Aesthetically, I think we can agree that as ecosystems go, coral reefs are not too shabby. Coral reefs are characteristically colorful and visually dynamic, and coral reef ecosystems are brimming with diverse, multi-colored fish. The photo below captures just one example of the variations in shape, color, and size that make coral reefs so unique. In this series of blog posts, I’ll be exploring the world of color on coral reefs. I’ll be writing about coral reef pigmentation, colorful reef fish, and ultimately why reef colors have important implications on ecosystem function.
Learning about corals is especially important today. Rising ocean temperatures and increasing ocean acidification have led to widespread coral disease1. Many coral species provide homes for symbiotic algae. That is, algae (called zooxanthellae) and corals have formed a partnership. Zooxanthellae get to live in coral skeletons and in return provide corals with food and protection from disease. When corals are stressed—by UV light, increasing sea surface temperatures, or a variety of other factors—they expel their symbiotic algae. If corals go too long without their algal partners, they die. If we want the planet’s coral reefs to remain beautiful for years to come, we need to first learn about the complex and balanced biodiversity, in all of its colorful beauty.
Figure 1. Coral reef ecosystems are highly diverse and highly productive. This image shows multiple species of coral and coral reef fish in clear, shallow water. Source: Wikimedia Commons
We’ll start from the beginning. Where do the colors on coral reefs come from? Endosymbiotic zooxanthellae, the tiny algae that live in coral skeletons and photosynthesize, are critical for keeping corals well fed. Zooxanthellae are also responsible for the underlying brownish hues in coral colonies2. Zooxanthellae cells contain chlorophyll (green pigment) in varying concentrations. The intensity and wavelength of the light hitting a specific coral also alter the perceived color. Depending on the light in the surrounding water, corals can appear a wide variety of colors.
A study on Montastrea cavernosa corals sought to shed light on color diversity. Researchers found that color diversity in these corals can be explained by varying levels of gene expression. Specifically, genes code for proteins similar to green fluorescent proteins (GFP). When these corals’ genes cause proteins to be expressed, they emit at three general wavebands: cyan, green, and red3. So, fluorescent proteins are also super important contributors to the variety of colors on reefs.
But what makes reef colors evolutionarily or ecologically important? A study by Salih et al. published in Nature describes how fluorescent pigments (FPs) in corals are photoprotective. Think sunscreen. These FPs scatter light and filter out damaging UV light, effectively protecting the corals from damage4. These scientists studied corals on the Great Barrier Reef and found that 124 species in 16 sampled families contained fluorescent morphs as well as non-fluorescent morphs. The different colors observed in different species are generally outlined in the figure below. Fluorescent morphs were especially common in shallow sites and were also more common in sunny areas than shady areas.
Figure 2. Fluorescent pigments found in blue, green, yellow, and red combinations (a, c, e, g). In Acropora nobilis (a, b) blue was found to be dominant. In Pocillopora damicornis (c, d) green was dominant. Porites cylindrica (e, f) showed emissions of all FPs. Source: Salih et al.
The figure below is from the same study. Graph A shows that corals with lots of fluorescent pigments recovered faster and more completely than non-fluorescent Acropora palifera when exposed to full sunlight. Graph B shows a significant correlation between bleaching resistance, measured by the biomass of symbiotic algae, and concentration of fluorescent pigments. Together, these graphs evidence the benefits of colorful, fluorescent pigments. Salih writes that “by changing their optical properties with the help of these [green fluorescent protein]-like pigments, coral polyps are able to optimize the photosynthetic activity of their tissues for the better survival of the organism.”
Figure 3. a) For the Acropora palifera species, the maximum yield of dinoflagellates (zooxanthellae) over time. NF stands for non-fluorescent, BF stands for brown medium fluorescent, and GF stands for green highly fluorescent. The highly fluorescent corals recovered best and most rapidly. b) Positive correlation between FPs concentration and increased zooxanthellae biomass. Source: Salih et al.
Not only are corals colorful, but the fish that inhabit coral reefs are brilliantly colored as well. Various studies have explored the benefits of bright patterns on reef fish. John Endler asserts that color patterns in reef fish are important for communication within the same species for courtship or mating, to confuse or escape visual predators, and can advertise danger5.
Colorful reef fish are often camouflaged, blending into their surroundings, to avoid being eaten by predators. A study described in Behavioral Ecology investigated the effects of coral bleaching and habitat degradation on coral reef fishes’ susceptibility to predation. Predation rates on coral-dwelling damselfish were found to be 17% higher on bleached or dead coral colonies6.
Coral reef fish and coral reefs are interdependent; in order for the whole ecosystem to function, the colors of each must remain in balance. Coral bleaching, the loss of coral pigmentation, can lead to the collapse of the entire underwater ecosystem. In the next blog post, I’ll talk more about coral reef fish, specifically how they respond to changes in coral reef coloration.
1 “NOAA Declares Third Ever Global Coral Bleaching Event.” NOAA. National Oceanic and Atmospheric Administration, 8 Oct 2015. Web. 16 Feb 2017.
2 Factors That Influence Coloration. “Factors That Influence Coloration.” N.p., n.d. Web. 16 Feb 2017.
3 Ilya V. Kelmanson, Mikhail V. Matz; Molecular Basis and Evolutionary Origins of Color Diversity in Great Star Coral Montastraea cavernosa (Scleractinia: Faviida). Mol Biol Evol 2003; 20 (7): 1125-1133. doi: 10.1093/molbev/msg130
4 Salih, Anya, et al. “Fluorescent pigments in corals are photoprotective” Nature. 14 Dec 2000. Web. 19 Feb 2017.
5 Sabbagh, Stephanie M. “Significance of Colors and Patterns of Coral Reef Fishes: An Overview.” ReefCI. N.p., 31 Oct. 2013. Web. 12 Feb 2017.
6 Coker, Darren J. et al. “Coral bleaching and habitat degradation increase susceptibility to predation for coral-dwelling fishes.” Behavioral Ecology. 19 August 2009. Web. 19 Feb 2017.