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
Deep-water coral than tropical corals because there are no zooxanthellae to feed them. "Lophelia" has a linear polyp extension of about per year. By contrast, branching shallow-water corals, such as "Acropora", may exceed 10–20 cm/yr. Reef structure growth estimates are about per year. Scientists have also found "Lophelia" colonies on oil installations in the North Sea. Using coral age-dating methods, scientists have estimated that some living deep-water corals date back at least 10,000 years. Deep-water corals use nematocysts on their tentacles to stun prey. Deep-water corals feed on zooplankton, crustaceans and even krill. Coral can reproduce sexually or asexually. In asexual reproduction