Coral reef A fringing reef forms around an extinct volcanic island as the island and ocean floor subsides. As the subsidence continues, the fringing reef becomes a barrier reef and ultimately an atoll reef. Darwin predicted that underneath each lagoon would be a bedrock base, the remains of the original volcano. Subsequent research supported this hypothesis. Darwin's theory followed from his understanding that coral polyps thrive in the tropics where the water is agitated, but can only live within a limited depth range, starting just below low tide. Where the level of the underlying earth allows, the corals grow around the coast
Coral reef fish too late. The clown triggerfish has strong jaws for crushing and eating sea urchins, crustaceans and hard-shell molluscs. Its ventral (lower) surface has large, white spots on a dark background, and its dorsal (upper) surface has black spots on yellow. This is a form of countershading: from below, the white spots look like the lighted surface of the water above; and from above, the fish blends more with the coral reef below. The brightly painted yellow mouth may deter potential predators. Many reef fish species have evolved different feeding strategies accompanied by specialized mouths, jaws and teeth particularly suited to