The reef has great species diversity with a high level of endemism. Many groups have been under sampled and insufficiently studied, especially when considering hard bottoms of the intermediate coral reefs and external slopes of the barrier reef. This diversity includes oceanic and continental reefs forming islands, atolls, uplifted reefs, immerged reefs, fringing reefs, barrier reefs, patch reefs and shallow or deep lagoons, is home to endangered dugongs ("Dugong dugon") and is an important nesting site for green sea turtle ("Chelonia mydas"). New Caledonia has a remarkable marine fauna due to the abundance of relic organisms from the Mesozoic., for example some sponges of the Lithistideae and the Tetractinellideae which are considered living fossils due to their closeness to Cretaceous species. The endemic mollusc "Nautilus macromphalus" is one of only four "Nautilus" species known in the world, the only living group of cephalopods with an external shell.. This mollusc seems to be identical to its Paleozoic ancestor. "Cephalodiscus graptolitoides", described in 1993, is also considered a living fossil member of the graptolites, previously thought to be extinct for over 300 million years.