Coral is hard, with lots of tiny holes.
It is in different shapes.
It is made by tiny animals called polyps.
Thousands of them build big coral reefs in the sea.
Thousands of colourful fish and other creatures live around a reef.
Coral polyps are invertebrates (creatures without backbones), related to jellyfish and sea anemones.
Coral is a structure made by millions of tiny carnivorous (meat eating) animals called polyps (say poll-ip). They are invertebrates (creatures without backbones), related to jellyfish and sea anemones.
A large formation of coral is called a coral reef.
Coral reefs are the largest structures created by any group of animals in the world. They have existed on earth for over 200 million years. Put together, all existing coral reefs cover less than 1% of the ocean floor.
Coral reefs are rich ecosystems, supporting thousands of kinds of marine and plant life living and feeding in them.
The polyp is related to the anemone, and consists of a stomach with a mouth at one end.
The mouth is surrounded by a number of tentacles. These tentacles resemble feet, which is how they get their name: 'polyp' is a Greek word meaning 'many feet.'
The tentacles are covered with tiny stinging cells, and when a small creature brushes against the tentacles, it is killed. The prey is then brought into the stomach to be digested.
Polyps cannot move from their limestone homes, but they do hide in their limestone 'home' or extend themselves out of it.
They mostly feed at night.
Polyps live in groups called colonies.
Coral polyps cannot live in polluted water or water carrying soil from the land.
They grow in warm (18-33ºC), clear, fairly shallow water because they need sunlight to survive.
How do polyps build a coral reef?
A reef begins when a polyp attaches itself to a rock and then divides into many polyps. A colony is formed, and acts like a single living thing rather than tens of thousands of individuals.
Each polyp builds a case of limestone around itself, using calcium from the water. It is like a house, with a floor and walls. This remains after the polyp has died and forms a foundation for another polyp to build a house on, putting a floor on the roof of the old one.
When these limestone formations increase, they are called a coral reef.
In shallow water algae live in the tissues of the polyps . The algae use sunlight and the waste products of the polyp to make oxygen and food for the polyp. This kind of algae is called zooxanthellae (say zoh-uh-zan-thel-ee). The algae need sunshine to survive, so do not live in deeper water where less sunshine filters through.
Coral colonies have different shapes. The shape is determined by its environment. Coral that is smooth and rounded grows in places where there is stronger wave action, and branched coral generally grows in calmer, deeper water. The limestone skeleton is white. The pieces of coral you buy are dead coral that has been painted.
Different kinds of coral reef
There are fringing reefs, barrier reefs and atolls.
Fringing reefs are found along coastlines. A fringing reef is formed where coral polyps settle and grow close to the shore. If the land sinks or the sea rises, the polyps build upwards and a barrier reef is formed.
Atolls (islands of dead coral) form in a barrier reef surrounding a sinking island. A lagoon is formed after the island has sunk, with a ring of atolls around it.
How do polyps reproduce?
A polyp reproduces in either of two ways: by dividing its own body to form two polyps, or by producing sperm and eggs. Just after the full moon in November, eggs and sperm are released from coral polyps and float about for a few days. A small number of eggs will fertilise, hatch into larvae, and settle on the reef to begin new colonies.
Colonies grow rapidly. One polyp can become a colony of 25,000 polyps in 3 years.
Creatures that live around coral reefs
As many as 200 different kinds of fish may live in one hectare of coral reef. They range in size from tiny gobies about 10 millimetres long to 20 metre long whale sharks, the largest fish of all. Coral reef fish have a great variety of shapes, which depend on its feeding and habitat in the coral. There are also anemones, jellyfish, rays, sea turtles and other marine life.
Coral reefs are under threat
Coral reefs are very delicately balanced. They are very sensitive to water temperature and light.
When water is too warm, corals will expel the algae (zooxanthellae) living in their tissues . Zooxanthellae convert sunlight into food for the coral polyps and give coral its brilliant colours. Without this algae the coral turns completely white. This is called coral bleaching. When a coral bleaches, it is not dead but will eventually die off if the warmer waters continue. Scientists think the warmer water temperatures are due to climate change.
Water pollution clouds the water so that sunlight cannot penetrate as fully as it should. Water pollution is caused by agricultural pesticides and fertilisers running off into the oceans, oil and petrol from boats and ships, sewage, and poisons used in some fishing practices. Corals need clear water so that the sunlight filters through to its depths. Less sunlight means less food for the coral polyps.