The Signal Crayfish (Pacifastacus leniusculus), is an American crayfish indigenous to the western United States. Signal Crayfish were introduced in Swedish rivers and lakes to replace the native crayfish species there that were nearly exterminated from the crayfish plague that devastated the European crayfish stocks. This species has also been introduced to Japan and a number of other countries as well. Unfortunately, this species has also been introduced carelessly to some countries without considering the fact that the signal crayfish also carries the crayfish plague, and thus causes the eventual extinction of native crayfish.
Members of this species are up to 6 inches in length. They are bluish-brown to reddish-brown in color with robust large smooth claws. They have a white patch near the claw hinge. Like all crayfish, they are solitary animals and are omnivorous, although their diet is mainly vegetarian they will eat anything from decaying roots and leaves to meat, including crayfish smaller than themselves.