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Montana Field Guide

Montana Field Guides

Virile Crayfish - Orconectes virilis

Aquatic Invasive Species

Global Rank: G5
State Rank: S5
* (see State Rank Reason below)

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State Rank Reason (see State Rank above)
Very common in most major rivers and expanding its range.
 
General Description
A medium-large crayfish (5-6 inches max length) that is native to eastern Montana but has been invading westward the last 30 years. Most distinguishing features are the red/orange tips on the ends of the claws. The rostrum is acuminate, acarinate, margins slightly converging and terminating in spines or sharply angular shoulders; cervical spines present; areola narrow with 2-3 punctations in narrowest part; male with hooks on ischia of 3rd pereiopods; male 1st pleopod terminating in 2 somewhat divergent slighly curved elements constituting 50% of total length of pleopod, central projection longer and with distal third smoothly curved so that apex directed caudally, lacking shoulder on cephalic margin of pleopod (Hobbs 1976). [LENGTH: 70mm Carapace Length to 150mm Total Length].

Diagnostic Characteristics
Narrow aerola (longitudinal space band) on the top of carapace (Signal Crayfish has a wide band). Red or orange tips of claws whereas Orconectes immunis has blue tips or no tip coloration. O. immunis has a large notch in the gap of the chelea (claw) where O. virilis is smooth.

Range Comments
O. virilis has a widespread North American native range from Alberta to Quebec, Canada, and in the United States it is native from eastern Montana and Utah to Arkansas, north to the Great Lakes, and east to New York. It is introduced in the United States in parts of the Southwest, Southeast, the mid-Atlantic, and the Northeast.

Summary of Observations Submitted for Montana
Number of Observations: 143

(Click on the following maps and charts to see full sized version) Map Help and Descriptions
Relative Density

Recency

 

(Observations spanning multiple months or years are excluded from time charts)



Migration
Somewhat invasive migratory pattern--this species is moving across the state of Montana in a westerly (east to west) direction and upstream into drainages (upper Missouri and Yellowstone) were they did not exist before.

Habitat
According to (Collicut 1998), O. virilis can be found in lakes, rivers, streams and and ponds. They are found in permanent bodies of water deep enough not to freeze solid or experience low oxygen levels. O. virilis requires shelter in the form of rocks, logs, or thick vegetation in which to hide from predators during daylight hours. In Montana, this species needs perennial water bodies and can survive in both flowing streams and rivers or pooled-up prairie streams, prefers cobbles and large woody debris for protection but can inhabit some silted areas if aquatic vegetation is present.

Food Habits
This species is omnivorous and a scavenger, feeds by shredding large pieces of orangic materials and consuming them. Pieces of bait fish in a minnow trap is a sure way to catch these crayfish.

Ecology
The shredding activity of this crayfish can have substantial affects on the food web by providing smaller organic materials to invertbrates and fishes feeding lower and smaller on the food chain, but also compete for food resources with other shredders.

Management
This species is included on the Global Invasive Species database, because of its invasive migratory pattern and highly competitive nature; this species is moving across Montana in a westerly (east to west) direction and upstream into drainages were they did not exists before. Although native to eastern Montana their spread has affected other watersheds and native species where they invade. Most recent reported sightings show them to be in the Thompson Chain of Lakes, the furthest western populations in Montana. These newest locations were most likely started with "bucket biology" techniques; a serious management issue spreading Aquatic Nuisance Species.

Threats or Limiting Factors
Low dissolved oxygen seems to be a threat, as are intermittent stream conditions, but otherwise are fairly tolerant and resilient to human induced changes including expanding into altered habitats (below dams) and enhanced benthic substrates (the addition of rip rap for wing dams and bridge abutments).

References
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Citation for data on this website:
Virile Crayfish — Orconectes virilis.  Montana Field Guide.  .  Retrieved on , from