When hard corals grow in one specific area and gather as one huge colony, they form a coral reef. The reefs are formed by the limestone skeletons of these tiny coral polyps, consisting of a fleshy sack with a ring of tentacles. These corals are the subphylum Anthhoza.
One might find it hard to imagine that such non-locomotive skeleton-like creatures are in fact alive and that they even have very active symbiotic relationships with other creatures found under the sea.
Thus, it may be important to unravel an underwater mystery occurring between the corals and the rest of the sea life.
An indispensable symbiotic relationship is present among these reef-building corals and zoxanthellae, a type of brown algae scientifically known as symbiodinium microadriaticum. Within the hermatypic corals (the coral's skin) dwell millions of these single-celled objects. The zoxanthellae help the coral in the production of calcium carbonate or what we commonly know as limestone. They do this by assisting in the production of sugars and oxygen through a photosynthetic process, a process attributed to any living organism existing on the earth. Due to the zooxanthellae's ability to aid the coral in its production of calcium and sugar for its continuous body formation, corals fast growth can be credited to the tiny zooxanthellaes.
Furthermore, the zooxanthallae is not only responsible for the skeletal growth of the corals, it also gives it its other aesthetic characteristic – a greenish color.
A phenomenon called coral bleaching, in which the coral's outer color from turns from greens to totally white, may occur. Some factors that may cause coral bleaching are changes in environmental conditions, such as sudden warming of the temperature and a change in the salinity of the sea water. Also, diseases, often cause by pollution, may cause the polyps to expel the little green algae and thus the discoloration of the corals. Since these algae are not only responsible for the outer color of the corals but for the continuous photosynthesis as well, bleaching may often result in the coral's death.
What really are the geographical qualities that are ideal for coral growth and survival? First, corals need warm sea water, between 18 ° C and 30 ° C. Cold water can only hinder the development of corals. This explains why they are usually found in the tropical areas and not in the cold corners of the earth. In fact, most coral reefs are found where sea temperatures are warmest, mostly between 30 degrees north and south latitude.
Another important aspect for the growth of corals is a good access to sunlight. As we all know, light from the sun is needed for photosynthesis, which in the case of the corals are performed by the green algae. The clearer and shallow the water, the better are the corals' growth. There should not be too much sediment in the water as they can smother the polyps. Too much fresh water also prevails coral growth development; This explains why it is impossible to find a colony of corals in the mouth of the river.
Coral reefs are habitat of up to 2 million varieties of marine plants and animals, resulting to a very extraordinary diversity. Coral reefs are known to be home to a number of species of tropical and reef marine fish, either swimming right adjust the corals or swimming right above or surrounding area of the reef. Reefs are also found to be a large variety of other sea organisms such as spongers, echinoderms, mollusks, and a whole lot more. These creatures are part of the food chain in the reef, all contributing greatly in the remarkable underwater ecosystem that keeps each coral alive.