Counter-illumination

Counter-illumination is a method of active camouflage seen in marine animals such as firefly squid and midshipman fish, and in military prototypes, producing light to match their backgrounds.

Marine animals of the mesopelagic (mid-water) zone tend to appear dark against the bright water surface when seen from below. They can camouflage themselves, often from predators but also from their prey, by producing light through bioluminescence on their lower surfaces, reducing the contrast of their silhouettes against the background. Counter-illumination differs from countershading, which uses only pigments such as melanin to reduce the appearance of shadows. It is one of the dominant types of aquatic camouflage, along with transparency and silvering. All three methods make animals in open water resemble their environment.

Counter-illumination has not so far come into widespread military use, but during the Second World War it was trialled in ships in the Canadian Diffused lighting camouflage project and in the American Yehudi lights project.

Among marine animals, counter-illumination camouflage occurs where bioluminescent light from an organism's ventral surface is matched to the light radiating from the environment. The bioluminescence is used to obscure the organism's silhouette produced by the down-welling light. Counter-illumination differs from countershading, also used by many marine animals, which uses pigments to darken the upper side of the body. Counter-illumination goes further than countershading, actually brightening the under side of the body.

...
Wikipedia