Coral reefs are formations of invertebrate animals whose pelagic larvae eventually settle down and become sedentary. They then grow and build up layers by depositing a hard outside surface around their bodies, forming a structure which is characteristic of each species. The compound animal inside puts out feathery tentacles from within the hard surface, to catch passing food in the moving currents.
Unfortunately, corals can be vulnerable to physical impacts, such as from fisheries, and to climatic changes, such as prolonged periods of warmer water.
The two photographs were provided by Derek Tittensor. The first shows a part of the Meso-American reef. It is a complex environment that provides lots of niches and habitat. The second picture shows a reef from the Tropical Pacific, over which two grey reef sharks are patrolling.