Corals are a wonderful addition to any aquarium but they’re not the simple creatures which at first they may appear to be. A myriad of stunning and colourful corals are available to buy commercially today and if you’ve ever wondered what makes them the colour that they are, read on to get the lowdown on the fascinating process which decides their fate.

The colouring of corals is basically down to the “Zooxanthellae” within each coral and also down to the light which they absorb. Zooxanthellae are actually single-celled plants which make their homes inside the cells of other creatures. They’re part of the same group that plankton belong to but also live within jellyfish, giant clams and anemones as well as a number of other sea dwelling organisms.

What Zooxanthellae get out of living inside corals is a mutually beneficial relationship whereby both organisms thrive due to the presence of the other; corals offer the Zooxanthellae a safe and protected environment to live in as well as the compounds necessary for photosynthesis whilst the products of this photosynthesis provide the coral with food…it’s a perfect symbiotic relationship and one which has been going on for millions upon millions of years.

The nutrients provided to the corals by the Zooxanthellae mean that the corals can create their skeleton…which is what makes up a healthy coral reef. It is only recently that marine biologists realised that there are more than once species of Zooxanthellae amongst corals….there are in fact nine and possibly even more waiting to be discovered.

When kept in an aquarium, corals are more sensitive to light conditions and when lighting conditions are poor, corals will decrease the number of Zooxanthellae cells which they are host to which in turn affects their colour intensity. The process of photosynthesis is also affected by the light provided, so when the light is poor, the Zooxanthellae produce less “chlorophyll” and photosynthesis is not efficient meaning that there is less food for the coral.

The light spectrum affecting corals and their colour can be understood by the following;

• Lower Kelvin (more yellow) warmer colour
• Daylight bulbs (6500K) will offer light equivalent to the equator at noon
• Higher Kelvin will produce more white to blue light
• Actinic (higher in the spectrum) lights produce more colours

Corals are extremely sensitive to UVA and UVB light; this is because in the ocean, UV is naturally filtered out to a large degree before it reaches them…too much can destroy their DNA and RNA. The colours produced by coral’s Zooxanthellae (blue, pink, purple) are a form of protection from UV light…so corals which reside in the shallows are brighter due to their proximity to the surface and the UV light…more UV light equals more colour.

Before exposing your corals to any new light source, consider it carefully and take it slowly. Move them slowly and allow them to adjust their Zooxanthellae in their own time…this will give far better results than a stressed out coral which has been forced into a new situation too quickly.

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