Several species of small crab around our shores have the ability to decorate their upper surfaces with plants and animals growing on the rocks of their main habitats. Over time this "planted" material grows and thus provides excellent camouflage against passing large fish such as cod and pollack. However, all crabs periodically have to change their shells as they grow so at that stage, the camouflage is lost, leaving the animal much more vulnerable until it can develop a new covering.
Commonly referred to as a decorator crab, this particular species, the Great Spider Crab, Hyas areneus, is common around the UK but less so in the south west.
See the following photograph for another crab that behaves similarly.
North Sea - St. Abbs Marine Reserve