The Scorpion Spider Crab, Inachus dorsettensis, is often referred to generically as a Decorator Crab. This species and some other types of crab are not particularly robust and are attractive prey to many fish. To counter this they have the ability to secrete a sort of mucous into which they can "plant" invertebrate life that they pluck from the surrounding rocks. This actually grows in situ and can almost envelope the crab eventually, making it very hard to see when it stays still. The whole strategy breaks down however when the crab has grown and needs to change its shell. The new shell is completely clean and the whole process is then repeated.
St. Abbs Marine Reserve, Berwickshire. E. Scotland