Signal crayfish

The signal crayfish is a large, robust crustacean, typically reaching 6–9 cm in carapace length and up to 16–18 cm total, with a smooth, dark brown to reddish-brown body. Its most distinctive feature is the bright white or pale blue patch on the claw hinge, resembling a signal flag, which gives the species its common name. The claws are large and smooth, lacking the bumps or ridges found on some native European crayfish. Juveniles are more uniform in color and may be mistaken for other species, but the white patch is usually visible even in small individuals. This species is often confused with the noble crayfish (Astacus astacus) in Europe, but the signal crayfish has a more slender claw and the white patch is diagnostic.

Native to the Pacific Northwest of North America, from British Columbia south to northern California and east to Idaho and Nevada, the signal crayfish inhabits streams, rivers, lakes, and reservoirs with cool, well-oxygenated water. It is an opportunistic omnivore, feeding on aquatic plants, detritus, insects, snails, fish eggs, and even small fish, and it plays a key role in nutrient cycling in its native ecosystems. The species is relatively long-lived, with a lifespan of 5–8 years, and reaches sexual maturity at 2–3 years. Females carry eggs under their tails for several months, with hatching occurring in late spring. In its native range, it coexists with other crayfish species, but it often dominates in disturbed habitats.

Introduced to Europe, Japan, and other regions since the 1960s for aquaculture and the aquarium trade, the signal crayfish has become one of the world's most damaging invasive species. It outcompetes native crayfish, such as the white-clawed crayfish (Austropotamobius pallipes) in Europe, through aggression, faster growth, and higher fecundity. Critically, it is a carrier of the crayfish plague fungus (Aphanomyces astaci), which is lethal to European native crayfish but harmless to the signal crayfish itself. The species also burrows into riverbanks, causing erosion and increased turbidity, and consumes large amounts of aquatic vegetation, altering entire ecosystems. In Japan, it has invaded many rivers and is considered a major pest, with control efforts including trapping and the use of biocides.

Beyond its well-known invasive status, the signal crayfish has several lesser-known facets. It was introduced to Sweden in the 1960s to replace the native noble crayfish after plague outbreaks, but it has since spread to most of Europe. In the United Kingdom, it is illegal to keep or release signal crayfish without a license, and eradication programs have used pheromone traps and even electric fishing. The species is also farmed commercially in several countries, with a market for its meat, and it is considered a delicacy in some regions. Interestingly, the signal crayfish can tolerate brackish water, allowing it to spread through estuaries. In its native range, it is an important prey for otters, herons, and raccoons, and it is also used as bait for fishing. Recent research has explored using its exoskeleton for chitin extraction, a sustainable source of biopolymer.

Carapace

The hard upper shell of a crustacean.

Crayfish plague

A fungal disease caused by Aphanomyces astaci, lethal to European crayfish.

Fecundity

The ability to produce many offspring.