INTRODUCTION

Nobody knows exactly how many corals are present worldwide. It is believed somewhere around 800 to 1000 species are present.
Sometimes the same coral species is called by different names in different countries. 206 species of corals are known from Indian reefs, with a majority of them occurring in the Andaman and Nicobar Islands.Some coral species are unique to India while others are found in the Oceans of different parts of the world.
Many people think corals are plants because they remain fixed to rocks, are colourful to look at, and many of the branched ones resemble small trees. But actually a coral is a simple animal like the sea anemone we find on the rocky beaches.
The term 'coral' is the skeletal part of the animal while the animal itself is the "polyp". The external surface of the polyp has some special cells called calico blasts that can take dissolved calcium from seawater and construct a solid skeleton.

DIFFERENT SHAPES AND COLOURS OF CORALS
The shape and the size of a coral colony is genetically determined. The common coral shapes seen are as follows:
Massive (stone like)
Foliaceous(leaf like)
Branching(tree like or flower like)
Encrusting(coating like)

At greater depths, the pressure is high and the coral skeleton tends to grow more flat. The growth forms tend to change because the corals are continuously looking out for sunlight.
Sunlight is very important for the growth of a coral because like a plant it prepares its food by the process of photosynthesis.
All corals on the reef have microscopic plants (called Zooxanthellae) living inside their cells. These algae have chlorophyll pigments and produce organic matter by photosynthesis. So the coral has to remain near the surface where sunlight is abundant.
Corals appear coloured due to the presence of algal pigments. In the absence of these pigments the coral tissue would be a transparent layer through which the skeleton can be seen.

CORALS ARE CARNIVOROUS AND NOCTURNAL
Although corals can prepare their own food by photosynthesis they are not totally satisfied hence they catch tiny animals from the seawater and swallow them.This is their main food because whatever they get from the algae is supplementary food.
Some corals can also feed on detritus(dead organic matter) also and some others can take up dissolved organic matter directly from the seawater. The versatility in their feeding habits helps the corals to survive and thrive in oceanic deserts.
Corals are fixed to rocky bases in the reef and hence they cannot move from place to place in search of food like other animals, instead the food comes to them along with the water currents.
Corals feed on tiny microscopic animals called plankton which drift along with currents and when they pass close by the corals catch them with their tentacles, sting them by using their stinging cells and paralyze them by injecting venom and finally swallow them.
Since all corals are nocturnal they are very active in the night when the plankton animals come to the surface layer from deep waters to where they go during the daytime.

STINGING CELLS:-of corals are also called nematocysts which are present in their tentacles and near the mouth.These stinging cells come in at least 20 varieties that is small, big, long and short.They are filled with poisonous substance.

SOLITARY CORALS
Some deep sea corals which do have zooxanthellae do not need light and can live in deep sea and lead a solitary life. These corals are also called asahermatypes and they occur in all latitudes.
Corals which have zooxanthellae need light and hence remain near the surface of the sea. These are called as hermatypic corals, and can be seen only in sub tropical seas and form colonies like coral reefs.