Bioluminescence is such a cool natural phenomenon. Victoria Gill's article for the BBC, Snails flash a green alarm light, discusses the newest organism to be discovered with bioluminescent abilities. Basically, bioluminescence is the ability to glow neon colors like a living glow-stick.
Organisms use their glow to scare off predators, and when that doesn't work, to put the spotlight on the organism that is attacking them so that perhaps another predator will come after it.
The bioluminescent snail are interesting because the actual part of the snail that glows is located inside the shell. The opaque nature of the shell amplifies the color so that its glow is increased and the glow spreads to the entire shell.