Between depths of two hundred and one thousand meters lies the ocean's mesopelagic or 'twilight' zone, where sunlight is too faint to support photosynthesis yet sufficient for predators to detect silhouettes. To survive in this hazardous dimness, more than ninety percent of mesopelagic animals have evolved bioluminescent organs called photophores. While some deep-sea squids and viperfish deploy intense bioluminescent flashes to startle approaching predators, many species utilize counter-illumination. By emitting downward-directed light that precisely matches the intensity, color, and wavelength of downwelling solar light filtering from the surface, these organisms effectively obliterate their silhouettes from predators lurking beneath them. This optical camouflage allows them to remain completely invisible against the downwelling surface shimmer.