In the dark twilight zone of the deep ocean, somewhere between 600 and 800 meters below the surface, there are fish looking up through their transparent heads.
Their eyes are like huge spherical lenses on silver barrel-like tubes, which earned them their common name barreleye (Macropinna microstoma).
The green tint (which actually comes from a yellow pigment) acts as sunglasses to help them spot their prey.
There is nowhere for any animal to hide in these ocean depths, and many animals that live there have bright bellies that act as camouflage and protect them from bottom predators.
However, these particular fish are a step ahead. The pigment in their eyes allows them to distinguish between sunlight and bioluminescence, Bruce Robison, a deep-sea biologist at the Monterey Bay Aquarium Research Institute in California, told the Guardian.
It helps these fish get a clear view of the animals they are trying to cover.
The fish’s tubular eyes are extremely sensitive and take in a lot of light, which is useful at these depths.
“It always struck me that their eyes were pointing up, but the field of view didn’t include their mouths,” says Robison. “Imagine trying to eat scraps of food floating in front of you while staring at the ceiling,” he says as an example.
But years later, Robison and his colleagues were able to get a close-up look at a live barreleye through the high-definition cameras of a remote-controlled system.
“Suddenly it lit up and there I saw that they can rotate their eyes,” he said. This means that fish can track prey drifting in the water until it is right in front of their mouth.
Seeing the fish live, Robison also noticed something else that had eluded scientists until then.
“It had this canopy over its eyes like fighter jets have,” he said, referring to the transparent front part of its body, which had been detached from all specimens scientists had previously found.
He believes this canopy probably helps protect the fish’s eyes as it steals food from between the tentacles of siphonophores, animals that float in the deep sea like drift nets.
Scientists have spotted barreleyes with a mixture of food in their stomachs, including siphonophore tentacles, as well as other animals that siphonophores feed on, including copepods.
Their tactic may be to swim up to the siphonophores and nibble on small prey caught in their tentacles, using the transparent shield to protect their green eyes from stings.
However, it is not easy or common to meet these fish in the wild. In his 30-year career, Robison says he’s only seen these 6-inch fish live 8 times. “We spend a lot of time exploring down there, so I can say with some confidence that it’s pretty rare,” he says.
With information from the Guardian