Counterillumination

Counterillumination is a camouflage strategy used by some marine organisms in dimly lit ocean zones. It involves emitting light from ventral photophores to match the intensity and color of the downwelling light from above, effectively masking the animal’s silhouette from predators looking upward.

The light is produced by specialized organs called photophores, arranged in species-specific patterns on the body. Emission is adjusted to track ambient light with depth, so brightness and hue approximate the background conditions. In many species the emitted light is predominantly blue-green to align with seawater’s light transmission window. Control of photophore output often involves neural or hormonal mechanisms and can respond to changes in depth, time of day, and surrounding light.

Taxa known to use counterillumination include lanternfishes (Myctophidae), hatchetfishes (Sternoptychidae and related groups), and certain squids and crustaceans. Photophore arrangements commonly form ventral stripes or patches that produce a diffuse glow, helping to conceal the organism’s form from predators below.

Function and ecological significance: Counterillumination reduces detection by predators from below and can influence vertical distribution and survival during diel vertical migration. In some species, the ventral glow may also play a role in intraspecific communication or schooling, though camouflage is the primary function. The effectiveness of counterillumination depends on depth, background light, and the viewing angle, and it remains an active area of research in marine biology and sensory ecology. See also bioluminescence, photophore, camouflage.