Signal crayfish © Pete Sibley
North American signal crayfish
Signal crayfish are from North America; they were introduced to Sweden and Finland in 1960s and then distributed throughout Europe (including Britain). They are the most abundant of the introduced crayfish in the UK and now occur in many rivers in England, Wales and Scotland and are spreading along rivers, streams and canals, becoming very abundant. Signal crayfish carry crayfish plague and compete with the White-clawed crayfish for shelter. They have a ferocious appetite and a considerable impact on other freshwater animals as well as damaging our river banks through burrowing.
FACTFILE: Male Signal crayfish are usually up to 16cm in body length (females up to 12cm) and they have smooth with generally bluish-brown to reddish-brown body colour or light- to dark-brown.The best way to recognise them is their distinctive smooth claws with a white-turquoise patch on top of their claws at the joint between the two fingers of the claw, which they open wide to “signal” to other crayfish. The underside of their claws is brick red.
Signal crayfish © D.M. Holdich