concept

Counter-illumination

A form of camouflage by light: an organism glows on its underside to match the dim light filtering from above, erasing its own dark silhouette when seen from below. In the ocean's twilight zone, a non-glowing animal shows up as a dark shape against the brighter water overhead - so many midwater fish and squid grow belly "lamps" tuned to match the background, making themselves invisible. This same pressure is the inferred reason Pandora's fauna had to glow to blend into a luminous forest background.