Signal Crayfish {Pacifastacus leniusculus}, a species introduced to the UK from North America. Signal Crayfish can transmit a fungal disease 'crayfish plague' to native Birtish white-clawed crayfish, posing a serious threat to the species. Even if they don't carry the crayfish plague, the signal crayish are far larger and more aggressive than the native species, and can easily out-compete them. Photographed on a white background, Derbyshire, UK.