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Great Barrier Reef
Coral Facts
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  Great Barrier Reef - Coral Facts

Coral Facts  

The world's first coral reefs occurred about 500 million years ago, and the first closeAerial Picture of a coral reef relatives of modern corals developed in southern Europe about 230 million years ago. By comparison, the Great Barrier Reef is relatively young at just 500,000 years old. The current reef's structure is much younger at less than around 8,000 years old.

Most modern reefs have formed on hard surfaces in the ocean, such as a base of an old reef that died during a period when sea level was lower, or the edge of a rocky island. Depending on how they start out, several types of reefs can form. Some coral reefs form in the deep ocean and are called atolls. The theories on how coral reefs form were first put forward by Charles Darwin (of The Origin of Species fame) who proposed that atolls form around the edges of high volcanic islands that gradually submerge beneath the sea with changes in sea level or subsidence of the land. Thus an atoll starts life as a fringing reef, then becomes more of a ring growing on the shrinking land-mass, until the land disappears and just the coral circle remains. In some cases, the coral growth is unable to keep pace with the sinking island, and sunken dead reefs have been found

Class: Anthozoa [includes corals, anemones and sea pens]

Habitat: Coral reefs are found in shallow water, ranging to depths of 60 m. Some species prefer either cooler temperate water while others are found along tropical reefs, such as the Great Barrier Reef, with waters ranging in temperatures from 18 - 33 °C. 
Living in colonies: They generally occur in large numbers as colonies of individual polyps linked by tissue. Resources, such as food, are then shared amongst the individuals in the colony. 

Coral Size: Individual polyps range from 3 - 56 mm in diameter or height; while colony size varies from 75 mm -1500 mm (1.5 m) in width, height or length. 

Some corals have a mutualistic relationship with algae called zooxanthellae. A mutualistic relationship is one where both parties benefit from their partnership. The algae use sunlight and the polyp’s waste products to make oxygen and food. These substances leak into the surrounding tissues of the polyp and can provide up to 98% of the polyp’s dietary requirements. These corals are found in shallow water, as they require sunlight to survive. They are generally fawn, brown or green in colour, due to the yellow-brown colour of the zooxanthellae. 

Stinging cells: All Cnidarians have characteristic stinging cells called nematocysts in the tentacles and body wall. Each nematocyst cell contains a coiled thread under pressure, which is ejected from the cell when triggered by touch. These stinging cells are used for catching prey and for defense, some having barbed ends connected to poison sacs, while others are sticky. 


Coral polyps: Coral reefs consist of hundreds and thousands of soft-bodied, invertebrate animals, having no backbone. These animals are called coral polyps. The individual polyp is radially symmetrical and has a tubular body with tentacles surrounding the mouth at the upper end. Each polyp’s body wall consists of two layers of cells, an outer layer called the ectoderm and an inner endoderm  layer. A gelatinous material called mesogloea is found in between these two layers. 

Hard corals: Hard corals build reefs by growing atop the stony skeletons of previous coral colonies. They consist of limestone cases made by coral polyps extracting calcium from seawater. These limestone cases form a ‘house’ for the coral polyp, consisting of a floor, outer walls and a number of internal partitions. Inside corals’ clear outer tissues live microscopic algae, which transform sunlight into sugars through Photosynthesis The hosts help themselves to some of the sugars and gain some colour through the process.

Reproduction: Corals reproduce in two ways: asexually and sexually. Coral Spawning Some corals divide to form new individuals. This is known as asexual reproduction. Sexual reproduction takes place as mass spawning, where polyps release millions of eggs and sperm. Polyps are either male or female or both male and female. After the eggs and sperm are released, they float to the surface. The fertilised eggs that escape predation by other animals hatch into larvae and drift with the plankton. The tiny percent that survive and settle on the reef then begin new coral colonies. 


Major Natural Predator:

The Crown-of-Thorns Seastar was once an animal of great controversy causing terrible damage to the coral reefs of the Great Barrier Reef. Many believed that this seastar was a pest species invading the coral reef habitat predating on and killing corals in great numbers. Research has indicated that the Crown-of-Thorns Seastar is only found in this habitatPicture of Crown of Thorns Starfish and a native species to Australian waters. The role that the sea star plays by eating coral polyps forms a population control, making more room for new coral reefs to form. Plague proportions are thought to coincide with rainfall and increases in nutrients from rivers during floods, and often occur cyclically every 17 years. Further scientific studies are still in progress to determine whether these plaques can be controlled.

Scuba diving & snorkeling From Cairns and Port Douglas are the most  sought after outing onto the reef. With so much to see like ship wrecks, coral gardens, the thousands of species of marine animals and under water canyons. Cairns Great Barrier Reef has many qualified scuba diving courses available and are amongst the safest in the world, with strict guidelines that trainers must follow. You can easily become a certified scuba diver and enjoy all the underwater attractions that the reef has to offer. 

Information Sources: Australian Marine Life, by G.J.Edgar
Steve Parish: Amazing Facts about Australian Marine Life Encyclopedia Britannica and




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