Coral reef islands and atolls. The reef face is the zone above the reef floor or the reef drop-off. This zone is often the reef's most diverse area. Coral and calcareous algae provide complex habitats and areas that offer protection, such as cracks and crevices. Invertebrates and epiphytic algae provide much of the food for other organisms. A common feature on this forereef zone is spur and groove formations that serve to transport sediment downslope. The reef flat is the sandy-bottomed flat, which can be behind the main reef, containing chunks of coral. This zone may border a lagoon and serve as
Open brain coral lobed. They typically have bilateral symmetry. During the day when the polyp is closed, the coral is covered by a mantle that extends beyond the skeleton, but can retract when disturbed. Polyps and mantle are very fleshy. Colonies can be blue, green, yellow, brown, and are often vibrantly colored. The open brain coral is known to host a species of gall crab, "Lithoscaptus" "semperi." Open brain corals can be found throughout the Indo-Pacific, from the Red Sea to New Caledonia. They are found up to a maximum depth of 40 meters. Open brain corals are less common directly in coral