Overview
The Signal Crayfish (Pacifastacus leniusculus) is a large, formidable freshwater crustacean native to the cool, crystal-clear streams and mountain lakes of the Pacific Northwest. Named for the conspicuous bright white or turquoise-tinged patch at the hinge of its massive claws—which looks like a railway signal flag when the claws are raised defensively—it is a cornerstone ecological engineer in northwestern aquatic food webs.
Biology and Physical Characteristics
Pacifastacus leniusculus has a heavy, smooth carapace colored rich chocolate-brown to reddish-olive, lacking the prominent spines of many other crayfish. Its defining anatomical feature is a pair of massive, robust chelae (claws) that are remarkably smooth and broad, decorated with a brilliant snow-white or pale turquoise spot at the dactyl hinge and bright reddish-orange undersides.
Behavior and Ecology
A nocturnal keystone omnivore, the Signal Crayfish hides during the day beneath flat river stones, submerged tree roots, or deep earthen burrows. At dusk, it emerges onto gravel stream beds, processing huge quantities of decaying riparian leaf litter and controlling snail and insect populations. In autumn, mating occurs, and females carry hundreds of dark eggs glued to their swimmerets throughout the winter until spring.
Conservation and Human Impact
Assessed as Least Concern in its native North American range, the Signal Crayfish is thriving and widely harvested for traditional seafood boils. However, its deliberate introduction to Europe in the 1960s proved disastrous: carrying the fungal crayfish plague pathogen (Aphanomyces astaci) to which it is immune, it decimated native European crayfish species.
Interesting Factoid
The Signal Crayfish earns its name from its bright white claw hinges: when threatened by a trout or river otter, it throws its claws wide into the air, flashing its glowing white patches like warning signal flags to deter predators.