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The Indonesian/Philippines archipelago has the world’s greatest concentration of reefs and the greatest coral diversity. Other area of reef concentration are the Great Barrier Reef of Australia, the Red Sea and the Caribbean, the latter having a much lower diversity than all major Indo-Pacific regions. Distribution of the world’s coral reefs (red). Blue areas indicate areas in which zooxanthellate corals occur. Distribution of the world’s coral reefs (red). Blue areas indicate areas in which zooxanthellate corals occur. The reefs and coral faunas of geological intervals are a combination of inheritance from the past and the outcome of more contemporary events. The distribution of modern reefs are an inheritance of the Tethys-dominated world of the Eocene, subsequently modified by the obliteration of both the Tethys Seaway (Miocene) and the Central American Seaway (Pliocene), see above.

Reefs and coral distributions

It is true that without corals there can be no coral reefs, however it is not true that without reefs there are no corals. Rather the opposite: there are at least as many corals (excluding azooxanthellate species) growing in environments that will not support reef development as there are in environments that will. This is because of the constraints of temperature, light availability, sediment load, depth and (lack of) turbulence, all of which may act independently or in various combinations to prevent reef growth. These environmental constraints clearly interlink to produce habitats in which corals might flourish yet do not produce skeletons quickly enough to overcome erosion, or where encrusting coralline algae do not grow. Without the cementing action of algae, coral debris remains unconsolidated and can be swept away by storm waves. Many places where corals are abundant illustrate this; for example, the sea off the northern coast of central Australia is shallow and moderately turbid. There are no reefs, yet 60% of all Great Barrier Reef species grow there, some forming colonies as big and healthy as those on any reef. Long-term ecological issues aside, almost all species of Indo-Pacific corals grow as well in non-reef habitats as they do on reefs.

Reefs and coral diversity

These is a widely held misconception that reef growth is somehow dependent on coral diversity, probably stemming from the observation that most reefs occur where coral diversity is at least moderately high. However, reefs of the Caribbean have a lower diversity than most Indo-Pacific reefs, and well-formed reefs of the Abrolhos Islands of Brazil have less than 20 species. Clipperton Atoll in the far eastern Pacific has actively growing reefs which provide significant habitat diversity yet only seven species of coral occur there, just three of which contribute to reef-building.

J.E.N. Veron