Hatchetfish are well camouflaged. Like many deep sea fishes, they have light-producing organs in rows along their bellies. These organs shine a pale blue light that matches daylight filtering down from above, and hides them from predators below.Hatchetfish can regulate the intensity and color of light from these organs to match the light filtering down. Each species of hatchetfish has its own particular pattern of lights.
This use of bioluminescent light is called "counterillumination," a common adaptation in midwater fishes and cephalopods. Some midwater predators hunt by looking up for the silhouettes of animals. Counterillumination makes hatchetfish almost invisible from below.