Signal Crayfish Pacifastacus leniusculus

The Signal Crayfish is native to freshwater of North America west of the Rocky Mountains. It has been introduced to several other countries where it has decimated populations of native crayfish both by out-competing them for resources and by carrying the fatal crayfish plague, to which it is largely immune. It also preys on frogs, newts, fish and invertebrates. It is classed as an undesirable invasive alien species in much of Europe, including the UK where it threatens the native White-clawed Crayfish, and in Japan and even California.

The body is dark brown on top and reddish brown underneath, with red undersides to the large claws, which have a white or turquoise marking on the "finger" joint after which the Signal Crayfish is named. It grows typically to some 10cm but can reach 20cm long.