How coral is formed

How coral is formed

by Preeti Sharma

Actually, Coral is a substance formed by the skeletons of the sea animals. It may look like a multi color rock, but it is not. Coral is a living animal and belongs to the large group of animals that comprises of, jelly fish, sea anemones and hydroids.

Coral is a colorful rock found underwater that is used for jewelry extensively. With their bright color range and nice quality, coral jewelry is appreciated throughout the world. You may admire the coral jewelry or may be proud of owing one, but have you ever thought how coral is formed. Not yet? So read on here to know the unique way how coral is formed.

coral reefs
image courtesy: National Geographic

What is Coral?

Actually, Coral is a substance formed by the skeletons of the sea animals. It may look like a multi color rock, but it is not. Coral is a living animal and belongs to the large group of animals that comprises of, jelly fish, sea anemones and hydroids. Corals ooze out a tough skeleton of limestone or calcium carbonate as protection for the coral Polyps.

Live coral animals make a colony of individual Polyps. Each polyp makes a strong and hard skeleton that as a shape like a cup. It is known as calyx that is made for security and support. The skeleton is connected to the polyps around it. Therefore, the skeletons of hundreds coral polyps dwell together forming one coral colony.

Corals are the sea animals that belong to the group of the Phylum Cnidaria with tentacles and stinging cells, also called nematocysts. They have a single opening in their body for the passing of food and waste. The living and healthy coral is colored brightly often.

There are diverse coral formations that look like large domes, branching trees, small irregular crusts, or tiny organ pipes. The living coral forming animals color the formations in beautiful hues of tan, orange, yellow, and green. The coral animals cannot live in cold water below 65 ° F. This is the reason why coral reefs are found in shallow and tropical seas, mostly.


The polyps are little carnivorous or meat eating animals that live in millions of number together. The polyps live together in large groups that are called colonies. The polyp is actually related to anemone. It consists of stomach and has a mouth at one end. The mouth is encircled by a good number of tentacles that are similar to feet. The polyp word is related to this fact actually, as in Greek the ‘Polyp’ word meaning ‘many feet’.

The tentacles of polyps are well covered with small stinging cells. So when a small animal gets close and brushed against the tentacles, it gets killed. The prey then goes off to the stomach and is digested. Polyps are immovable from their homes made of limestone. They come out at night to feed most of the time.

So, how do polyps form coral?

The colony of coral grows by the budding of a single coral polyp that splits in two. It makes a matching copy of itself. Most of the polyps that make coral live together in colonies. The polyps attach themselves together with each other. There is a flat sheet of tissue that joins one polyp to the middle of the body of another polyp. The half of coral polyp stretches out above, while the remaining half is down under the connecting sheet.

Continuously the polyps keep on creating the new skeletons at their bottom and sides. While doing this, they keep on going upwards and outwards from coral colony center. The coral polyps take away calcium present in the sea water and make their own limestone skeletons. In this way, they deposit limestone or calcium carbonate in the half region of their lower body halves.

As the polyps die, they go away as limestone skeletons. These skeletons in future become the base of various barriers and ridges that are known as Coral Reefs. The living polyps live on the top of the old and dead skeletons that they have made. In this way the whole colony continues to grow, with the new polyps getting created and the old ones dying off. As there are new polyps growing, the group makes the limestone structure large and larger.

The major part of the structure of single coral colony is actually made up of the dead skeletal material. A living and healthy coral has just a thin layer of living material that inhabits its surface.

How do polyps reproduce?

A coral polyp actually reproduces in using either of the two following ways:

* One way can be to divide its own body to make two polyps

* The other way is to produce sperm and eggs

The eggs and sperm come out from coral polyps at the time just after the full moon in the month of November. They keep floating for about a few days. There are a small number of eggs that get fertilized, originate into larvae and then get settled on the reef. Once they get settled, they start together to begin new colonies. The colonies formed by polyps grow fast. You may be amazed to not e the fact that in three years one polyp can make a colony of 25,000 polyps!

Coral reefs

A strip of curdled coral at or close to the surface of ocean is known as coral reef. The coral reefs are made mostly of coral colonies. They are formed by the animals or plants that grow within them, largely the stony corals. Reefs have a history and life span of about more than about 450 million years on earth. The big coral groups have round or branching masses. Over thousands of years the accumulation of skeletal material forms a coral reef.Coral reefs generally grow in low tropical water. They have algae that need light and they grow sound only near the surface of clear water. Also echinoderms, molluscs, protozoa, sea cucumbers and sponges add to the budding reef. The mats of blue and green bacteria work to ambush and alleviate the structure. Some reefs also have sea grasses and mangroves growing on top of them. As the corals break surface of water they make the coral islands.

Coral reefs are of four types, mainly:

The fringing reefs around islands
The barrier reefs near the landmass, for example the Great Barrier Reef of Australia
The atolls that are actually hollow rings, formed on underwater volcanoes, like the islands of Bermuda
The patch reefs are tiny circular and uneven reefs that rise from the floor of the lagoon or inside the atoll.

Now that you know about how coral is formed, are you a bit more interested in coral jewelry?