Hatchetfish are deep-sea fish found in the Pacific, Atlantic, and Indian oceans. They use a form of bioillumination to mimic light intensity from the surface, effectively cloaking it from poor-sighted predators. Creepy, but cool.
The ROV Hyper Dolphin caught this deep-sea jelly (Atolla wyvillei) on film east of Izu-Oshina Island, Japan. When attacked, it uses bioluminescence to "scream" for help—an amazing light show known as a burglar alarm display.