Coral Reefs

In tropical seas many kinds of coral animals and marine organisms such as coral polyps, calcareous algae, shell-forming creatures and lime-secreting plants line in a large colonies. Though they are very tiny creatures, their ability to secrete calcium carbonate within their tiny cells has given rise to a perculiar type of marine landforms. they exist in numerous species of many forms, colours and shapes. Under favourable conditions, they grow bin great profusion just below the water level. taking coral animals as a whole, the polyps are the most abundant and also the most important. each polyp resides in a tiny cup of coral and helps to form coral reefs. When they die, their limy skeletons are cemented into coralline limestone. There are also non-reef building species such as the `precious corals' of the Pacific Ocean and the `red coral' of the Mediterranean which mat survive in the colder and even the deeper waters. As a rule they thrive well only in the warmer tropical seas.
The reef-building corals survive best under the following conditions :-

• The water temperature must not fall below 68 degree F. This virtually limits the areal the tribution of corals to the tropical, and sub-tropical zones. Again they will not flourish where there are cold currents because of the upwelling of the cold water from the depths that cools the warm srface water. This explains why coral reefs are generally absent on the western coasts of the continents. On the other hand the warming effect of the warm currents e.g. the Gulf Stream, means that corals are found far to the north of the West Indies in the Atlantic Ocean. The Pacific  and the Indian Ocean, however have the most numerous coral reefs.
• The depth of the water should not exceed in fathoms or 180 feet, because beyond this depth sunlight is too faint for photosynthesis to take place. This is essential for the survival of the microscopic algae, on which the coral polyps depend. Shallow water of less than 100 feet is ideal. But there should always be plenty of water as polyps cannot survive far too long out of water.
• The water should be saltish and free from sediments. Corals therefore survive best in the moving ocean water well away from the silty coasts or muddy mouths of streams. The corals are best developed on the seaward side of the reef, where constantly moving waves, tides and currents maintain an abundant supply of cleat, oxygenated water. They also bring an adequate supply of food in the form of microscopic organisms. 

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