Coral Reef What is a Coral Reef?

Coral reefs are natural underwater ecosystems built from calcium carbonate structures made from the accumulation of the bodies of billions coral polyps. In fact, most of the coral structure is made up the calcified skeletons of dead coral polyps. Only the topmost layers are alive. With the right amount of sunlight, water conditions and temperature, and over long periods of time, corals form the structures of the beautiful reefs we see today.

What is a Coral? Is Coral a Plant?

While corals may appear like plants, they are actually made up of colonies of millions upon millions individual animals known as coral polyps. An individual polyp has a tubular body with tentacles surrounding its mouth at the upper end.

Corals can live individually or on large colonies containing thousands of polyps. The brain coral, for example, is made up of thousands of little polyps no bigger than the size of a pinhead of a straight pin.

Coral polyps are closely related to jellyfish and sea anemones.

Types of Coral Hard Coral & Soft Coral

There are two main types of coral — hard coral and soft coral and these come in a multitude of different shapes, sizes and colours.

Hard Coral

Hard coral, also known as stony coral, are mostly responsible for building coral reefs. They are referred to as hermatypic because they extract calcium carbonate from seawater to create a hard, durable exoskeleton that protects their soft, sac-like bodies. Another key characteristic of hard coral is that they have six (or multiples of six) smooth tentacles. The most common types of hard coral are brain coral and stag-horn coral. When hard corals die their hard carbonate exoskeletons are left behind. New coral polyps grow on top of the skeletons of their ancestors, and over time the process repeats itself over and over again, with each generation adding their own exoskeleton to the existing coral structure. These accumulate over long periods of time to form large solid structures referred to as a coral reef.

Soft Coral

Soft corals, such as the fan coral, are so named because they are flexible and lack a solid skeleton like that of hard corals. Instead, they have spike-like structures called spicules with which they support themselves. They tend to be more spectacular and more brightly coloured, with bright pinks and mauves rarely seen in hard coral. Soft corals have eight tentacles (as opposed to the six in hard corals) and generally have a feathery, spongy texture.

Many different species of fish, prawns and sea slugs make their homes amongst the branches of soft corals. Some even camouflage themselves by adapting the colours and patterns of their host coral thus making it harder for predators to detect them.

Soft corals, lacking the hard outer casing of the hard corals, are more susceptible to being eaten by other animals. They protect themselves by producing toxic chemicals in their tissues that make them unappetising or even poisonous to other animals. Their spiky spicules also act like thorns to deter attackers. Soft corals are relatively fast-growing and may double or triple the size in just a year.

How Does a Coral Reef Get Its Colour? Coral Colouring

Hello Zooxanthellae

Most of the colour in coral is due to the presence of tiny symbiotic algae, called zooxanthellae, which live inside the coral and give them their distinctive shades of colours ranging from yellow to brown. The coral can regulate the population of Zooxanthellae algae it hosts. It does this by controlling the amount of light and nutrients it makes available to the zooxanthellae. The coral and zooxanthellae live in a mutually beneficial relationship. The coral provided the algae with a protective environment and nutrients and in return the zooxanthellae uses photosynthesis to produce glucose, glycerol, and amino acids which it shares with the coral. It has been estimated that the zooxanthellae shares as much as 90% of this material with the coral. This mutual relationship is vital for a healthy coral reef.

Other Colouring Agents

Not all coral colour is due to the presence of zooxanthellae. Some corals which usually live closer to the surface of the ocean, have their own natural pigmentation to protect them from the damages of direct sunlight. These corals usually have bright pink, blue and purple colours.

Why do Corals Look Washed-Out Underwater?

A coral may look 'washed out' and rather drab underwater (when scuba diving for example).  This is because as water depth increases the visible colours in the light spectrum decreases. This causes some corals to look 'washed out'. Exposure to artificial light, like a camera flash, sometimes displays the true vibrant colours of many corals.

Coral Diet What Do Corals Eat?

Coral polyps are usually nocturnal. They stay inside their protective cases during the day and extend their tentacles out to feed in the night.

Whilst corals get most of their nutrients from the zooxanthellae that live within them, they also feed on various small organisms, from microscopic plankton to tiny fish. The polyp's tentacles grab onto their prey and kill them using stinging cells called nematocysts. The tentacles then contract to bring the prey into the stomach. Once digested, the stomach reopens and the waste products are eliminated.

Coral Spawning Video

One of the most spectacular occurrences in nature is coral spawning when large groups of corals collectively release their eggs and sperms. This synchronised release usually occurs after a full moon and when the water has risen to the right temperature. By simultaneously releasing their eggs and sperm the corals increase the possibility of cross-fertilization of their eggs.

Coral Reproduction Coral Spawning

While corals can be either male or female, many corals are hermaphrodites. While cross-fertilization is the predominated method of fertilization, some corals and are capable of self-fertilising their eggs and even cloning themselves.

Means of Reproduction

Spawning - where eggs and sperm are released by coral polyps into the water at the same time to increase the chances of cross fertilization (see video).
Brooding - Here the egg and sperm are fertilised within the parent coral polyp and the resulting baby coral known as a larvae is released in to the surrounding water when it is relatively well developed.
Budding - is where the young polyp grows out of an adult polyp. The new polyp is a clone of its parent.
Parthenogenesis - in this process the egg grows into a new coral without fertilization and the offspring is a clone of the parent.
Coral Bail - is when a piece breaks off from its parent, then regrows any missing body parts to become a fully viable new polyp. It is a clone of its parent.

Formation of New Coral Colony How is a New Coral Reef Formed?

A coral egg, once fertilised, develops into larva called a planula that drifts around in the water until it finds a suitable place to establish itself. It takes only a single coral polyp floating in the sea to attach itself successfully to a suitable surface, such as a rock, to start up a brand new coral colony.

Natural Threats to Coral Reefs What Endangers a Coral Reef?


Coral is very sensitive changes to temperature, nutrient and water quality. Just a 1 or 2 degree change in the water temperature has been known to stress coral and cause coral bleaching. It is feared that global warming may severely impact the reef.

Crown of Thorns

These starfish destroy coral by eating it. It was once thought of as a great risk to the reef but recent research has shown that it is actually native to this habitat and may act as a means of natural population control by eating coral polyps and making room for new coral to form.

Once in about every 17 years or so crown of thorns starfish appear in plague proportion, denuding vast tracts of the reef. It is thought that these outbreaks are related to increased rainfall and nutrient flows from flooded rivers.

Coral Bleaching

The symbiotic algae which usually resides with the coral can sometimes put strain on the host causing it to eject the algae. Mass expulsions of the algae is known as coral bleaching (because the algae contribute to coral's colouration). This ejection seems to increase the polyp's chances of dealing with short-term stress. If the stressful conditions persist, the polyp eventually dies.

Coral Bleaching What is Coral Bleaching?

What is Coral Bleaching?

In times of physical stress, usually caused by environmental factors, the coral may resort to a mass expulsion of its zooxanthellae population. Because it is the zooxanthellae that give most corals their colour; their loss causes the coral's tissues to become transparent revealing its white skeleton. This whitish appearance is referred to as coral bleaching. A coral can survive for about a month without its zooxanthellae as it will slowly starves to death.

Causes of Coral Bleaching

The major cause for coral stress and subsequent mass coral bleaching is increases in sea water temperatures in the coral's environment. To a lesser extent the following can also cause coral bleaching; cyclones, large freshwater inflows from flooded river on land and pollutants originating from human activity such as fertilizer and pesticide drain-off.

Is Coral Bleaching Normal?

Coral bleaching have been naturally occurring events as the earth's ocean temperature fluctuate from time to time. However since 1871 the water temperature in the Great Barrier Reef area, for example, has increased by 0.67 degree Celsius. Some scientist believe this is a result of global warming caused by humans.

Can Reefs Survive Coral Bleaching?

Bleaching events have occurred in the past and over time coral reefs have recovered from such events. Scientist, however, fear that global warming may permanently raise sea water temperatures which would then have catastrophic effects on reefs. It should be kept in mind however that as sea temperatures rise other areas in turn may become more hospitable to coral growth. Sufficient unbiased evidence has not yet become available.

Human Threats to Coral Reefs How Humans Damage Coral Reefs

Coral reefs take thousands of years to form. People can easily damage them by:

• Walking on them
• Dropping anchors on them
• Dragging diving gear over them
• Breaking them and taking them as souvenirs
• Knocking and grounding boats on them

Fortunately strict guidelines are presently enforced on all tour operators to ensure the protection of this vital natural asset.

Environmental Threats to Coral Reefs

Oil Spills and Ships Running Aground

Busy shipping channels traverse the reef and occasionally mishaps occur. Most recently bulk coal carrier Shen Neng 1 ran aground about 70km east of Great Keppel Island damaging its hulk and leaked a quantity of fuel oil into the water (The light blue area in the photograph on the left is caused by sand stirred up by the grounded vessel and not the actual fuel oil which was quickly contained).

Dumping and River Run-offs on Great Barrier Reef

Sediments, nutrients, fertilisers, pesticides, toxic chemicals, sewage, rubbish, detergents, heavy metals and oil run into rivers and out to the Great Barrier Reef, where they can threaten plants and animals on the reef.


Related Article: Animals that live in the Great Barrier Reef

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