Invertebrates
Signal Crayfish
Pacifastacus leniusculus
In a lowland English river, a signal crayfish crawls out of a burrow in the bank, its big claws marked with the pale patch that gave it its name.
Overview
The signal crayfish, Pacifastacus leniusculus, is a large freshwater crayfish native to north-western North America and now widespread and invasive across Europe. Its size, burrowing, and carriage of crayfish plague make it one of the most damaging freshwater invaders on the continent.
A farmed crayfish that escaped
Brought to Europe to be farmed, the signal crayfish escaped into rivers and spread, replacing the native crayfish whose populations it helped to kill.
Status
Least Concern in its native range, and invasive in Europe.
Signal Crayfish: common questions
Why is it called the signal crayfish?
It has a white to bluish-white patch at the joint of each large claw, which looks like a signal.
Why is the signal crayfish a problem?
It carries crayfish plague, which kills native European crayfish, and it also digs into river banks and eats eggs and young of other animals.
How did it reach Europe?
It was introduced from the 1960s to be farmed for food, and escaped into the wild.
Where is the signal crayfish native?
It is native to rivers and lakes of north-western North America.