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Brain coral (Platygyra daedalea) underwater, UV light. Brain coral photographed using near-ultraviolet (UV) flashlight. The UV light excits pigments within the coral and causes them to emit a fluorescent glow (green). It is thought the glow attracts symbiotic algae which in turn allows the coral to grow in deeper water. Fluorescence may also offer protection against harmful ultraviolet light from the sun in shallower water. (KEYSTONE/SCIENCE PHOTO LIBRARY/EYE OF SCIENCE/SCIENCE PHOTO LIBRARY)