In the dim light of dusk, the world appears gray. That’s because we, like most vertebrates, have just on type of rod opsin, the light-sensitive pigment in our eyes that lets us see in low light. During the day, a different set of pigments in cone cells pick up a broader range of wavelengths, giving us color vision.

Now, an international team of researchers has discovered that fish living in the dark depths of the oceans nearly a mile below the surface may use rod opsins — which grant us l