Montana Field Guide

The signal crayfish, Pacifastacus leniusculus, is an large North American crayfish indigenous to the western United States. Members of this species are up to 15 cm in length. They are bluish-brown to reddish-brown in color with robust large smooth claws. They have a white to pale blue-green patch near the claw hinge (see photo). Like all crayfish, they are solitary and omnivorous, although their diet is mainly vegetarian they will eat anything from decaying roots and leaves to meat, including crayfish smaller than themselves.

This crayfish prefers large cobbles or woody debris to hide under, and if softer sediment banks are available will create burrows. It is found in streams to large rivers and is even larger and more abundant in reserviors and lakes along the mainstem Clark Fork river.

This species is omnivorous and a scavenger, feeds by shredding large pieces of orangic materials and shredding them up.

Pacifastacus leniusculus is very active and migrates up and down rivers, as well as moving overland around obstacles. However, their rate of colonization is relatively slow and may only be about 1 km yr-1. In one stream in England it took 17 years for them to spread 12 km downstream (Stanton 2004).

Pacifastacus leniusculus has a typical life cycle as a member of the crayfish family Astacidae. The eggs hatch into miniature crayfish that stay with the mother for three stages, the third stage gradually becoming more and more independent of the mother. Juveniles undergo as many as 11 moults during their first year, but by age 3 this is reduced to two moults per year, and by age 4 onwards to one moult per year (Lewis and Horton 1996).

Abundant in the Lower Clark Fork and Kootenai Drainages, big harvests have been reported in the Noxon Reservior areas.

No threats to Montana's populations of Pacifastacus leniusculus have been identified, it is an aggressive competitor and has been responsible for displacing indigenous crayfish species wherever it has been introduced.