What is a Barrier Coral Reef

Over four hundred (or about 1/3) of coral species on earth are to be found in the Great Barrier Reef World Heritage Area. The visible part of the barrier reef coral is just a thin layer of living coral believed to be about 8000-years-old, which has accumulated over the top of dead corals and algae over the past 1/2 million years years.

The Great Barrier Reef coral contains many different species.. Hard corals are formed when a limestone covered poly divides leaving behind its hard outer skeleton. In the first few years coral growth is slow, with most hard corals growing at a rate of 1-1.5cm a year.

Hard corals are one of the main building blocks that are involved in the structure of the seventh natural wonder of the world - the Great Barrier Reef. The barrier reef corals found on Queensland reefs not only provide a habitat for all the flora and fauna species but allow divers and snorkellers the opportunity to experience an array of colour, shape and movement that is not replicated anywhere else in the world.

Hard corals can be further separated into 2 sub-groups. The zooxanthellate (reef-building or hermatypic) corals depend on zooxanthellae algae for nutrients. These shallow water corals perform a vital reef-building function. They are generally found in clear water less than 50 metres deep as the algae requires light for the essential process of photosynthesis.

Azooxanthellate which are the deep water or ahermatypic corals do not contain zooxanthellae and therefore acquire nutrition solely from filtering plankton from seawater. Azooxanthellate are isolated, solitary or colonial forms and they rarely build big structures. Many of these coral species are present in non-reef environments such as in coastal areas like Moreton Bay in Queensland.

Exotic coral structures formed like staghorns, tabletops, fans and brains create an environment for an innumerable array of fish, enchinoderms, molluscs and microoganisms such as algae and plankton.

favia coral

tubipora coral

pineapple red coral

Soft corals also contribute to the reef's solid structure,though less significantly, but nevertheless make a vital contribution to this delicately-balanced ecosystem. In the same way as hard corals of the reef, most soft coral species are in a symbiotic relationship with microscopic single-celled algae (zooxanthellae) that reside within their tissues, conveying food to the host coral.

This crucial relationship only flourishes in clear warm shallow waters with temperatures of above 18 C. Significant temperature changes can result in the coral expelling the algae, resulting in the death of the polyps or coral bleaching.

Coral spawning or reproduction occurs annually and at night in late spring or early summer. During this amazing event, egg-engorged corals simultaneously release masses of pink eggs and sperm into the sea which then becomes free-floating larvae. It is not possible to predict exactly when coral spawning will occur but it is thought to be linked to the phases of the moon and water temperature.

The Crown of Thorns Starfish is the coral's most voracious natural predator. Outbreaks of these starfish have in the past stripped affected reefs of nearly all living coral. Fortunately new corals re-establish after an outbreak and the outbreaks are thought to be a natural phenomenon.

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