Coral What is a Coral?

Corals are tiny sea creatures know as polyps. Colonies made up of millions upon millions of individual polyps form what we know as coral reefs.

These coral reefs are natural underwater ecosystems built up from the calcified skeletons of dead coral polyps. Only the topmost layers are alive. Coral reefs are home to some of the most diverse ecosystems in the world.

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Coral - Description What is a Coral?

A coral polyp has a soft tubular body with tentacles surrounding its mouth at the upper end. It attaches itself to a hard surface or the dead skeletons of other polyps and secretes a hard outer skeleton of limestone (calcium carbonate) to protect itself.

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.

Coral polyps are closely related to jellyfish and sea anemones.

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Types of Coral Hard Coral & Soft Coral

There are two main types of coral — hard coral and soft coral. 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 its 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 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.

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Coral Colouring How Does a Coral Reef Get Its Colour?

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 use photosynthesis to produce glucose, glycerol, and amino acids which it shares with the coral. It has been estimated that the zooxanthellae share as much as 90% of this material with the coral. This mutual relationship is vital for a healthy coral reef.

Not all coral colour is due to the presence of zooxanthellae. Some corals that usually live closer to the ocean's surface have their natural pigmentation to protect them from the damages of direct sunlight. These corals generally 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.

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Coral - Habitat Where do Corals Live?

Coral reefs occupy approximately 284,300 sq km of the earth’s oceans. The Indo-Pacific region from the Red Sea to the Pacific Ocean accounts for nearly 92% of all coral in the world. The Great Barrier Reef off the north-eastern coast of Australia is the largest and most diverse coral reef in the world.

Reefs are more abundant in areas that are subject to strong wave action. Waves carry food, nutrients, and oxygen to the reef; distribute coral larvae; and prevent sediment from settling on the coral reef.

The optimum water temperature for coral is 20–28 °C. Few corals are found in waters with temperature below 18 °C.

Reef-building corals grow to depths of up to 50 meters where sunlight penetrates. Some deep-water corals are found in colder latitudes, but very little is know about them.

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Coral - Diet What Do Corals Eat?

Shallow-water 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 coral 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.

Deep-sea corals live in colder, deeper waters and don't have zooxanthellae. They capture and eat plankton and organic matter.

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Coral - Reproduction Coral Spawning

Corals can be either male or female, but 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.

There are a number of ways coral propagate themselves:

• 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 larva 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 fertilisation, 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.

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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.

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Natural Threats to Coral Reefs What Endangers a Coral Reef?

Coral is very sensitive to changes in temperature, nutrient and water quality. Just a 1-2 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.

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.

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.

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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, then it will slowly starves to death.

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 fertiliser and pesticide drain-off.

Coral bleaching has been a naturally occurring events as the earth's ocean temperature fluctuates 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 scientists believe this is a result of global warming caused by humans.

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 seawater 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.

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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.

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Environmental Threats to Coral Reefs

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).

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.

Animals that live in the Great Barrier Reef

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