counterilluminate

Counterillumination is a camouflage strategy used by many marine animals to conceal their silhouette against the brighter surface waters when viewed from below. This technique involves the emission of light from the underside of the animal's body, matching the intensity and color of the downwelling sunlight. Organisms that employ counterillumination typically possess photophores, specialized light-producing organs, strategically placed on their ventral surface.

The effectiveness of counterillumination relies on the principle of matching the light emitted to the ambient light conditions. As light penetrates the ocean, it is absorbed and scattered, with blue wavelengths traveling farthest. Therefore, many counterilluminating species produce blue light. The intensity of the emitted light can also be adjusted by the animal to compensate for variations in light intensity throughout the day, water depth, and turbidity.

This form of camouflage is particularly common in mesopelagic (twilight zone) and epipelagic (sunlight zone) environments where light levels are sufficient for visual predators to detect prey from below. Animals found at depths ranging from the surface down to approximately 1000 meters, including various fish, squid, and crustaceans, are known to utilize counterillumination. By effectively blending their ventral outline with the background light, these organisms reduce their conspicuousness and enhance their survival rates against predators that hunt by looking upwards.