FACTORS AFFECTING THE GROWTH OF CORALS

TEMPERATURE:

Plays an important role in the growth and survival of the corals.
They grow well at temperatures greater than 20 degrees centigrade and can thrive even at 35 degrees centigrade as in the RED SEA, PERSIAN GULF or in GULF OF KACHCHH.
Due to this temperature factor and their preference for warm waters, majority of corals are found in the tropical belt.

The other factors affecting the growth of corals are :
Firm substratum to settle like natural surfaces like rocks, coral stones or molluscan shells. Sometimes they grow on metallic structures like hulls of sunken boats and ships.
Salinity of the seawater which contains 35 g of dissolved salt per liter is ideal for the corals to thrive in contrast to fresh water which has only a few mg of dissolved salts.
Water Turbidity caused by high sedimentation affect corals because the skeletons of corals are fixed and open above.
If the sea water is less turbid the corals can clean themselves and survive.
High sedimentation also reduces light penetration in the sea, thus reducing photosynthesis by zooxanthellae and the coral growth.

Corals grow very slowly especially the brain resembling massive corals because the calcium carbonate deposition is a slow process and the growth occurs in all directions. Their growth rate is a centimeter per year.
The branching corals have relatively a faster growth rate and their skeletons are less dense than those of massive ones. The growth rate of branching corals is 18 to 20 centimeters per year.
Growth of the corals is not uniform throughout the year as it depends upon the changing weather conditions.
When the conditions are warm with abundant sunlight corals grow fast, when there is heavy rainfall during monsoon,their growth slows down.


INDICATION OF DARK AND LIGHT BANDS ON CORALS
Dark and white bands present on coral reveal important things like the age of the coral.
When a coral grows fast, it deposits more calcium into the skeleton, which becomes dense and dark and when the growth is slow calcium deposition becomes lighter.
Since seasonal changes, such as those between warm and rainy months occur generally once a year, a dark and light band together would correspond to a one year's growth and hence by counting the number of light and dark bands present on a portion of coral colony the AGE OF THE CORAL can be known.
It is believed some coral colonies are believed to be more than 100 years.

Apart from finding the age of the coral the light and dark bands of the coral also indicate the index of rainfall. This is because the corals contain calcium deposits along with a variety of materials like metals, humic acids and some natural elements like carbon and oxygen.
Among these humic acids come from land sources via the river flow into the sea. So more the rainfall, the more humic acids come with the river flow, the more their presence occurs in the coral skeleton.
When the coral skeleton is looked under the ultra violet light, the presence of humic acids is seen as fluorescent bands.
THE INTENSITY OF FLUORESCENCE is thus an INDEX OF RAINFALL. By knowing the age of the growth band at the location of the fluorescent band, one can deduce whether the monsoon was strong or week at that time.

Light and dark bands are helpful in deducing the fluctuating temperature of the sea water.
At higher temperatures the lighter isotopes of Oxygen escape from the seawater leaving behind the heavier ones.
So, if we find more heavier isotopes of oxygen in the coral skeleton at a particular growth band, then it can be concluded that seawater temperature was higher that year and by how much more it was.
Thus corals act as indicators of PAST CLIMATE CHANGES. CORALS AS PRECIOUS

GEMS/STONES USED IN JEWELLERY.
The RED CORAL, is called 'Corallum rubrum, and is not a reef coral but a stony coral with zooxanthellare. The red colour comes from a natural pigmentation of the skeleton and remains even after the death of the polyp.It is not porous like a reef coral but is dense and compact so, it can be shaped or machined without breakage, and cut into coral beads to be used in jewellery.
The Red coral occurs only in the Mediterranean Sea and off Japan from shallow depths up to 1000 meters. They are collected by skin diving by fisher manor by dragging wooden frames over the seabed and breaking off the branches.Apart from RED CORALS, the BLACK CORALS and THE BLUE CORALS also retain their colour after death.

PREDATORS AND DISEASES SUFFERED BY CORALS
Some fishes like parrot fishes and butterfly fishes bite and nibble the corals. The most dangerous predator of corals is the CROWN-OF-THORN STARFISH as it exclusively feeds on corals. This starfish envelops the whole coral with its arms and sucks out all of the coral tissue, leaving only the bare skeleton.

Corals mainly suffer from two types of disease which are:
THE BLACK BAND DISEASE: It is widely prevalent and this infection is caused by a bacterium wherein the affected coral tissues become blackish and look like a dark band among healthy tissues.
THE WHITE BAND DISEASE: It is less prevalent and causes the affected tissues look like a white band around the lesion.
Recently scientists recorded a new disease known as the PINK LINE DISEASE from the corals in Lakshadweep.The cause of this disease is not known and it is suspected that the mortality associated with the 1998 EL-NINO event has a role in this.