Coral Reef - What are Corals?

Corals live in the sea, too, but they stay in one place all their life. 

Corals that stretch over large areas are called reefs and can be very colorful.
Each reef is made up of millions of individual coral animals, called polyps.
Polyps may live singly or together in large groups called colonies.
Each tiny polyp has a body shaped like a hollow cylinder, or tube.
This is attached at one end to the seabed, the reef, or even another polyp. At the other end, the polyp's mouth is surrounded by tiny tentacles.
Polyps eat tiny plants and animals that they filter from the seawater with their tentacles.

Jellyfish and corals belong to a group of invertebrates called coelenterates. The term coelenterate means "hollowbody."
Some coral animals under the sea look just like miniature trees.
They wave gently in the water.
Corals grow in hundreds of different shapes and sizes.
Many corals look like leafy plants, such as lettuce, or like mushrooms. Corals grow together in huge colonies that spread out like beautiful underwater gardens.
Each coral shape is made up of the remains of lots of small animals called polyps. A polyp has a soft body that's protected by a small, hard "skeleton." This is made of limestone and shaped like a cup. The "skeletons" of lots of polyps join to make a coral.
When a polyp dies, it leaves its "skeleton" behind, and new young polyps grow on top of it. This is how coral reefs and coral islands are formed. Living coral grows in many different colors, but the remains of dead polyps are usually white. Most of a coral reef is made up of limestone from the bodies of dead polyps.
The biggest coral reef in the world is in the sea of the northeast coast of Australia called the Great Barrier Reef, it is about 1,250 miles (2,010 kilometers) long.
Yet the Great Barrier Reef has been formed by polyps that range in diameter from less than 1 inch (2.5 centimeters) to 1 foot (30 centimeters).

Life in a coral reef

Coral reefs and coral islands grow only in warm, clear, shallow waters. If you were lucky enough to swim by a coral reef, you would see thousands of colorful fish, crabs and other animals moving through the underwater garden.
Clams and sponges attach themselves to the coral.
Octopuses and eels hide in caves that form in the reef.
Sea cucumbers help to keep the reef clean by eating small animals and plants, such as algae, that grow in between the corals.
Some of the animals that live among the coral are harmful to the reef.
Some worms and moilusks tunnel into the limestone and weaken it.
Some fish and starfish feed on the living coral.
If all the living coral is eaten away, the sea quickly destroys the rock that's left behind.
Parts of the Great Barrier Reef are in danger because thousands of crown-of-thorns starfish are eating away large areas of the living coral there.