There are many environmental reasons why corals can change color. In home reef aquariums however, the most common cause of color change is mainly light.
Corals respond to light by regulating the number of cells responsible for receiving light, as well as pigments that help protect against strong light. In fact, a coral’s color change is the coral’s natural response to a particular light source. The coral will strive to achieve a balance where the number of cells and the amount of pigment match the coral’s needs both for nutrition and for protection purposes.
Many coral species have the ability to adapt to different light intensities. For example, corals photosynthesize with special, light-sensitive cells called Zooxanthellae cells. This is a single-celled microalgae that lives in symbiosis with corals. These symbiotic cells contain chlorophyll and provide nutrients to the coral in exchange for protection. To ensure a constant supply of needed nutrients, the host will regulate the number of Zooxanthellae cells and the amount of chlorophyll in those cells. One of the basic criteria for regulation of Zooxanthellae cells and chlorophyll is light intensity.
If the light is more intense than the coral is used to, one of two things can happen. Some Zooxanthellae cells will be expelled from the coral or the amount of chlorophyll in those cells will decrease. If the excess of Zooxanthellae cells in the light environment is too strong, it will be dangerous for corals. Under intense light, oxygen produced as a by-product of Zooxanthellae photosynthesis can accumulate as toxins in corals.
If the excess of Zooxanthellae cells in the light environment is too strong, it will be dangerous for corals.
Conversely, if the light intensity is lower than what corals normally receive, the photosynthetic Zooxanthellae cells will not be able to produce enough nutrients for the coral. Then the number of Zooxanthellae cells and the amount of chlorophyll in those cells will increase to try to capture more light energy.
So how do Zooxanthellae cells and chlorophyll concentration affect the color of corals? Zooxanthellae cells range in color from golden yellow to brown, and the large number of these cells give corals their brown color. In other words, light intensity changes the color of corals by affecting the concentration of both Zooxanthellae cells and the amount of chlorophyll present in those cells.
Therefore in low light conditions, photosynthetic corals will have a darker brown color because corals have more microbial cells to produce more nutrients. If the same coral is placed under a strong light source, the Zooxanthellae cells will be ejected and the reduced amount of chlorophyll will make the coral appear brighter.
The light spectrum or color temperature of the aquarium light will also change the appearance of the coral. In general, the color temperature of light with a lower Kelvin will be “warmer” while the color temperature of a higher Kelvin will be “cooler”. Different lighting fixtures with different color temperatures will impart different color effects to the coral.
For example, light in the blue range produces fluorescent colors that are blinding and cannot be seen under the full spectrum. While many people prefer diverse combinations of spectrums. A typical aquarium lighting system consists of 50% white with high Kelvin and 50% blue, photochromic lights.
In nature, ultraviolet rays (UV-A and UV-B) can penetrate the ocean’s surface, but are partially filtered through water. Both UV-A and UV-B rays cause damage to DNA and RNA in coral tissue.
Corals in shallow water will easily lose pigment during transportation.
Of course many coral species have adapted and reduced the effects of these harmful rays. These corals develop protective pigments, usually blue, purple, or pink. Most corals that contain these pigments live in shallow water, where UV-A and UV-B rays are higher than in deeper waters.
In home reef aquariums using light from halogen lamps, we will have to protect the corals from UV rays. Corals that do not have protective pigments and corals in shallow water are prone to loss of pigmentation during transport, and are particularly susceptible to UV rays. Fortunately, blocking UV rays from entering the aquarium is also quite simple as just using glass aquarium covers and properly installed.
The loss of colorful pigments is not necessarily a sign that the coral is unhealthy, it is simply that the coral is learning to adapt to its new environment.
A common misconception among many aquarists is that a change in the color of the coral is a sign that the coral is in trouble. Many times the color change is just the result of the coral adjusting to light intensity, spectrum, and changes in UV light from the new environment.
As such, it is important to consider the color of new corals and understand the effects of light on them. So you don’t need to worry too much because corals will know how to adapt to new light conditions. But everything will take time for them to adjust to the most suitable color.