Modern coral reefs
Modern reefs are not necessarily coral-dominated. Some, forming intertidal structures in the Caribbean and Bermuda, are made by vermetid ‘worms’ (molluscs). There are also widespread subtidal structures dominated by serpulid worms (annelids) and odd reefs everywhere owe their existence to other organisms, especially oysters. However, in terms of quantity, reefs dominated by organisms other than scleractinian corals and their allies (including a few small taxa such as the blue coral Heliopora and the fire coral Millepora) are insignificant and have little in common with coral reefs other than in the material of which they are made. Such structures excluded, it is still necessary to narrow the term ‘coral reef’, for some deep ocean corals form extensive structures, built by one type of coral (Lophelia), which are commonly called reefs. These have none of the characteristics of coral reefs as geological structures (they do not form solid limestone) or as biological structures (they do not depend on photosynthesis nor are they biologically diverse); they warrant a separate name, ‘bioherms’.Although coral reefs are principally made of calcium carbonate derived from coral, corals are incapable of building reefs on their own. Coral skeletons need to be cemented into solid rock by coralline algae. Coralline algae generally have a wider distribution range than corals, but those that cement coral debris into reefs flourish only in shallow, turbulent, well-lit environments and thus it is they as much as corals that determine where highly consolidated reefs grow. They may also have a dominant influence on how fast they grow, or if they grow at all in deeper water.
Although most (90-95%) of terrestrial limestone is derived from reefs built by a variety of animals in shallow marine environments, this by no means reflects the principal source of carbonates on Earth. Around 90% of all today’s marine carbonates are deep-sea sediments derived primarily from plankton, mostly foraminifera and coccolithophores. Due to their deep water location these are rarely uplifted to form geological rock formations on land, rather they are either dissolved in the oceans or are subducted into the Earth’s mantle. Another 5% of carbonates of mixed composition are found on continental slopes. In a few regions these have been consolidated and uplifted or otherwise exposed on land. Only about 5% of all carbonates today are of coral reef origin, although this small proportion includes vast tracts of mountain slopes like the calcareous reefs of Austria, formed from millions of years of accretion. The proportion of total carbonates which are represented by living reefs is probably less than 1%.