| Normally one imagines tropical seas as being rich in life, but in actual fact the warm and transparent waters of the tropics are very poor in nutritious substances and thus are under populated by fish and other organisms. |  |
 | A coral reef is, on the other hand, an explosion of life, with a large number of animal and plant species continually competing for space, which makes it an extremely rich and complex environment. |
| On top of this other calcareous layers are laid down, forming over a few million years, the great coral formations. Single colonies can reach huge dimensions, weighing dozens of tons, with a volume that would fiE the hving room of a normal apartment. |  |
 | To express it in an analogue in dry land terms, one could call it an oasis in a large desert. The secret of such thriving luxuriant life lies in the particular biology of the hard corals, also called coral builders, animals that generally form colonies. These organisms aie able to extract calcium carbonate from the water, with which they make their skeleton. |
| One of the most striking aspects of the corals is the incredible variety of shapes that they can form: spherical, pillar-shaped, fan-shaped or branched. Furthermore, the external morphology of the same species may be more delicate or solid depending on whether it grows in calm or rough waters. |  |
 | They also show great variety in colour, with shades of yellow and pink, green and purple, brown and blue. The colouring lasts as long as the corals are living; after their death they loose the surface tis- sue and only the white calcare~ ous skeletons, covered with little holes where the polyps lived re- main. These are the living part of the great colonies. Each single polyp looks like a little contrac- tile sac with an annular crown of tentacles arranged round an opening which acts as a mouth. The tentacles have stinging cells which eject a filament similar to an arrow and a toxin capable of stunnmg small prey like the mi- croscopic shell fish on which the polyps feed. Most of the species hunt by night, while during the day the polyps remain retracted in their holes. |
| An important characteristic of the corals which bui!,:1 large reefs is the presence of particu- lar unicellular algae which live inside their tissues. Although the precise details of the workings of this unusual relationship based on a mutual exchange of favours are not yet entirely clear, it is known for certain that these algae aid the depostion of calcium carbonate for the construction of the corals, that they remove waste substances produced by the polyps, turning them into other substances which are useful to the colonies, and that they also produce Oxygen in greater quanties than is consumed. |  |
 | The proof of the importance of these minute plants is that the corals that are without them only form colonies of a modest size. Other organisms able to secrete calcareous substances, like "fire corals" and many s pecies of "coral-algad" which are all-important in the building of the coral formations due to their cernenting function, contribute to the construction of the reefs. |
| The great coral reefs are usually to be found in shallow waters, only rarely as deep as 1 00 metres, to allow the algae in them the necessary light to photo-synthesise. Furthermore, the corals require warm water (between 20' and 35'), and cannot tolerate low salinity or very turbid waters. |  |
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