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)
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
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 ,,
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
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 ,
Diet similar to adults
Feeds on a variety of live and dead plant and animal material
Often eat and destroy macrophytes (large water plants)
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 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 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 are laid from mid-March to mid-April
Females were found carrying between 28 and 707 eggs with an average of 347
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
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
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 ,
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
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
Low tolerance to toxic water conditions
Gills are especially vulnerable in poor water conditions
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
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 ,
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 ,
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)
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
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