Tropical warm seas contain the great reefs of coral growing in shallow water. The so-called coral islands found in the south seas, of which the Maldives is a prime example, are made from the limestone skeletons of millions of these invertebrate marine organisms which belong to the class Anthozoa (anthos flower, zoion animal) and which live in colonies, sharing the work of feeding, cleaning, and defence.

Each unit of the coral is known as a polyp which secretes the stony skeleton which is almost pure calcium carbonate. Corals along with sea anemones and jellyfish are collectively called coelenterates, Greek for cavity, which aptly describes their basic body structure. All coelenterates are simply a hollow sac with a mouth at one end surrounded by tentacles. These tentacles armed with stinging cells help them to paralyze small swimming animals which are then pushed into its mouth. The polyps of some of the corals do not have long tentacles, thereby passing the food to its mouth by the cilia coating the tentacles. Corals feed by photosynthesis of marine algae living on coral tissue. They also follow a carnivorous diet, plankton being the favourite.

There are several kinds of corals of which the stony coral (Order Madreporaria or Scleractinia), numbering around a thousand species, is the most widely distributed. Living either solitarily or in colonies, stony corals occur in all tropical oceans, growing best upto 30 meters. These corals are also sometimes seen at an incredible depth of 20,000 feet. Stony corals make up vast coral reefs and atolls, some of them as old as 70 million years. When one polyp dies another takes its place and continues to grow until the reef is created. However since they by nature are slow in their growth (averaging about 5 to 28 mm a year), a damaged or destroyed reef requires a long time to rebuild. The most common and popular types of stony corals include the mushroom, the brain, the staghorn, and the star corals, all named so for their distinctive shapes. The 1,200-odd species of horny corals (Gorgonacea) thrive in shallow tropical waters. Their branches, sometimes ribbon-like, can grow to lengths of occasionally upto 10 feet. The rose coral belonging to this class is used in making interesting pieces of jewellery. The one living species of blue coral (Coenothecalia) occurs on the reefs of stony corals. Colonial by nature, they can be found in the Pacific and Indian oceans where they form large lumps, some of them spanning two meters in diameter.

Other kinds of corals include the black and thorny corals (Antipatharia), numbering a hundred species; and soft corals, a widely distributed group. Soft corals are not true corals. One important distinguishing factor is their tentacles which, instead of being simple as in true corals, are fringed and each polyp has eight tentacles instead of the usual six or a multiple of six. A close relative of soft corals is the beautiful organ-pipe coral, which when it expands resembles a delicate flower. The organ-pipe consists of a mass of vertical tubes, joined at intervals throughout their length by thin horizontal plates. The purplish skeleton contains a pale lilac-coloured polyp.

Though almost all corals are sedentary, i.e. animals permanently fixed to the substratum, there is a species of walking coral. Though dubbed so, it actually does not move on its own, but represents a fine example of symbiosis. The Heteropsammia michelinii has a marine worm living in its limy skeleton and as this worm moves around, foraging for food, it drags the small one-inch-long coral with it.

The reef islands of the Maldives is the result of a process of coral formation known as accretion. Rubbles of reef rock broken off from the reef by heavy cyclonic storms and waves create reef- top shoals. Due to the normal action of waves and ocean currents other materials begin to gradually accumulate. Beaches develop around these shallows with the wind heaping up the lighter materials into dunes. The material being almost entirely made of calcium carbonate readily dissolves in rainwater and the dissolved lime is then redeposited around the loose materials, cementing it together. Soon such newly-formed islands are inhabited and colonised by plants and animals.

Corals exemplify some of the richest known ecological communities supporting countless other living organisms, intertwined in the complex web of food and resource competition. Seaweeds grow on the skeletons of dead coral together with sponges and other animals. Molluscs graze on algae while predators such as reef sharks, groupers, rock cod, lionfish, scorpionfish, stonefish, and eels lurk amongst the coral jungles which provides them with a perfect hunting ground. At times corals themselves end up in the beak-like jaws of parrot fish or other predators like butterfly fish, commonly-found denizens of this spectacular microcosm.

Reefs prevent soil erosion. And with their brilliant hues and sculpted shapes corals add beauty, enticing people to dive underwater to appreciate some of nature's most magnificent of creations. Corals constitute a very delicate ecosystem which requires our tender care and nurturing.

Visitors are advised not to pick up corals from the sea or attempt to export them.