Such marine formations consist of the cemented skeletons of living organisms, known as hermatypes (reef-forming) and their dead remains.
However, these reefs were built by different hermatypes from those we are familiar with in present-day coral reefs.
Today, although some coral species are found outside the described boundaries, they are not hermatypes since they survive and grow as isolated colonies that never develop into continuous, stable reef structures.
Although some coral species such as the deep water Red Sea coral, Leptoseris fragilis, may live at depths over 325 ft (100 m), they are not hermatypes, since they do not form reefs.
They can be classified by their degree of association with the reef, the trophic level they occupy, whether they are builders (hermatypes) or not, whether they contribute to the workings of the system, or whether they are bioeroders that attack it.
The Octocorallians as a group, unlike the Hexacorallians, do not have calcium carbonate skeletons and, with few exceptions, are not hermatypes. Their skeletal elements may either be characteristic, flexible individual spicules, or hard, horn-like skeletons used in some cases as jewellery, especially the red coral, Corallium rubrum and the black corals (Antipathes).
The various soft corals compete with Hexacorallians and other hermatypes for suitable substrates for settling.