This cool video shows how underwater creatures express fluorescent proteins. When struck by ultraviolet light they emit green light back. Luckily we've managed to sequence the genomes of these proteins and express them in whatever we choose to make; like glowing mice. We've determined to a large extent how they function but why they exist in nature is anyone's guess. The video suggests that they may be a way of protecting against harmful UV light since the pigments absorb the radiation, thereby preventing harm to the creature that expresses the protein. Right now words will probably just take away from the beauty exhibited by these creatures, so enjoy the video!
Video credit: BBC Earth