Powderblue surgeonfish (Acanthurus leucosternon) grazing on algae on a coral reef. This fish is found on shallow coral reefs in the tropical Indian Ocean. It feeds almost exclusively on algae and can reach over 50 centimetres in length. The name surgeonfish arises due to the presence of a sharp spine (yellow) by the base of the tail, which is used to deter predators. Photographed in the Andaman Sea, Thailand.