Their hard calcareous skeletons are produced by the polyps themselves. Each polyp in a colony is suspended inside a tiny cup of skeleton and is connected by a thin layer of tissue which overlies the surface of the colony. Small single-celled algae called zooxanthellae are of great benefit to corals. As these nutrients are produced via photosynthesis the corals are able to build their skeletons much faster in the light than in the dark. Corals also possess chemical compounds which prevent them from being affected by ultraviolet radiation when exposed at low tide or d u ring prolonged periods in very shallow water.
Corals reproduce in several different ways.
Sexual reproduction also occurs. In some corals the eggs are fertilised internally. These are then brooded within the body of the parent and at the appropriate time are released as planulae. To ensure that sufficient fertilisation takes place many corals on the Great Barrier Reef spawn together on one night of the year. Whilst corals occur throughout the world they are most abundant in the tropics where water temperatures are highest.