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| Acropora species |
A reef is mostly made of Calcium Carbonate, the majority of which is the old skeletons of countless generations of hard corals,
built up over many hundreds of years. This makes the hard coral arguably the most important class of corals for the continued survival of a reef.
You could be excused for imagining that a hard coral is a single animal. In truth they are colonies of individual animals. Each animal, called a polyp, lives in its own calcium carbonate skeleton called a corallite. Each successive generation of polyps living on the skeleton build up of the previous generations. This reef building abilty gives hard corals the label hermatypic.
All hard corals are to a greater or lesser extent predatory, catching food with their tentacles that are lined with stinging cells. However the majority of hard corals have also formed a symbiotic relationship (beneficial to both parties) with zooxanthellae, single-celled algae that live within their tissues. The algae carry out photosynthesis sharing the sugars and oxygen produced with the coral polyp. The coral returns the favour by providing protection, useful waste products like carbon dioxide and nitate plus simple minerals.
These corals with a symbiotic relationship make up the majority of hard corals and live close to the surface, where there is plenty of sunlight, . The extra source of food enables them to grow faster, than hard corals that don't have a symbiotic relationship (called ahermatypes) arguably making them the main reef builders. This is why the term hermatypic is often used exclusively to describe hard corals containing zooxanthellae
It should be noted that many other animals also produce calcium carbonate skeletons, including some soft corals.