Pacifastacus leniusculus

Signal Crayfish

Family: Astacidae · Class: Malacostraca · Order: Decapoda

The signal crayfish is a large freshwater crustacean native to western North America, measuring up to 180 mm (7.1 inches) in total length. Adults typically reach 60-120 mm (2.4-4.7 inches) in body length and can weigh up to 250 grams. The species is characterized by its robust build, smooth carapace, and distinctive white or pale blue patches on the underside of its claws, which give the species its common name. The body coloration ranges from olive green to reddish-brown dorsally, with lighter coloration ventrally. Large specimens develop pronounced chelae (claws) with the right claw typically larger than the left. Signal crayfish are endemic to western North America, with their native range extending from British Columbia south to central California and eastward to Utah (USFWS 2015). In California, the species historically occurred in the Sacramento and San Joaquin river drainages, coastal streams from Sonoma County to Monterey County, and select tributaries of the Klamath and Colorado rivers. The species has been widely introduced outside its native range and is now considered invasive in Europe and parts of eastern North America. The species inhabits cold, clear freshwater environments including streams, rivers, lakes, and reservoirs. Signal crayfish prefer rocky or gravel substrates with adequate cover such as fallen logs, undercut banks, and aquatic vegetation. They typically occur in waters with temperatures below 25°C (77°F) and dissolved oxygen levels above 3 mg/L. Adults are found at depths ranging from shallow margins to over 30 meters in lakes, with juveniles preferring shallower areas with dense cover. Signal crayfish are opportunistic omnivores, feeding on aquatic invertebrates, fish eggs, algae, detritus, and occasionally small fish. Adults are primarily nocturnal, emerging from cover at dusk to forage. The species exhibits aggressive territorial behavior, particularly during breeding season. Mating occurs in fall when males deposit spermatophores on females. Females carry fertilized eggs under their abdomen for 6-7 months, releasing 100-400 juveniles in late spring. Sexual maturity is reached at 2-3 years of age, and individuals can live up to 10 years. While not federally listed, signal crayfish populations face threats from habitat degradation, water diversions, and pollution within their native range. Ironically, the species poses significant ecological risks where introduced, outcompeting native crayfish species and altering aquatic ecosystems (USFWS 2015). In California, signal crayfish threaten the endangered Shasta crayfish through direct competition and habitat displacement. The species carries crayfish plague (Aphanomyces astaci), a pathogenic water mold that is lethal to many native crayfish species. Conservation efforts focus on preventing further introductions while protecting remaining native populations through habitat restoration and water quality improvements.

Data from The California Species Project — 14,000+ California species with verified data from CNPS, CDFW, USFWS, and more.