Coral is the common collective name for a large class of marine invertebrate polyps with six or eight tentacles. The polyps secrete calcium carbonate which forms an exoskeleton into which the polyp withdraws for protection from predators. While a very few such animals are solitary, the great majority are "colonial" in that they form colonies in which their individual skeleton structures are joined together in a common mass. Colonial corals can survive at great depths, as long as there is a sufficient supply of plankton for food. However, others, commonly called reef-building corals, can only exist in warm, shallow seas.
All reef-building coral species enjoy a symbiotic relationship with species of algae called zooxanthellae. Residing within the coral polyps' tissues, these photosynthetic one-celled plants are the ultimate source of the energy for the polyp to produce and exude calcium carbonate. Their relationship is such that the corals cannot survive without the zooxanthellae and the latter cannot survive without the corals. Over time, coral colonies expand both laterally and vertically, the collective skeletal structures forming reefs. There are three types of reefs: fringing reefs, extending outward directly from the shore; barrier reefs, occurring further offshore with a channel between the reef and the shoreline; and atolls, essentially horseshoe-shaped coral islands enclosing a lagoon. These large structures are not simply communities of minute polyps. They also diversify the shallow water habitat. Plants growing on coral surfaces feed herbivorous fish. The convolutions of coral structure provide safe havens from predators. And some predators (e.g., moray eels) use these structures to hide in while awaiting passing prey.
Reef-building corals and their associated zooxanthellae are particularly sensitive to their environments. They tolerate a sustained temperature range of only 25-29 degrees Celsius. Amino acids in the polyp dictate the upper level of ultra-violet (UV) toleration. When exceeded the colony starts to die. There is also unconfirmed evidence that a chain of biochemical events may cause colony polyps to expel their endo-symbionts, in effect killing themselves. [Coral Reef bleaching]