To protect algae that live within them and provide much of their sustenance, corals living in shallow waters fluoresce to protect the algae from damaging ultraviolet radiation, producing proteins that act as sunblock. But corals that live in deep, dark waters, away from the damaging sunlight, also fluoresce when there is no need to protect themselves. This has puzzled biologists for some time, until researchers from the University of Southampton figured out that deep-sea corals fluoresce for the opposite reason, but also to benefit the algae. The algae, which need to photosynthesise to survive, are too deep for the sun's rays to reach them, but the corals have used their fluorescing to produce their own sunlight in the near-darkness of the deep oceans, providing the algae with the required light for photosynthesis to occur. To find out more go to Deep water corals glow in the dark to survive.