Signal crayfish

Pacifastacus leniusculus, commonly known as the signal crayfish, is a species of freshwater crayfish in the family Astacidae, native to western North America. They are thought to have originated from the Columbia River Basin. Physically, they are characterized by a smooth carapace and a distinctive white to pale blue-green patch at the base of the claw joint. Reminiscent of the white flags that signalmen used for directing trains, this light patch is responsible for the species' common name. Due to its heightened aggression and opportunistic polytrophic diet,P. leniusculus has become an established invasive species found across North America, Europe, and Japan, displacing many of the regions’ native species. During the twentieth century, P. leniusculus was intentionally introduced to many regions of Europe to replace declining populations of the native Astacus astacus, which had been affected by outbreaks of crayfish plague. At the time of introduction, it was not widely known that North American crayfish species, including P. leniusculus, commonly act as asymptomatic carriers. As a result, P. leniusculus facilitated the continued spread of crayfish plague across European freshwater ecosystems, continuing to decimate native crayfish.