Definition of signal crayfish in US English:

signal crayfish

noun

  • A freshwater crayfish, Pacifastacus leniusculus (family Astacidae), with a bright patch on the underside of each claw, native to the western United States and introduced in Europe and Japan, where it now threatens native wildlife.

Origin

1980s; earliest use found in Ambio. After Swedish signalkräfta.