Corals are some of the most delicate and popular living organisms in the ocean. They are admired for their complex but beautiful structures, their provision of habitats for other sea life and of course, their mesmerizing colors. Their colorful nature makes corals a popular addition as aquarium organism. However, corals are notoriously sensitive organisms and sometimes lose their famous vivid colors when transplanted to at home aquariums. Many aquarists panic when this happens, fearing that their corals are dying or unhealthy. Coloration is not always an indicator of health; however, here is a simple guide to help explain why corals have their remarkable colors, how to maintain coloration in an aquarium and why corals may lose their color.
Coral’s coloration is mostly due to photosynthetic pigments, fluorescent proteins and non-fluorescent chromoproteins (Bruckner). The largest influence on these factors is light. Corals have a symbiotic relationship with algae called zooxanthellae. These tiny creatures are responsible for supplying corals with their color. In exchange, corals provide a place of residence and protection (Bruckner). Zooxanthellae contain chlorophyll, which supplies the coral with its color, as well as nutrients that help it survive. Corals often have several types of zooxanthellae known as clades, each of which have their own tolerance for temperature and light (Bruckner). Corals regulate the population of zooxanthellae and how much chlorophyll each cell contains (Live Aquaria). The major factor in the regulation of zooxanthellae is the intensity of light.
As the intensity of light increase, corals expel zooxanthellae or reduce the amount of chlorophyll in each cell. Because zooxanthellae produce higher amounts of oxygen in brighter environments, their population must be reduced (Foster, Smith). High concentrations of oxygen can be deadly to corals. On the other hand, when the intensity of light is low, zooxanthellae are unable to produce enough nutrients to sustain its host coral. Subsequently, corals will increase the number of zooxanthellae and the amount of chlorophyll in each cell (Live Aquaria). This increases the surface area for capturing additional light energy helping corals to display their wonderful coloration.
What does this mean for coloration? The zooxanthellae cells range in color from yellow-green to brown. The higher the population of cells, the coral will appear brown, which is the case in low intensity lighting (Foster, Smith). The opposite is true with an increase in light intensity. With a lower concentration of zooxanthellae cells, the coral appears lighter or yellow-green. Some corals can also fluoresce red under certain conditions. Another pigment found in zooxanthellae, phycoerythrin, allows some corals to give off a bright orange color (Bruckner). As mentioned earlier, chromoproteins, non-fluorescent reflective proteins, are also responsible for the coloration of corals. There are about two-dozen chromoproteins that can be identified in color corals. These chromoproteins result in variations of red, purple, blue and mauve (Bruckner).
Aquarium lighting also has an effect on the coloration of corals in a tank. Bulbs with different Kelvin ratings will produce different intensities of light. Some aquarists use bulbs that emit actinic blue light, which accentuates fluorescent colors that are unable to be seen under regular lighting (Live Aquaria). Typically, a 50-50 mixture of white light at a higher Kelvin rating and actinic blue light are used for aquariums. UV light also plays a part in coral coloration. Corals dwelling in shallow ocean waters, where UV light exposure is higher, are more likely to have blue, purple or pink pigments due to their protective qualities (Live Aquaria). In aquariums, glass is placed over metal halide lighting to help reflect UV light from entering the tank. Previously shallow dwelling corals that are transferred to aquariums tend to lose their bright coloration during transport or after they adapt to lower UV conditions within the aquarium (Foster, Smith).
Many aquarists mistake this loss of color in newly received corals as an unhealthy or dying coral. However, this change is mostly due to changing light intensity, spectrum or change in UV light (Foster, Smith). Thus, it is important to research and have as much knowledge as possible about your corals, the conditions they prefer, under which intensity or spectrum of light they will flourish. This will allow you to adapt any newly received corals to their new aquarium home and give your tanks the desired pop in color!
Bruckner, Andrew. “Fiji’s Colorful Corals Are Indication of Coral Health, Reef Resilience.” Living Oceans Foundation. Khaled Bin Sultan Living Oceans Foundation, 04 Mar. 2014.
“Factors That Influence Coral Coloration.” Live Aquaria. Petco Wellness, 2016. Web. 21 Nov. 2016.
Foster, and Smith. “Colors of Corals: Light, Chlorophyll, and Other Pigments.” Animal Planet. Veterinary & Aquatic Services Department, 15 May 2012.