Spinecheek anemonefish (Premnas biaculeatus) in its host anemone. This fish is found on the coral reefs of the tropical Indo-Pacific region. It lives in the stinging tentacles of a sea anemone, to which it is immune. This affords it protection against predators, which would be stung by the anemone. The fish picks parasites from its host anemone, thus forming a symbiotic (mutually beneficial) relationship with it. The spinecheek anemonefish can reach a length of over 16 centimetres. It takes its name from a sharp spike on its gill-covering (operculum), which deters predators. It is sometimes sold in the aquarium trade. Photographed off Indonesian Borneo.