Coral heads are made up of many small animals called polyps. These animals live in shells. They can be as small as the head of a pin, or as wide as 30cm.
Corals need sunlight to make food. They only live as far down in the sea as the sunlight can reach. This means they do not live below 50 metres. Corals do not make their own food. There is a little organism that lives inside them that feeds them. These little organisms also give the corals their colour. The clearer the water, the faster thecorals grow. This is because the sunlight can shine better through clear water.
Types of corals
There are more than 1,000 types of corals. Some look like wrinkled brains, table tops, cabbages or reindeer horns. They wave their tentacles in the water to catch food.
Coral reefs are thought to cover 284,300 square kilometres. Most of these reefs, or 91.9 per cent, are found in the Indo-Pacific region. Southeast Asia - part of the Indo- Pacific region - has about 32 per cent. The largest single reef is the Great Barrier Reef off Australia. It is 2,000 kilometres long.
Types of reefs
There are three main types of coral reefs: barrier reefs, fringing reefs and atolls. They all take many, many years to form. The Great Barrier Reef is about 20,000 years old. A fringing reef can take 10,000 years to form. An atoll can take up to 30 million years to form.
Coral reefs are very valuable for tourism and for fishing. Fish like to live near the reefs. The reefs also protect small islands from big waves.
Reefs are harmed by pollution, coral mining and overfishing. Rising sea levels also kill reefs. They need shallow water so the sun can reach the coral. The Agriculture, Fisheries and Conservation Department has a coral watch programme. Visit www.afcd.gov.hk  for more information.
Now do this
1How many main types of coral reefs are there?
2 Besides water, what does coral need to survive?
3 Which of the reefs takes the longest time to form?
a. the barrier reef
b. the fringe reef
c. the atoll