A coral reef is an enormous mass of limestone that is organically built

A coral reef is an enormous mass of limestone that is organically built. Coral reefs and atolls are formed by an accumulation of the skeletal remains of very small sea animals and plants that secrete lime.

In the tropical region many sea coasts are characterized by the presence of rocky reefs, often of great size, rising up to the surface of the water. What is commonly called a coral reef was not built by corals alone.

There are numerous other organisms which contribute to the growth of a reef. Among these organisms there are various forms of calcareous algae, stromatoporoids, gastropods, echinoderms, foraminifera, and mollusca.

The coral is formed by creatures which are very like the sea anemones. Some of these sea creatures live apart on its own little cup of coral, but the true reef-building coral polyps are joined to each other in great colonies.

These coral polyps are of many colours-red, yellow, green, pink, purple and white, and of various shapes, sometimes smooth and rounded, or like masses of sponge, or branched in clusters, or even like pieces of lace.

But they all have a hard skeleton made of calcium carbonate. As each coral dies this skeleton accumulates to form white coral limestone.

Calcareous algae or nullipores precipitate calcium carbonate within them and help to cement the spaces between dead corals. Besides, the skeletal remains of starfish, sea- urchins and sea-cucumbers also contribute in building up the reef.