Corals are marine invertebrate animals, usually found at the base of tropical and sub-tropical seas or oceans, like the Indian Ocean, Pacific Ocean, the Red Sea etc. . They are small and soft-bodied and bear relation to sea-anemones and jellyfish. They occur in dense colonies, and the individual branches of a single coral are often referred to as ‘polyps’.

General Anatomy

The body of a polyp is symmetrical and is shaped like a tube.  The open end of the tube serves as the mouth. There are 6-8 tentacles surrounding the mouth, through which it takes it food.  The tentacles possess stinging cells(Nematocysts) to paralyse the prey. The body of the polyp is composed of  two distinct layers; the outer epidermis and the inner gastrodermis. The space between these layers is filled with a jelly-like substance called mesoglea.

Classification of corals

Corals are classified into Hermatypic or soft corals and Ahermatypic or hard corals. This classification is mainly based on whether they produce reefs or not.

Hermatypic Corals: These are the type of corals which build coral reefs from the calcium carbonate produced from algae, called ‘zooxanthellae’, with which they form a symbiotic relationship. The formation of the coral reef starts when a polyp attaches itself to a rock on the sea-floor. It now secretes a skeleton, producing a cup-like structure called the calyx. The walls surrounding the cup are the theca while the floor is known as basal plate, from which thin septa arise. The calyx and the polyp together are known as Corallite. Two adjacent calyxes are connected by a skeletal structure called coenosteum, while the polyps are connected by a ‘coenosarc’. The famous Great Barrier Reef near Australia is an example of this type. Hermatypic or hard corals are very colourful, owing to their host algae. They lose their color when the ‘zooxanthellae’ die, and this is referred to as “Coral bleaching”.

Ahermatypic corals: Also called soft corals, these do not associate themselves to algae, hence are not able to produce any calcium carbonate and do not build reefs. Instead, their body is supported with variously-shaped small structures called sclerites. Examples of soft corals are sea-fan, leather corals and tree corals.

Uses of corals

As Gemstone: The use of corals as a gemstone is a well-known fact. Red corals [Botanical Name: Corallium rubrum] found largely in the Mediterranean Sea, are used in the manufacture of the famous gemstone ‘Coral’(Moonga in Hindi). They are available mainly in rounded and oval shapes.  The stones are rarely available even in black and white colours which are made from Black Corals and White Corals respectively. Coral Beads are also made from this species of Coral.

In Food and Health: The calcium, extracted form certain corals, is used as a calcium supplement and also for the treatment of many diseases.  Many other chemicals are also extracted from corals.

In Ecology: The coral reefs act as a natural barrier to rough seas, thus protecting the lagoons and other islands. It has been found that coral reefs support nearly 25% of all marine life.