Saltwater Aquarium Guide -- Fast Facts

Fast Facts (page 19)

There are many reasons why corals may change thier coloration, but one of the most common and dramatic is due to the qualities of light that the corals receive. These include; intensity, spectrum, and amount of UV light the corals receive.

A coral contains symbiotic chlorophyll-containing cells called zooxanthellae cells. The coral provides these cells with protection, and in turn, the cells provide the coral with nutrients as a result of photosynthesis. The numbers of zooxanthellae cells and amount of chlorophyll vary in response to light intensity so that the coral continues to receive the necessary nutrients. If the lighting is more intense than the coral is accustomed to, either some zooxanthellae cells will be expelled from the coral, or the amount of chlorophyll within those cells will be reduced. Having an excess number of these cells in a brightly lit situation can be dangerous to a coral because the zooxanthellae cells can produce too much oxygen. Oxygen at high concentrations can become toxic to corals. On the other hand, if the intensity of light is lower than what the coral usually receives, the zooxanthellae cells will not be able to provide the necessary nutrients. In this case, the number of zooxanthellae cells will increase as well as the amount of chlorophyll within those cells.

The resulting color change of a coral in response to different light intensities is due to the concentration of both the zooxanthellae cells and the amount of chlorophyll present within those cells. The color of these cells ranges from a golden-yellow to brown. The higher the concentration of these cells within the coral, the more the coral will take on a brown appearance. In other words, if the lighting is less intense than the coral is accustomed to, it will take on a darker brown appearance. On the other hand, when the same coral is placed under intense lighting, some of the zooxanthellae cells will be expelled and the amount of chlorophyll reduced, giving the coral a lighter appearance.

The spectrum of light that corals receive will alter their appearance in the aquarium. Certain colors, such as fluorescent red or orange, that are not visible in daylight become very apparent under actinic lighting. Mixes of bulbs with different spectrums, along with different lighting systems, will produce varying effects on the colors of the same or similar corals. A typical lighting system for a reef aquarium will provide lighting that contains a mixture of approximately 50% white and 50% blue actinic lighting. This mixture of lighting provides the wavelengths of light that will bring out the fluorescent colors, as well as the spectrum that makes the aquarium appear natural in color. more---->

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