In the ocean’s shadowy twilight zone, between 600 and 800 metres beneath the surface, there are fish that gaze upwards through their transparent heads with eyes like mesmerising emerald orbs. These domes are huge spherical lenses that sit on a pair of long, silvery eye tubes – hence its common name, the barreleye fish (Macropinna microstoma).
The green tint (which actually comes from a yellow pigment) acts as sunglasses, of a sort, to help them track down their prey. There’s nowhere to hide in the open waters of the deep ocean and many animals living here have glowing bellies that disguise their silhouette and protect them – bioluminescent prey is hard to spot against the dim blue sunlight trickling down. But barreleyes are one step ahead.
