Signal Crayfish

Signal Crayfish originate form eastern North America; they were introduced to Sweden and Finland in 1960s as a potential food source and then spread throughout Europe reaching Britain in 1976. They're now the most abundant of the introduced crayfish in the UK and now occur in many rivers in England, Wales and Scotland including the River Bure, where Pete and Chris were sampling invertebrates, and captured this one. 

Signal Crayfish carry crayfish plague and compete with our native 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. 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 (see extra photos), which they open wide to “signal” to other crayfish. The underside of their claws is brick red, illustrated well in this image.  

As Signal Crayfish are listed in Section 9 of the Wildlife and Countryside Act, it's illegal to return them to the river, even when they've been caught accidentally. The law says that they should be killed, but there seems to be some disagreement as to the most humane way to do this, although freezing is suggested by the Environment Agency. As few people have a freezer to hand on the river bank, these were brought back home, but I couldn't resist a photo-opportunity before they meet their fate. I believe they're very good eating, but unfortunately we don't  have enough for a decent meal!