Bioluminescence can provide effective camouflage. In the twilit depths, the silhouette of an animal against the sunlight filtering down is an easy target. This is why many open-ocean predators like the hatchetfish have upturned eyes and mouths. But the hatchetfish itself is prey to bigger fish swimming below it. It’s narrow silhouette and silver sides make it harder to see, but it also emits light from its belly that is a perfect match in color and intensity to sunlight filtering down. If a cloud dims the sunlight, the fish dims its bioluminescence—a trick called counterillumination.