Calico Crayfish - Orconectes immunis
FWP Conservation Tier
Rostrum acuminate, acarinate and with converging margins lacking marginal spines or shoulders; cervical spines present; areola narrow with 2-3 punctations in narrowest part; male with hooks on ischia of 3rd pereiopods; male first pleopod terminating in 2 subparallel elements <25% length of pleopod, curved throughout length but distal 1/3 more pronounced so that apices of both elements directed at about 90 degrees to main axis of pleopod (Page, 1985). [LENGTH: to 50 TCL; to 95 TL] [WIDTH: to 25]
O. immunis have characteristically long, slender chelae with a very splotchy coloration, but these features are a bit difficult to use as diagnostic characters without a few reference specimens. The easiest way to distinguish between O. immunis and O. virilis is the notch near the base of the dactyl, or 'thumb' of the cheliped--O. immunis has it, O. virilis does not.
O. immunis has a widespread North American native range from southern Quebec, Canada and New England westward across the upper Midwest to Wyoming and eastern Colorado and the Dakotas and south to extreme northwestern Tennessee (Hobbs, 1989; Pflieger, 1996). Montana's eastern plains populations are the furthest reported western extent this species ranges.
Summary of Observations Submitted for Montana
Number of Observations:
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(Records associated with a range of dates are excluded from time charts)
Does not seem to migrate any large distance and often burrows and maintains a very small home range
The species is always in, at best, sluggish flow (unlike O. virilis which can tolerate moderately flowing rivers); burrows during drying periods (i.e., tertiary burrower). In Missouri, it occurs in shallow ditches and sloughs on the broad, flat flood plains of large to medium-sized rivers and in the isolated pools of intermittent headwater streams draining level upland prairies. Wide seasonal fluctuations in water area and depth (with many areas becoming entirely dry during late summer), deep mud bottoms and absence of strong flow or current are common characteristics of these habitats. However, large populations also occurred in habitats where high turbidity provided the only cover (Pflieger, 1996).
This species is an omnivorous detritivore and a scavenger, feeding by shredding large pieces of orangic materials and shredding them up.
In eastrern Montana, this species may be becoming rare and out-competed due to the invasive nature of the Virile Crayfish
Threats or Limiting Factors
Invasive crayfish species Orconectes virilis