CORAL & POLYPS - how coral is formed

What is Coral?
Coral is made by millions of tiny carnivorous (say: car-niv-or-us)
animals called polyps.(say: pol-lips) Carniverous means to eat meat.
Polyps live in groups called colonies.
Each polyp builds a case of limestone around itself.
Coral reefs are made of thousands of these limestone cases.

Polyps

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 polyp's stomach to be digested.

Polyps cannot move from their limestone homes. They mostly feed at night.


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 about 3 years.









Building up a coral reefEach 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 it 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. The algae need sunshine to survive, so do not live in deeper water where less sunshine filters through.

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. 

They grow in warm (18-33ºC), clear, fairly shallow water. The coral cannot grow in polluted water or water carrying soil from the land.