The amazing array of coral on the Great Barrier Reef is responsible for many of the bright and beautiful colours that this natural icon is internationally renowned for.
About 600 different types of coral can be found in the Great Barrier Reef Marine Park, and all of them come in a variety of shapes, sizes and colours.
Despite looking like plants, these corals are actually colonies of very small animals called coral polyps which are closely related to jellyfish.
There are two main types of corals — hard and soft.
Hard corals act as building blocks for the Reef. They form when colonies of coral polyps produce limestone skeletons to support themselves.
In most cases, a hard coral consists of hundreds, thousands or even millions of individual coral polyps living together as a colony.They have six (or multiples of six) smooth tentacles.
Common types of hard coral on the Reef include brain coral and staghorn coral.
Soft corals are flexible because they lack a solid skeleton which means they are often mistaken for plants.
Instead they are supported by tiny limestone spike-like structures called spicules.
Apart from their swaying bodies and jelly-like feel, soft corals also have eight tentacles on each polyp. The tentacles have a feathery appearance, whereas hard corals have smooth tentacles.
Soft corals tend to be brightly coloured, with bright pinks and mauves rarely seen in hard corals.
A number of animals, such as different species of fish, prawns and sea slugs, like to make their home in the branches of soft corals. Often, these animals are camouflaged by having the identical colour pattern to the soft coral that they live on.
Soft corals are always in danger of being eaten by other animals such as fish, snails and crustaceans. They fight back by producing chemicals in their tissues that make them distasteful or even poisonous to those animals. Soft corals also have spiky spicules which function like thorns on a rose bush.
How fast do corals grow?
The exact rate at which coral colonies grow varies.
Colonies of boulder coral — which can live up to one thousand years — are likely to be the longest living corals on the Great Barrier Reef. These corals grow in height at about one centimetre each year.
Some branching coral species, such as staghorn corals, can grow up to 30 centimetres each year, while the porites (stony corals with finger-like structures) grow at an annual average of one to three millimetres.
Soft corals grow relatively quickly and may double or triple the size of their colonies over a year.
Where do corals get their colouring from?
Some corals have pigments (fluorescent proteins) in their tissues that give them their orange, yellow, green, blue, red and purple colours.
Others get their golden-brown colour from the algae, called zooxanthallae, that live within their tissues.
What do corals feed on?
Corals use a variety of methods to obtain their food.
Most of their nutrients come from the zooxanthellae. Like plants, zooxanthellae use the sun to make food for themselves and the coral. This is why it is important for corals to live in clear, shallow waters where they can get lots of sunlight.
Corals also eat plankton — these are tiny animals or plants which drift around in the water. Some corals also consume very small fish.
To catch these animals, the corals use their tentacles to paralyse their prey with specialised stinging cells called nematocysts. They can also feed on tiny plants or from the zooxanthellae that live within their cells.