A pocketful of glowing bacteria
Animals that produce light are called bioluminescent. The light is usually blue, a wavelength that travels through water best and is visible to most marine organisms. Watch our splitfin flashlightfish. About every 30 seconds you'll see the flash of a fish's light organ. It's a bean-shaped pocket under either eye, filled with billions of symbiotic bacteria that live on sugar and oxygen from the fish's blood and emit a biochemical light as a by-product. Each pocket has a black membrane that can be raised, like an upside-down eyelid, to block nonstop illumination that could attract predators.
Blink to evade predators or attract a mate
When a school of flashlightfish is pursued by a predator, the fish use a “blink-and-run” defense. Taking off, the fish flash up to 75 times a minute and zigzag so that the predator can’t zero in on any individual. At some point, the school changes direction, the fish conceal their lights in unison, and they swim away safely. The light is also a visual Morse code for attracting a mate and for communicating within schools of 20 to 200 fish.