Female signal crayfish carrying eggs. Signal crayfish (Pacifastacus leniusculus) are an invasive species that have been introduced from the USA into Europe and Japan. Each female lays around 200?00 eggs in the autumn, and they are carried under the female's tail until they are ready to hatch the following spring. The eggs hatch into juveniles, which pass through three stages (two moults) before leaving their mother. Sexual maturity is reached after two to three years, and their lifespan can be up to 20 years. This signal crayfish was photographed in the Buskin River watershed, Kodiak Island, Alaska, USA, in July 2017. Signal crayfish are not native to Alaska, but the population there was discovered in 2001 and is now well established and breeding.