Eggs hatch into very small, pink individuals with large heads 
Carapace length is about 4.5 mm at time of hatching 
Young grow quickly undergoing several molts and quickly resembling adults 
,
Information Last Reviewed Spring 2007
The Northern crayfish originated within the North Central United States and has since spread into most of the states, as well as portions of Canada. Presence of this species, outside of its native habitat, has had a negative impact on all aquatic ecosystems it has entered.
Crayfish are especially difficult to identify. For proper identification the specimen needs to be a Form I male or an adult male during the breeding season. Crayfish males can molt or lose their shells after the breeding season and the new shell will look like a juvenile or quasi-juvenile crayfish (Form II male) until tubercles and color patterns appear that make the individuals easier to identify. Because of the difficulty in identifying most crayfish species it is important to go through all of the characters described below, before notifying the authorities about the presence of a possible invasive species. The Carnegie Museum of Natural History has a viable key for identifying most crayfish to genus: (http://iz.carnegiemnh.org/crayfish/Keys/identification.htm)
Phylum: Arthropoda
Subphylum: Crustacea
Class: Malacostraca
Order: Decapoda
Family: Cambaridae
Juvenile Morphology
Eggs hatch into very small, pink individuals with large heads
Carapace length is about 4.5 mm at time of hatching
Young grow quickly undergoing several molts and quickly resembling adults ,
Adult O. virilis
Top Image modified from: http://www.ces.purdeu.edu/extmedia/AS/AS-500.html
Bottom Image modified from: http://www.mdc.mo.gov/nathis/arthopo/crayfish/varcraw.htm
Adult Morphology
Overall reddish brown or olive brown color 
Numerous yellow bumps on the medial (side closest to the head) sides of the pinchers (#1 in figure)
Pinchers often have orange or orange-red tips (#2 in figure)
Dark specks are often found on pinchers (#3 in figure)
Paired black blotches along the abdomen, especially prominent in young and individuals that have recently molted
Adults reach a size of 45–125 mm ,
Males usually grow larger than females ,,
Distinguishing Characteristics
Most often confused with the spothanded crayfish (Orconectes punctimanus) and the papershell crayfish (Orconectes immunis) ,
,
Spothanded crayfish does not have dark specks on pinchers but may have a black spot at the base of the moveable finger (#3 in figure)
Most spothanded crayfish have a "narrow crescent-shaped saddle mark" at the back end of the carapace. This mark is not present on the northern crayfish (#4 in figure)
Papershell crayfish have a pale area along the midline of the carapace and abdomen while the northern crayfish is usually uniform in color
The pinchers of the papershell crayfish are often purple or pink while those of the northern crayfish are usually green or blue-green
Behavior
Adults are primarily nocturnal ,
Maximum lifespan is about three years
Less aggressive than similar species 
In northern locations (Michigan, Canada, Iowa) the northern crayfish has been documented moving into deeper water in early fall to escape winter freezing ,
When occupying deeper waters, the crayfish are inactive and become covered with silt ,
Juveniles
Omnivorous
Diet similar to adults 
Adults
Omnivorous
Feeds on a variety of live and dead plant and animal material
Often eat and destroy macrophytes (large water plants)
Growth
Young undergo several molts during first few months of life
Growth is quick but not as quick as some similar species ,
Reaches a length of 23–56 mm in the first year of life
A length of 58–84 mm is attained by the end of the second year
Maturity
Maturity is reached during the second summer of life ,
Maturity is usually reached around 24 mm
In northwest Iowa, adult males molt from their mating form into their non-mating form in mid-May to mid-June
These same adult males undergo a second molt back into their breeding form in July or August
In southwest Iowa the first molt occurred earlier (late April or May) and the second molt did not occur until late August 
Breeding
Breeding begins in early July and extends until the adults retreat to deep water and become inactive ,,
Breeding sometimes occurs for a brief period in the spring when the water begins warming ,,
Males may construct special tunnels in which mating and brooding of the eggs and young may occur
Rarely interbreeds with other crayfish species
Eggs
Eggs are laid from mid-March to mid-April
Females were found carrying between 28 and 707 eggs with an average of 347
Juveniles
Young emerge from mid-May until about the third week in June ,
Juveniles are believed to complete their first molt about nine days after they hatch
Young leave parental care about 13 – 14 days after hatching
Preferred Environment
Primarily found in streams and usually in areas with rocky bottoms ,
Prefer fertile, warm, moderately turbid streams with lots of cover
Prefer cobble substrate and rocky crevices in streams ,
Often uses rocks, logs, and other organic debris as cover ,
Occasionally dig pits in river banks and under rocks especially when water levels are low
Temperature
Can tolerate temperatures 0° C and 32° C
Temperatures between 24° C and 25° C are preferred 
Locomotion is slowed or stopped at temperatures below 10° C
Quiescence occurs in low temperatures ,
pH
Not able to successfully withstand acidic conditions 
In an experimental lake area in Ontario, Canada, populations declined considerably when subjected to a pH of 5.37
Young juveniles are more sensitive to pH than older juveniles and adults 
Calcium uptake is limited at low pH levels (< 5.8) and inhibited below 4, causing softer shells for recently molted individuals 
Oxygen
Not very tolerant to low oxygen levels
In experimental conditions with low oxygen levels, all O. virilis died within 24 hours with most dying after just 3 ½ hours
Salinity
Water Quality
Low tolerance to toxic water conditions 
Gills are especially vulnerable in poor water conditions
Native Range
Has one of the largest natural ranges of all North American crayfishes 
Native range extends from New Hampshire and Maine, across the Midwest including much of the Mississippi River and its tributaries, as far west as Colorado, south to Texas and north to Ontario, Canada 
Native populations are found in: Arkansas, Colorado, Illinois, Indiana, Iowa, Kansas, Kentucky, Maine, Michigan, Minnesota, Missouri, Montana, Nebraska, New Hampshire, New York, North Dakota, Ohio, Oklahoma, South Dakota, Texas, Utah, Wisconsin, Wyoming ,,
North American Distribution
Introduced:
Alabama, Arizona, California, Connecticut, Maryland, Massachusetts, Mississippi, New Jersey, New Mexico, Pennsylvania, Rhode Island, Tennessee, Vermont, Virginia, West Virginia ,
Probable Means of Introduction
Escaped or released by fishers using them as bait 
Currently expanding from their native and introduced water bodies ,
Negative
Males cause a decrease in the length and growth of some macrophytes 
Drastically modify the aquatic macrophyte and macroinvertebrate communities, which in turn may lead to a decline and reconfiguration of fish communities ,
,
Greatly reduce the number of snails in the community ,
Cause a reduction in the number of small invertebrates present
May consume eggs of sunfish, bluegill, and trout leading to reduced population sizes ,
Positive
Often harvested for human consumption
Serves as a food source for many fish, birds, and mammals
Increased food supply for game fish including largemouth bass (Micropterus salmoides)
Control Measures
Protect sport-fish populations in order to increase the level of predation on the crayfish
The rusty crayfish (Orconectes rusticus), another invasive crayfish, almost always outcompetes and reduces the population size of the northern crayfish (Orconectes virilis) 
,
Continued education by state and federal agencies outlining the harmful effects of using and releasing nonnative bait crayfish into new water bodies
Aiken, D.E. 1965. Distribution and ecology of three species of crayfish from New Hampshire. American Midland Naturalist. 73:240-244.
Bergman, D.A. and P.A. Moore. 2003. Field observations of intraspecific agonistic behavior of two crayfish species, Orconectes rusticus and Orconectes virilis, in different habitats. Biological Bulletin. 205:26-35.
Bovbjerg, R.V. 1970. Ecological isolation and competition exclusion in two crayfish (Orconectes virilis and Orconectes immunis). Ecology 51:225-236.
Caldwell, M.J. and R.V. Bovbjerg. 1969. Natural history of the two crayfish of northwestern Iowa, Orconectes virilis and O. immunis. Iowa Academy of Science Proceedings 76:463-472.
Capelli, G.M. and B.L. Munjal. 1982. Aggressive interactions and resource competition in relation to species displacement among crayfish of the genus Orconectes. Journal of Crustacean Biology. 2:486-492
Chambers, P.A., J.M. Hanson, J.M. Burke, and E.E. Prepas. 1990. The impact of the crayfish Orconectes virilis on aquatic macrophytes. Freshwater Biology. 24:81
DiDonato, G.T. and D.M. Lodge. 1993. Species replacements among Orconectes crayfishes in Wisconsin lakes: The role of predation by fish. Canadian Journal of Fisheries and Aquatic Sciences. 50:1484-1488
Dorn, N.J. and G.G. Mittelbach. 2004. Effects of a native crayfish (Orconectes virilis) on the reproductive success and nesting behavior of sunfish (Lepomis spp.). Canadian Journal of Fisheries and Aquatic Sciences 61:2135-2143.
France, R.L. 1984. Comparative tolerance to low pH of three stages of the crayish Orconectes virilis. Canadian Journal of Zoology 62:2360-2363.
Hanson, J.M., P.A. Chambers, and E.E. Prepas. 1990. Selective foraging by the crayfish Orconectes virilis and its impact on macroinvertebrates. Freshwater Biology. 24:69-80
Hazlett, B., D. Rittschof, and D. Rubenstein. 1974. Behavioral biology of the crayfish Orconectes virilis I. Home range. American Midland Naturalist. 92:301-319.
Hill, A.M. and D.M. Lodge. 1994. Diel changes in resource demand: Competition and predation in species replacement among crayfishes. Ecology. 75:2118-2126
Horns, W.H. and J.J. Magnuson. 1981. Crayfish predation on lake trout eggs in Trout Lake, Wisconsin. Rapp. P.-v. Reun. Cons. Int. Explor. Mer. 178:299-303
Lodge, D.M. and Lorman, J.G. 1987. Reductions in submersed macrophyte biomass and species richness by the crayfish Orconectes rusticus. Canadian Journal of Fisheries and Aquatic Sciences. 44:591-597
Lodge, D.M., A.L. Beckel, and J.J. Magnuson. 1985. Lake-bottom tyrant. Natural History. 94:32-37
Lodge, D.M., M.W. Kershner, J.E. Aloi, and A.P. Covich. 1994. Effects of an omnivorous crayfish (Orconectes rusticus) on a freshwater littoral food web. Ecology. 75:1265-1281
Lodge, D.M., C.A. Taylor, D.M. Holdich, J. Skurdal. 2000. Nonindigenous crayfishes threaten North American freshwater biodiversity: Lessons from Europe. Fisheries. 25:7-20
Malley, D.F. 1980. Decreased survival and calcium uptake by the crayfish Orconectes virilis in low pH. Canadian Journal of Fisheries and Aquatic Sciences 37:364-372.
Mirenda, R.J. 1986. Toxicity and accumulation of cadmium in the crayfish, Orconectes virilis. Environmental Contamination and Toxicology 15:401-407.
Momot, W.T. 1967. Population dynamics and productivity of the crayfish, Orconectes virilis, in a marl lake. American Midland Naturalist. 78:55-81.
Peck, S.K. 1985. Effects of aggressive interaction on temperature selection by the crayfish Orconectes virilis. American Midland Naturalist 114:159-167.
Pflieger, W.L. 1996. The Crayfishes of Missouri. Missouri Department of Conservation. Jefferson City, Missouri. 152p
Ruiz, G.M., P. Fofonoff, A.H. Hines, and E. D. Grosholz. 1999. Non-indigenous species as stressors in estuarine and marine communities: Assessing invasion impacts and interactions. Limnology and Oceanography 44:950-972.
Schindler, D.W., and M.A. Turner. 1982. Biological, chemical and physical responses of lakes to experimental acidification. Water, Air and Soil Pollution 18:259-271.
Soderback, B. 1994. Interactions among juveniles of two freshwater crayfish species and a predator fish. Oecologia. 100:229-235.
Wetzel, J.E., W.J. Poly, and J.W. Fetzner Jr. 2004. Morphological and genetic comparisons of golden crayfish, Orconectes luteus, and rusty crayfish, O. rusticus, with range corrections in Iowa and Minnesota. Journal of Crustacean Biology. 24:603-617
Additional Literature
Berrill, M. 1978. Distribution and ecology of crayfish in the Kawartha Lakes region of southern Ontario. Canadian Journal of Zoology. 56:166-177.
Bouchard, R.W. and H.W. Robinson. 1980. An inventory of the decapod crustaceans (crayfishes and shrimps) of Arkansas with a discussion of their habitats. Arkansas Academy of Science Proceedings 34:22-30.
Butler, M.J. and R.A. Stein. 1985. An analysis of the mechanisms governing species replacements in crayfish. Oecologia. 66:168-177.
Capelli, G.M. 1982. Displacement of northern Wisconsin crayfish by Orconectes rusticus (Girard). Limnology and Oceanography. 27:741-745.
Lodge, D.M., T.K. Kratz, and G.M. Capelli. 1986. Long-term dynamics of three crayfish species in Trout Lake, Wisconsin. Canadian Journal of Fisheries and Aquatic Sciences 43:993-998.
Luttenton, M.R., M.J. Horgan, and D.M. Lodge. 1998. Effects of three Orconectes crayfishes on epilithic microalgae: A laboratory experiment. Crustaceana 71:845-855.
Pennak, R.W. 1989. Fresh-water invertebrates of the United States. Protozoa to Mollusca, 3rd ed. John Wiley and Sons, Inc. New York. 628p.
Perry, W.L., J.L. Feder, and D.M. Lodge. 2001. Implications of hybridization between introduced and resident Orconectes crayfishes. Conservation Biology. 15:1656-1666.
Taylor, C.A. 2000. Systematic studies of the Orconectes juvenilis comples (Decapoda: Cambaridae), with descriptions of two new species. Journal of Crustacean Biology 16: 547-551.
http://www.issg.org/database/species/ecology.asp?si=218&fr=1&sts
http://www.ncwildlife.org/pg07_WildlifeSpeciesCon/nccrayfishes/o_virilis/o_virilis.html