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3(d) Structure, Classification and Function of Corals

Uniqueness: The Great Barrier Reef is not just the largest coral system in the world, it is the one thought to have the highest biodiversity. That is, more kinds plants and animals than any other ecosystem.
Since the early Aborigines first saw it some 40 000 years ago, people have been using it, studying it, and investigating the different life forms in it.  The Reef extends for 2300 kilometres. There is nothing quite like it in size, rarity, complexity and beauty and it is one of the Seven Natural Wonders of the World. That is why it has been declared a World Heritage Site and a Marine National Park, under the governing body of the Great Barrier Reef Marine Park Authority (GBRMPA).

Structure: A coral reef is composed of calcium carbonate, or limestone. This is absorbed from the water by colonies of coral polyps and coralline algae. Most the underlying foundation of the reef is dead, made up of layer upon layer of coral skeletons. The living reef is built over the top of this, by tiny coral polyps adding new limestone to the massive base structure.

The polyps make skeletons (or corallites) of calcium carbonate around themselves. The beautiful colour of corals comes from the colourful tentacles of the coral polyps and the zooxanthellae algae that live in the tissues of many species.

close-up photograph showing the tube-like corallites of the bottle-brush staghorn coral Acropora echinate
A close-up photograph showing the
tube-like corallites of the bottle-brush staghorn coral Acropora echinate, taken near the Ribbon Reefs, far northern Great Barrier Reef. The coral polyps live within these corallites and are generally only seen extended at night. Image: Dr. Paul Muir, QM.

Classification: Though a coral polyp looks like a plant, it’s really an animal, or rather, a colony of animals, and is classified into the Phylum Cnidaria (also called Phylum Coelenterata). Phylum Cnidaria is further subdivided into three classes: the jellyfish (Class Scyphozoa); the hydrozoans (Class Hydrozoa); and the corals and sea anemones (Class Anthozoa).

Corals are an ancient group having a simple, radially-symmetrical body with a single opening that serves as both a mouth and anus. The body is made up of two layers of cells, separated by a jelly-like layer with no internal organs. Corals possess specialised stinging cells called nematocysts on their retractable tentacles that are used to catch food.

Obtaining Food: Colder climates tend to experience major oceanic upwelling which brings nutrients to the surface.  This helps the growth of plankton. Compared with these plankton-rich waters, tropical seas do not provide plentiful feeding grounds for small carnivores, such as coral polyps. For some time, people wondered how such small creatures could flourish and build up such spectacular reefs. The mystery was solved when large numbers of microscopic algae were found living in their tissues. These algae, called zooxanthellae, live symbiotically within hermatypic corals.

Zooxanthellae absorb the nitrogen wastes produced by the coral. The algae use these nutrients together with sunlight and carbon dioxide to make sugars in the process of photosynthesis. Some of this food is absorbed by the coral polyp so the coral benefits in having food provided and having its waste products metabolised by the algae. The sugars produced by the zooxanthellae make up 98 per cent of the food for the coral. The algae benefit as the coral provide a safe, sunny place to live. This type of association is called mutualism. Therefore, without having to do any work at all, the coral is kept clean and well fed, and the zooxanthellae with their brilliant reds, oranges and browns, give corals their beautiful colour. External stressors such as increased temperature and acidity lead to zooxanthellae being shed from the coral, resulting in coral bleaching.

Giant Clam from Hayman Island with zooxanthellae algae living in the mantle tissues of the clam. Image: Pierre Pouliquin, Creative Commons.
Giant Clam from Hayman Island

As well as providing shelter and food for a wide range of invertebrate and fish species, corals may have some direct commercial value. Several unique chemical compounds have been isolated from them. These have potential application in the development of new therapeutic drugs and in industrial processes.

Queensland Museum scientists at the Museum of Tropical Queensland campus in Townsville, AIMS (Australian Institute of Marine Science), Reef HQ Aquarium Townsville (Reef Head-Quarters Aquarium), and GBRMPA (Great Barrier Reef Marine Park Authority), are studying the corals of sub-tropical and inshore Queensland to provide a baseline for monitoring changes in coral health and distribution, predicted to result from global warming and climate change.

Much of the reef is made up of Acropora corals. Here is a wide variety, including tables, bushes, plates, and branching thickets as seen at Addu Atoll in the Maldives. Image: Dr. Paul Muir, QM.
Addu Atoll in the Maldives


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