What is "signal crayfish"

n. An American crayfish, ''Pacifastacus leniusculus'', having a large claw that it waves to attract a mate.

The signal crayfish, Pacifastacus leniusculus, is a North American species of crayfish. It was introduced to Europe in the 1960s to supplement the Scandinavian Astacus astacus fisheries, which were being damaged by crayfish plague, but the imports turned out to be a carrier of that disease. The signal crayfish is now considered an invasive species across Europe and Japan, ousting native species there.