Pacifastacus leniusculus, commonly known as the signal crayfish, is a species of freshwater crayfish native to the western United States. It is characterized by its large size, typically reaching up to 15 cm in length, and its distinctive coloration, which includes a dark green or brown body with bright blue claws. This species prefers habitats with clean, flowing water and abundant vegetation. Signal crayfish are important in their ecosystems, serving as both predators and prey. They feed on a variety of organic materials, including plants, insects, and detritus. However, they are also known for being invasive in some regions, where they can outcompete native species like Astacus astacus and disrupt local ecosystems.