

Natural Food Sources

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Food SourceFood Source DatasheetLife StageContribution to Total Food Intake (%)Details
Aquatic macrophytes All Stages
Fish eggs All Stages
Macro-invertebrates All Stages


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C - Temperate/Mesothermal climate Preferred Average temp. of coldest month > 0°C and < 18°C, mean warmest month > 10°C
Cf - Warm temperate climate, wet all year Preferred Warm average temp. > 10°C, Cold average temp. > 0°C, wet all year
Cs - Warm temperate climate with dry summer Preferred Warm average temp. > 10°C, Cold average temp. > 0°C, dry summers
Cw - Warm temperate climate with dry winter Preferred Warm temperate climate with dry winter (Warm average temp. > 10°C, Cold average temp. > 0°C, dry winters)
D - Continental/Microthermal climate Preferred Continental/Microthermal climate (Average temp. of coldest month < 0°C, mean warmest month > 10°C)
Df - Continental climate, wet all year Preferred Continental climate, wet all year (Warm average temp. > 10°C, coldest month < 0°C, wet all year)
Ds - Continental climate with dry summer Preferred Continental climate with dry summer (Warm average temp. > 10°C, coldest month < 0°C, dry summers)
Dw - Continental climate with dry winter Preferred Continental climate with dry winter (Warm average temp. > 10°C, coldest month < 0°C, dry winters)

Latitude/Altitude Ranges

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Latitude North (°N)Latitude South (°S)Altitude Lower (m)Altitude Upper (m)
54 28

Air Temperature

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Parameter Lower limit Upper limit
Mean maximum temperature of hottest month (ºC) 26
Mean minimum temperature of coldest month (ºC) 0

Water Tolerances

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ParameterMinimum ValueMaximum ValueTypical ValueStatusLife StageNotes
Dissolved oxygen (mg/l) 5 9 Optimum (Weagle KV and GW Ozburn, 1972)
Hardness (mg/l of Calcium Carbonate) 2.7 70 Optimum (Lawrence SG, 1981)
Water pH (pH) 6.5 9 Optimum (Malley DF, 1980; France RL, 1984)
Water temperature (ºC temperature) 0 26 Optimum (Lawrence SG, 1981)

Notes on Natural Enemies

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Crayfish provide an important function within aquatic ecosystems; they are largely omnivorous and provide a food source for a wide variety of fish, birds and mammals. A study in Michigan showed that Brook Trout (Salvelinus fontinalis) significantly preyed on crayfish that were under a year old. However, increasing or decreasing trout numbers did not affect crayfish density due to compensatory mortality found with low predation (Gowing and Momot, 1978).

Means of Movement and Dispersal

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Natural Dispersal (Non-Biotic)

Crayfish are mobile and will extend their range when resources become scarce. It is therefore possible that a natural movement will facilitate new areas being colonised.

Vector Transmission (Biotic)

There is a possibility of crayfish being moved between water bodies by birds or mammals, however the potential for long range, inter-catchment movements are limited. Female O. virilis are able to store sperm after mating so a single female, even if not carrying eggs could be enough to start a new population.

Accidental Introduction

Within the USA and Canada a large number of translocated crayfish populations have been attributed to the fishing bait industry; a good review of this can be found in Distefano RJ et al. (2009). As many as 49% of US and Canadian fisheries agencies reported aquatic resource problems linked to “bait bucked introductions”.

O. virilis is also widely used in schools and experimental facilities as a test animal, which heightens the risk of both accidental and intentional releases (Larson and Olden, 2008).

Intentional Introduction

It is thought that the two European O. virilis populations can be attributed to released aquarium specimens. Within England it is illegal to keep the species without a specific licence from the regulatory board, Natural England. There is also evidence that crayfish have been introduced to ponds to control weeds, and have consequently escaped (Larson and Olden, 2008).

Impact Summary

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Environment (generally) Negative

Economic Impact

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While there have been direct economic impacts of other introduced crayfish species such as damage to river banks, and flood defences; there are no specific examples for O. virilis.

Environmental Impact

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Impact on Habitats

While many impacts of invasive crayfish may be relatively generic there are a number of studies that have been carried out specifically on O. virilis. In a small channel near Kamerik, Netherlands there has been a significant shift from clear, macrophyte dominant waters to turbid, macrophyte poor waters (Soes, 2007). This shift has coincided with an increase in the abundance of the invasive O. virilis. A similar effect has been found in Arizona where streams have shifted from clear to muddy waters (Davidson et al., 2010). The effect of this species on macrophytes has been considered experimentally, and crayfish were found to significantly affect the biomass and density of four macrophyte species (Chambers et al., 1990a).

Impact on Biodiversity

Invasive crayfish are likely to have wide ranging ecological impacts, only some of which are currently understood. As a result of their broad diets and high population densities it is likely that there will be simultaneous effects on multiple trophic levels (Dorn and Wojdak, 2004). Experimental studies have shown O. virilis to have negative effects on the White Sands pupfish Cyprinodon tularosa, a threatened species in New Mexico. Rogowski and Stockwell (2006) found that O. virilis affected the reproduction and survival of pupfish and that the magnitude of these effects were dependent on the density of crayfish present.

The effects of introduced O. virilis are likely to be greatest where no indigenous crayfish are present, or where indigenous species are less gregarious. One such introduction was in Arizona, USA where no native crayfish species exist. Here introduced O. virilis have had negative impacts on a range of native and threatened species including: Chiricahua leopard frog (Rana chiricahuensis), juvenile desert suckers (Catostomus clarkii), the Sonora sucker (Catostomus insignis), Little Colorado spinedase (Lepidomeda vittata), and the critically endangered Three Forks spring snail (Pyrgulopsis trivialis) (Davidson et al., 2010). There is no evidence of hybridisation between O. virilis and other closely related species (Perry et al., 2001), although this can’t be ruled out completely.

All crayfish species endemic to North America are thought to be potential carriers of Aphanomyces astaci, commonly known as crayfish plague. This is lethal to all indigenous European crayfish species, and has caused widespread loss of the native White clawed crayfish (Austropotamobius pallipes) in England. The population of O. virilis from the River Lee, England was tested for crayfish plague and was found to have one of the highest infestation rates of any population of crayfish found in the UK. Many other North American crayfish species have been introduced into Europe and are widespread, so the specific threat of O. virilis spreading crayfish plague further is limited.

Threatened Species

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Threatened SpeciesConservation StatusWhere ThreatenedMechanismReferencesNotes
Cyprinodon tularosaVU (IUCN red list: Vulnerable) VU (IUCN red list: Vulnerable)New MexicoCompetition - monopolizing resources; Predation
Lepidomeda vittata (Little Colorado spinedace)EN (IUCN red list: Endangered) EN (IUCN red list: Endangered); USA ESA listing as threatened species USA ESA listing as threatened speciesArizonaCompetition - monopolizing resources; PredationIUCN, 2012
Pyrgulopsis trivialis (three forks springsnail)CR (IUCN red list: Critically endangered) CR (IUCN red list: Critically endangered); USA ESA listing as endangered species USA ESA listing as endangered speciesArizonaPredationIUCN, 2012
Rana chiricahuensis (Chiricahua leopard frog)VU (IUCN red list: Vulnerable) VU (IUCN red list: Vulnerable)Mexico; Arizona; New MexicoCompetition - monopolizing resources; PredationIUCN, 2012

Social Impact

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There is concern in Europe that large numbers of invasive crayfish, which are more aggressive and reach higher densities than native crayfish, may affect recreational angling.

Risk and Impact Factors

Top of page Invasiveness
  • Proved invasive outside its native range
  • Has a broad native range
  • Abundant in its native range
  • Highly adaptable to different environments
  • Capable of securing and ingesting a wide range of food
  • Highly mobile locally
  • Fast growing
  • Has high reproductive potential
  • Gregarious
  • Has high genetic variability
Impact outcomes
  • Altered trophic level
  • Changed gene pool/ selective loss of genotypes
  • Damaged ecosystem services
  • Ecosystem change/ habitat alteration
  • Modification of natural benthic communities
  • Reduced native biodiversity
  • Threat to/ loss of endangered species
  • Threat to/ loss of native species
Impact mechanisms
  • Competition - monopolizing resources
  • Herbivory/grazing/browsing
  • Hybridization
  • Predation
  • Rapid growth
Likelihood of entry/control
  • Highly likely to be transported internationally deliberately
  • Highly likely to be transported internationally illegally
  • Difficult to identify/detect as a commodity contaminant
  • Difficult to identify/detect in the field
  • Difficult/costly to control


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Economic Value

The species has been commercially harvested within its native range (Morgan and Momot, 1988), however it is not generally considered a crayfish of great economic importance.

Similarities to Other Species/Conditions

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It may be confused with a number of other Orconectes species, such as the papershell crayfish (Orconectes immunis), but can usually be distinguished by its broader, flattened tuberculate chela with straight margin of dactyl, and male gonopod morphology. Compared to the rusty crayfish (Orconectes rusticus), O. virilis is typically more blue in colour (without rust markings) with broader shorter chelae bearing distinct yellow tubercles. Whereas, the rusty crayfish has larger more elongated fingers of claws without tubercles (Hamr, 2013). Unlike the spothanded crayfish (Orconectes punctimanus), O. virilis does not have a narrow crescent-shaped saddle mark at the back end of its carapace. Dark specks on its pinchers may also distinguish these two species (USACE, 2013). When first found in England it was confused with Orconectes limosus.

Prevention and Control

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Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

SPS Measures

O. virilis have already been spread widely throughout Canada and the USA, largely attributed to “bait bucket introductions”. In 2009 only 4% of USA states and Canadian provinces banned the use of live crayfish bait although a higher percentage has some form of regulation in place. Where possible the use of live crayfish as fishing bait should be stringently regulated.

Public Awareness

Public awareness of non-native invasive crayfish varies considerably between regions and countries. Where possible water users should be educated; particularly recreational anglers who play a role in both introductions and detection of populations.


There is currently no known sanctioned method for eradication of crayfish populations, a number of methods have been tried including biocide (Peay et al., 2006) and trapping (Ibbotson et al., 1997). There has been considerable review of the subject, and while numbers of crayfish can be drastically reduced, no one method has led to long term eradication (Rogers and Holdich, 1998; Rogers and Loveridge, 2000; Smith and Wright, 2000; Wright and Williams, 2000; Peay, 2001; Hiley, 2002; Kozak and Policar, 2002; Stebbing et al., 2002; Hyatt, 2004; Hiley and Peay, 2006; Peay et al., 2006; Davidson et al., 2010).


Where possible inter-catchment, and inter-regional movement should be reduced. Currently O. virilis is limited to North America, Mexico and Northern Europe but there is potential for more introductions around the globe. In the UK, fact sheets have been prepared by the Environment Agency for anyone applying to trap crayfish in areas where O. virilis are found. It is hoped this will reduce the risk of accidental releases into the wild.


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Links to Websites

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Environment Agency (UK)
GISD/IASPMR: Invasive Alien Species Pathway Management Resource and DAISIE European Invasive Alien Species Gateway source for updated system data added to species habitat list.
Global register of Introduced and Invasive species (GRIIS) source for updated system data added to species habitat list.


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30/08/12 Original text by:

Adam Ellis, Ahern Ecology, 49 North Street, Wilton, Wiltshire, SP2 0HE, UK

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