Bioluminescence is the ability of an organism to produce light. A firefly, for example, uses bioluminescence as a mating call and attracts females with its glow. In the ocean, about 90% of fish and crustacean species found 200–1000 metres deep in the mesopelagic zone, are bioluminescent.
The mesopelagic zone is also known as the twilight zone because of the low ambient light. The diffused sunlight present here has uniform intensity, colour and direction. It offers no refuge from predators, no nooks or crannies to hide in. Predators below have a clear view of the shadows of the fish swimming above them, making them easy prey.
However, some fish have mastered the technique of counterillumination camouflage, where they light up their underside to match the intensity and wavelength of the surrounding light perfectly. This adaptation blurs the silhouette of the fish for the predators lurking below, and they hide in plain sight!
Besides helping the fish blend into its surroundings, the camouflage also acts as a lure. Some species have luminescent patterns that make them seem like they are a small fish. Tricked by this, when a fish approaches them to attack, they reveal their actual size and devour it!
Research shows that each animal uses bioluminescence differently, revealing a new hunting ground working on entirely different rules. While the anglerfish uses it as a lure, ostracods use it as a repellent. Dragonfish use it to avoid detection while swimming by lighting up their path with a red light that other fish cannot see. Acanthephyra purpurea, a type of shrimp, uses it to create a flash of light by spitting out bioluminescent ink at their predators, stunning them visually.
Edie Widder, an oceanographer, says of these creatures: “It's these rocket ships and explosions of sparks and spewing of what looks like blue smoke... it's magic.”