counterillumination

Counterillumination is a camouflage technique used by marine animals to avoid detection from predators below.

By producing light on their undersides through bioluminescence, creatures like squid and certain fish match the ambient light filtering down from the surface, eliminating the dark silhouette that would otherwise make them visible.

The strategy exploits the fact that ocean predators often hunt by looking upward for shadows against the brighter water above.