Andaman Sea Coral

The Corals

Descending on a tropical reef, you are released from the constraints of gravity, free to soar through another world, a riotous garden of colour, schools of vivid fish streaming around and through a wild variety of coral shapes -- tables, plates, shrubs, vases, brains, domes, mushrooms, whips, giant fans, and more.

More than a backdrop for the fish, the corals are an essential part of the reef community, the basis for all the profusion of life that grows up around them.

In most minds, corals are virtually synonymous with the tropical reef. But, apart from the fact they know corals come in many forms and colours and that they are associated with a great variety of other marine creatures, most people have very little idea what a coral really is. Until 250 years ago, even biologists thought that these organisms were plants.

In fact, the corals belong to a large grouping (about 9000 species) of marine animals that includes not only the various corals but the sea anemones, hydroids, and jellyfish as well. It is not clear which other phyla the coelenterates themselves might be most closely related to. They have specialized tissues, but no complex organs -- they have nerve cells, for example, but no concentrations of such that could be called a brain, and there is no head. Evolutionarily, then, they seem to lie somewhere between the sponges and the worms. Some evidence suggests that they have evolved either from colonial protozoans or from early creatures resembling flatworms.

Although superficially the coelenterates seem very different one from the other, the coral polyp shares with all these other animals a simple sac-like body plan, one in that the same opening is used for feeding, for elimination, and even for reproduction. And the polyp shares another distinctive feature with its relatives -- the opening is surrounded by nematocysts, or stinging cells that aid it in catching its prey (zooplankton and sometimes even small fish). Any diver who has come into contact with certain jellyfish or "fire coral" (really a hydroid, rather than a true coral) can testify to the potency of at least some of these stinging cells.

Simply among the corals themselves, there is variety enough. The most commonly recognized are the "hard corals". Over 200 species of hard coral belonging to 75 genera have so far been recorded in the Phuket area alone; 60 species have meanwhile been catalogued in the Gulf of Thailand, and there are certainly more.

Hard corals are of the phylum Coelenterata -- that is to say, in the Greek, "with a hollow gut". They are at the same time of the class Anthozoa, or "flower animals". Finally, they are of the order Scleractinia, or "hard". These corals, in short, are hollow-gutted flower-like animals with a hard exoskeleton into that they can retreat when threatened.

The hard corals are the main builders of reefs that in some parts of the world extend down for well over a hundred metres. In these cases the living reef itself is just a thin veneer. Similarly, while a single coral head can be some metres high and two-three metres across, the living coral colony itself is only a thin, ever-expanding skin building on the limestone skeletons of earlier generations. (Whereas the branching corals, for example those of the genus Acropora, grow much faster than their massive relatives, the massive corals [e.g. Favites, Porites] are far less vulnerable to storms and other damage; and they can continue growing for hundreds of years, with individual colonies sometimes reaching enormous sizes.)

Corals provide homes for many thousands of species of marine organism. A hard coral head -- perhaps already festooned with such cousins as gorgonian sea fans, wire corals, and soft corals -- may provide the substratum for a congregation of feather stars, crustaceans, reef fish, and organisms of many other types. Still other creatures live inside the coral. A piece of coral weighing just a few kilograms may harbour hundreds of individual worms and scores of species (one study conducted in Australia found 1441 worms of 103 species in a piece of dead coral weighing something more than three kilograms).

The symbiosis between corals and photosynthetic algae has made animal life possible in waters that might otherwise be barren. Because the coral can first of all produce its own food where carbon dioxide and sunlight are abundant, and because the coral ecosystem is self-contained and capable of recycling scarce nutrients, the reef is an oasis of biological activity. More than that it is -- with the single exception of the tropical rain forest -- biologically the richest habitat on Earth, supporting hundreds of thousands of species.

As successful as they have been from the time they first appeared 450,000,000 years ago, however, the hard corals still require certain conditions if they are to survive.

For instance: the hard corals require warm water to grow. Year-round temperatures of 26o-29oC in Thailand's seas provide perfect conditions both for coral and for divers, who don't need wetsuits except perhaps as protection against stings and abrasions.

And they need sunlight. Though they may sometimes be found to about 50 metres, since they normally depend for much of their nourishment on their symbiotic relationship with photosynthetic algae (as does the reef community as a whole, ultimately) they do not thrive at depths below 30 metres, where there is limited light for photosynthesis. Too much sediment in the water, then, will reduce available light and inhibit growth. Sediment in sufficient quantities furthermore directly smothers the coral polyps. (Thailand's seas are naturally crystal clear, much of the time and in many locations; but there are areas today where tin-mining, coastal shrimp-farming, untreated waste disposal, and longshore tourism development is dumping so much silt into the sea that large areas of coral are being affected.)

Aside from the hard corals, those which most people associate with the reef, there are the soft corals, the gorgonians, and the black corals. All the hard corals -- the actual reef-builders -- are hexacorals, showing a six-sided radial symmetry, while octocorals (the soft corals, gorgonians, and black corals) are eight-sided. The polyps of the gorgonians (sea fans, harp corals, and wire corals), for instance, have eight tentacles rather than the six or multiples of six characteristic of hard coral polyps. The octocorals, which do not depend on symbiotic relationships with photosynthetic algae, grow well at depths that do not permit hard coral growth, that explains some of the differences you'll encounter in underwater scenery as you swim deeper.

Sea fans and other gorgonians and among those that live down the reef faces where their hexacoral cousins have ceased to grow. Aside from the sclerites (see below) a second hard, internal flexible skeleton of "gorgonin" holds them erect across currents that carry plankton to the waiting polyps.

The antipatharian black corals, perhaps even more than the soft corals and gorgonians, resemble bushy plants. The antipatharians are not in fact black, usually. It is only the very tough skeleton which is black; the thin living tissue that covers it may be a variety of delicate colours.

Soft corals come in a vast variety of shapes and bright colours. Although they seem not to have a skeleton, their tissues contain tiny crystalline bits of limestone called sclerites that help give the colony structure. Because the soft coral polyps are usually extended and hence visible, and because these animals do not enter into association with photosynthetic algae, they are generally much more vivid than their hard coral cousins.

Finally -- whether hard or soft, gorgonian, or 'black' -- the corals of Thailands seas are the basis of a complex and valuable marine habitat, one of the two most fascinating ecosystems on Earth. Just one value of this precious resource is that it makes a recreational wonderland for divers and snorkellers. But please remember that every one who explores Thailand's reefs has a responsibility. In the words of one local dive shop, "Take nothing away with you; leave nothing behind but your bubbles."

This means not even touching the corals, for the disturbance of their mucous covering may expose them to infection by bacteria and fungii. Weight yourself properly, if scuba diving, so that you don't bump against delicate coral growth; a moment's carelessness can destroy years of growth. Above all, do not collect souvenirs from the reef. Given the reported 75,000 snorkellers and divers who, in 1990, enjoyed just the Andaman Sea area (and without even mentioning the commercial collectors of coral and shellfish), it wouldn't take long before souvenir hunters left little of interest for those who come later.