Wikipedia
Counter-illumination is a method of camouflage in which an animal (or possibly a military vehicle) produces light to match an illuminated background, such as the ocean surface or sky. It differs from countershading which uses only pigment, such as a vehicle's paint or the melanin in an animal's skin, to paint out the appearance of shadows. By adding the right amount of light through bioluminescence or electronically controlled lamps, counter-illumination can at best exactly match the brightness of the background, making for more effective camouflage than countershading can provide. Since counter-illumination allows the intensity of the generated light to be adjusted, camouflage can be actively adapted to varying lighting conditions and backgrounds.
Counter-illumination is one of the dominant methods of camouflage in marine organisms, along with transparency and silvering. All three methods make animals in open water resemble their environment.