Signal Crayfish
Category
Invasive Crustacean - Order Decapoda, Family Astacidae
Common Name
Signal Crayfish
Species Name
Pacifastacus leniusculus
Distribution / Status (Lake Tahoe)
Established invasive throughout Lake Tahoe littoral zone and tributaries; dominant crayfish species; target for removal in several management programs
Diagnostic Features
White “signal” patch on claw; smooth carapace without spines; robust body with olive-brown coloration
Body Length
Adults typically 6–12 cm total length (up to 16 cm including claws)
Color / Patterning
Variable brown, olive, or reddish-brown; claws often bluish or greenish with a distinct white to pale patch (“signal spot”) on the dorsal claw joint
Body Form
Robust body with smooth, hard exoskeleton; large chelae (claws) relative to body size
Rostrum (snout):
Triangular, with raised ridges along margins and a pointed tip
Eyes / Antennae
Dark, prominent eyes; long antennae extending outward past body
Claws (Chelae)
Asymmetrical and powerful; inner edge toothed; white patch (diagnostic) on outer surface near joint
Abdomen & Tail Fan
Segmented abdomen ending in broad tail fan used for swimming; underside often lighter in color
Operculum / Gills
Covered gills beneath carapace; respiration via gill filaments
Behavior / Habits
Burrowing and nocturnal; hides under rocks or in sediment burrows by day; active forager at night; highly aggressive and territorial
Feeding Ecology
Omnivorous; Feeds on detritus, macrophytes, benthic invertebrates, fish eggs, and carrion; can reduce macrophyte cover and prey on native amphibians and invertebrates
Habitat Type
Lakes, rivers, and streams with rocky or coarse substrates; requires well-oxygenated water and shelter such as boulders or woody debris
Depth Range
Littoral to sublittoral zones (0.5–10 m), occasionally deeper in summer
Temperature Range
Prefers 10–22 °C; tolerates cold alpine waters; less active below 5 °C
Mobility
Active crawler and swimmer; capable of upstream and overland migration between connected waters
Ecological Impact
Displaces native crayfish if present, increases turbidity through burrowing, alters macrophyte and invertebrate communities, preys on fish eggs
Sexual Dimorphism
Males have larger claws, extra set of larger “swimmer legs” just below walking legs, and narrower abdomen; females broader abdomen for egg carrying, extra swimmer legs absent
Reproduction
Mates in late fall; females carry 100–400 eggs under abdomen through winter; juveniles hatch in spring
References
Flint, R. W., & Goldman, C. R. (1977). Crayfish Growth in Lake Tahoe: Effects of Habitat Variation. Journal of the Fisheries Research Board of Canada, 34(1), 155–159. doi:10.1139/f77-022
Global Invasive Species Database (2025) Species profile: Pacifastacus leniusculus. Downloaded from http://www.iucngisd.org/gisd/species.php?sc=725 on 05-11-2025.
Invasive Species Northern Ireland. (n.d.). North American signal crayfish: Pacifastacus leniusculus. Invasive Species Northern Ireland.
Larson, E. R., Renshaw, M. A., Gantz, C. A., Umek, J., Chandra, S., Lodge, D. M., & Egan, S. P. (2017). Environmental DNA (eDNA) detects the invasive crayfishes Orconectes rusticus and Pacifastacus leniusculus in large lakes of North America. Hydrobiologia, 800(1), 173–185. doi:10.1007/s10750-017-3210-7
Makkonen, J., Kokko, H., Gökmen, G., Ward, J., Umek, J., Kortet, R., … Jussila, J. (2019). The signal crayfish (Pacifastacus leniusculus) in Lake Tahoe (USA) hosts multiple Aphanomyces species. Journal of Invertebrate Pathology, 166(February), 107218. doi:10.1016/j.jip.2019.107218
Supporting Partners
The Martis Fund, a collaborative project of Martis Camp landowners, DMB/Highlands Group (the developers of Martis Camp), Mountain Area Preservation (MAP), and Sierra Watch. (See martisfund.org for more information)
The Community Impact Fund, Ashley Quinn Fund & the Foresight Fund held at the Tahoe Community Foundation
Additional thanks to the Tahoe Regional Planning Agency for providing specimens for studio filming
