Light Made by Living Things

[A] Bioluminescence, the production of light by a living organism, is far more common in the ocean than on land. [B] The chemistry behind it is simple enough: a molecule called luciferin reacts with oxygen, and an enzyme speeds the reaction up. Almost all of the energy released leaves as light rather than heat, which is why the glow is cool to the touch. [C] What varies enormously is the use to which the light is put.

Some species use it to hide. [D] Many small fish that live in open water are lit faintly from below by rows of organs along the belly, and the brightness of these organs is tuned to match the dim daylight filtering down from the surface. A predator swimming underneath sees no silhouette. Other species use light to advertise. Certain deep-sea anglerfish dangle a glowing lure in front of the mouth, and small animals that approach it become the fish's next meal. A third strategy is closer to an alarm: when attacked, some shrimp eject a cloud of luminous fluid that clings to the attacker, making it conspicuous to larger hunters and buying the shrimp a moment to escape.