The Swimming Crab, Liocarcinus depurator, is present all around the UK and is found on sand or gravel. Its rear legs are modified into flat paddles which it beats up and down to take off from the bottom and move through the water extremely rapidly. The shell is broader than the length - maybe 5cms by 4cms and is a reddish-brown. A good method of identification of larger specimens is to check the rear paddles which are somewhat violet in colour. Often seen clasping in pairs as part of the mating procedure. Also now being referred to as the Harbour Crab.
St. Abbs Marine Reserve, North Sea