Bioluminescent Jellyfish
Depth: 700 m · Twilight Zone
Many deep-sea jellyfish can make their own light. This is called bioluminescence. They use their glow to attract prey or startle enemies, because down here it is otherwise pitch black.
For scientists
The glow usually comes from the molecule coelenterazine reacting with an enzyme and oxygen, cold light with almost no heat. Some jellyfish like Atolla flash a spinning burglar-alarm light when attacked, drawing in larger predators to eat the attacker. In the lightless deep, bioluminescence is the most common way to communicate.
Source: Wikipedia
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