Contained in neritic zone, in warm tropical waters, Coral Reefs constitue a conspicuous and distinctive biome.  The currents and waves constantly renew their nutrient supply and the photic zone that extent to the ocean floor allows photsynthesis.  The reefs are dominated by the structure of the corals that occupy it.  Corals are a diverse group of Cnidarians that secrete a calcitic hard external skeleton.  The varying shape of the reef systems form a substrate on which the newer corals and algae  grow.  The coral animals themselves are preditorial but require to be within the photic zone due their symbiotic relantionship with dionoflagellates.  They also rely on the photic zone to supply them with micro-organism and organic matter, which themselves are photosynthetic organisms.  the reef systems are the most diverse and productive biome on the planet.  This is dictated by the immense variety of micro-organisms, invertebrates and fish who m live among the reefs.  Found in the biome are herbivores which are predominately snails, sea urchins, and fishes.  The herbivores are in turn cosummed by predetory octopus and carnivorous fishes.  Reef communities are very old and are threatenned continuously by human encroachment.