The signal crayfish (Pacifastacus leniusculus) is a North American species of crayfish. It was introduced to Europe in the 1960s to supplement the North European Astacus astacus fisheries, which were being damaged by crayfish plague, but the imports turned out to be a carrier of that disease. The signal crayfish is now considered an invasive species across Europe, Japan, and California, ousting native species there. 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. Signal crayfish display correlation between boldness, aggression, and activity but the repeatability of these behaviors was not tested. Moreover, previous studies showed that the signal crayfish is a nocturnal species. Signal crayfish can be found in habitats ranging from clear, shallow coastal streams, to major rivers with high turbidity, as well as eutrophic and oligotrophic lakes and reservoirs. They have a broad and flexible diet that includes different trophic levels. As juveniles they typically feed on aquatic insects, shifting towards a more herbivorous diet as adults. They are also known to cannibalize other Signal Crayfish throughout their life history. Northern Pike (Esox lucius), Atlantic Salmon (Salmo salar), and Brown Trout (Salmo trutta) have all been found to consume Signal Crayfish in their introduced range. Mating occurs in autumn at 2-3 years old. Eggs are held over the winter attached to the underneath of the females tail, when hatched the young are still attached to their mother. Young are released in May-June. The eggs hatch into juveniles, which pass through three stages (two moults) before leaving their mother. Sexual maturity is reached after 2–3 years, and the lifespan can be up to 20 years.
