Common Hermit Crabs, Pagurus benhardus, are frequently found tussling with each other. This behaviour is caused by their need to change their shells as their soft bodies grow. Although they will inspect - and try on for size - empty shells, many shells are already occupied and they therefore try to eject the occupant. Hermit crabs are very vulnerable as they move from one shell to another and the switch is accomplished in a blink of the eye.
St. Abbs Marine Reserve, Berwickshire. E. Scotland