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Close up photo of the external limestone structure of a coral. Bioluminescent corals produce light from a protein, and for deep sea species, where the light is low due to the depth, this additional light helps with photosynthesis for the symbiotic algae which lives within the coral polyp (animal). The algae uses light to produce carbohydrate which provides food for itself and for the polyp, while waste from the polyp feeds the algae. Cairns Aquarium.