Several species of small crab around our shores have the ability to decorate their upper surfaces with plants and animals growing on 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.
Here, the Long-legged Spider Crab has developed a covering of hydroids matching those on the kelp frond on which it is resting.
Common around the UK and recorded from Norway to the Meditteranean.
North Sea - St. Abbs Marine Reserve