In addition to the calcium carbonate structure that the diverse corals species provide, numerous species of attached sponges and algae help to give the reefs their form. Tropical Coral reefs are fragile and diverse habitats that exist in sunlit waters along continental and island margins, with diversity greatest near the Equator.
Southeast Asian seas support over 450 species of scleractinian corals, the western Indian Ocean around 200, and the Caribbean only 50 species. Variation in reef fish and non-coral invertebrate diversity follosw a similar biogeographic pattern. Overall, the coral reef communities of Southeast Asian seas are the most diverse in the world, with the Sulu, Sulawesi, Banda, and Coral sea ecoregions being the most diverse on Earth.
The largest barrier reef in the world is the Great Barrier Reef. Other world-class barrier reefs include the barrier reefs of New Caledonia, the Mesoamerican barrier reef, and the large barrier reefs of Fiji. The largest coral atoll complexes occur in the Maldive-Lakshadweep ecoregion of the central Indian Ocean and in Micronesia.