The term, "twilight zone" represents a transition from a region that receives sunlight during the daylight hours, to a region that remains in perpetual darkness. The coral-reef twilight zone is roughly defined as coral-reef habitat at depths between about 200 feet (60 meters) and 500 feet (150 meters). The upper limit represents the approximate maximum depth to which stony corals tend to dominate the reef structure, and the lower limit represents the maximum depth at which significant photosynthesis occurs (the maximum depth to which the living coral reef extends).