Before safety lamps, British coal miners worked past explosive firedamp by the glow of dried fish skins, and Navy pilot Jim Lovell, before Apollo 13, found the carrier USS Shangri-La at night by following its glowing wake. Bioluminescence is cold light: an enzyme, luciferase, oxidizes a fuel, luciferin, and the excited molecule releases a photon. The crystal jelly Aequorea victoria instead uses aequorin, a calcium-triggered photoprotein, for sudden flashes. One luciferin, coelenterazine, appears across 11 animal phyla, yet the enzymes differ so much that the trait has evolved independently at least 94 times, first in octocorals about 540 million years ago and 27 times in ray-finned fishes alone. Howard Seliger proposed that pigment enzymes mutated into light emitters, while Rees argued that luciferin began as an antioxidant against reactive oxygen species and turned to light as animals moved deeper.

In the open ocean, with nowhere to hide, light became a weapon. The firefly squid erases its silhouette by counter-illumination, Octopoteuthis deletron sheds glowing arms as decoys, dinoflagellates flash as a burglar alarm that draws bigger predators to their grazers, and the barbeled dragonfish hunts with red light most deep-sea animals cannot see. The Hawaiian bobtail squid farms glowing Vibrio bacteria in a light organ that admits only the right species. Raphael Dubois identified luciferin and luciferase in the late 19th century by studying a mollusk’s siphons preserved in sugar, and Osamu Shimomura’s 1961 isolation of green fluorescent protein led to a 2008 Nobel Prize.

• Mycena and Armillaria fungi glow green, probably to attract spore-carrying insects.
• The railroad worm is the only known land organism that makes red light.
• GFP and luciferase serve as reporter genes that make tumors light up in lab animals.
• ATP bioluminescence swabs detect microbial contamination in seconds.
• The French company Glowee lit signs with Aliivibrio fischeri, but the bacteria lasted only about three days.