Coral reefs are famous for their beautiful colours, but in fact when I visit them I am usually surprised how colourful reefs AREN’T. Most healthy corals have a sort of dusty tan or pink hue to them, and it’s only on reefs with particularly high coral cover, superb water clarity and bright overhead sun that you really see a lot of colours “pop”. The same cannot be said under UV light, however, as this video shot in the Gulf of Aqaba demonstrates spectacularly (more videos here). Many corals (and sometimes other reef organisms, as you’ll see) fluoresce under UV light, and that lends to the reef a surreal and psychedelic feel if you SCUBA at night and bring a blacklight with you. Fluorescence occurs when a material absorbs one wavelength of light and re-emits it at another wavelength. In this case the corals absorb the UV, which we can’t see, and they re-emit it in the visible wavelengths that we can, and so the corals appear to glow or emit their own light, because they are. The effect is quite stunning.
It seems that “native” fluorescence like this may not actually serve an adaptive function. Corals aren’t subjected to those UV wavelengths much in nature, so there’s no selective force to promote the evolution of this kind of trait. Instead, it’s more likely a wonderful and beautiful coincidence that can be exploited to make spectacular YouTube videos like this one! The culprits are different protein pigments in the corals and their symbiotic algae (Symbiodinium spp.) that serve important biological functions for the coral, acting as electron donors in biochemical reactions, as anti-oxidants and other functions. But, they also just happen to fluoresce under the right wavelengths of illumination. This class of proteins includes the famous GFP, Green Fluorescent Protein, first isolated from jellyfish but these days genetically engineered into a whale range of organisms as a molecular biological tool.