Why corals change colors

September 24, 2011

Bleaching of corals

Corals are living animals having differential structures, known as colony, and beautiful colors. These are often mistaken for plants or rocks. Corals often have packed small algal cells with varied pigmentation distributed all round within the colony, which impart corals very attractive and contrast colors. These algae are known as zooxanthalle and these are in symbiotic relationship with the corals for their shelter as well as some basic requirements. They use the carbon dioxide released by corals and synthesize glucose as food material in the presence of sunlight, whereas corals are benefited with some of the glucose supply given by the zooxanthelle.

Sometimes corals seem to change their coloration of colony in an interesting way which can be defined as following:

1. Bleaching phenomenon

This is a stress condition for corals in response to prolonged high surrounding water temperature. Normally the change in temperature by 1-2 °C can be tolerable but beyond this limit, the symbiotic association between the algae and coral disintegrates. The break in symbiosis results in loss of the most of the zooxanthal cells from coral’s skeleton leaving a pale color of skeleton. Heavy loss of algae from the coral colony leaves it white in coloration. The phenomenon is called coral bleaching.

If the change in surrounding temperature is temporary, the algal cells again start photosynthesis as well as divides rapidly and coral colony recaptures its original color.

2. Change in Intensity of sunlight

The change in coral coloration is highly dependent upon the sunlight intensity. The concentration of chlorophyll within the algal cells as well as the rate of photosynthesis within the zooxanthalle both factors are controlled by the intensity of light reaches to them. High intensity of light creates darker pigmentation so that the harmful effect of the light could be minimized. Thus darker colors like dark blue, brown or dark green coloration is developed. Whereas, low intensity of light discourages the photosynthetic reaction within the algal cells. Thus light coloration occurs within the color colony.

3. Nature of spectrum of light

The color of corals also dependent upon the spectrum of the light source used. The spectrum of the light is measured in terms of Kelvin scale. Bluish light shows higher K rating, whereas the reddish goes towards the lower value in Kelvin scale. Sunlight at the noon time is rated nearly 5500-6000 K. In general, more corals are adapted to bluish light (10 000 to 20 000 K) and show their normal colors. Light with lower spectrum (towards redness) develops warmer colors. Higher spectrum induces to develop lighter colors (white or light colors) in coral body.

4. Presence of ultraviolet rays in light

Ultraviolet rays (UV-A and UV-B) are not visible by our eyes but are capable to generate most destructive effects inside the living organism and so to the zooxanthalle inside the coral body. It is capable of causing mutations by breaking DNA and RNA of the cell. So, the algae loses their natural color to mutate into light colors like white, sky blue, purple and light as well as very light green.

Further there is a point to understand that transformation into another color is not a symbol of unhealthy colony of corals but it is an adaptive phenomenon performed by zooxanthal cells residing inside the coral body. Keeping the above points into the mind, one can manage the proper conditions for better adaptability of the newly procured corals.

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