Discovered in the deep: the incredible fish with a transparent head

The rare barreleye fish tracks its prey with extremely light-sensitive rotating eyes encased in a see-through canopy

Discovered in the deep: the worm that eats bones

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.

The ocean is one of the world’s last truly wild spaces. It teems with fascinating species that sometimes seems to border on the absurd, from fish that look up through transparent heads to golden snails with iron armour. We know more about deep space than deep oceans, and science is only beginning to scratch the surface of the rich variety of life in the depths.

As mining companies push to industrialise the sea floor and global leaders continue to squabble over how to protect the high seas, the Guardian's Seascape series, Discovered in the deep, will profile some of the most recently discovered weird, wonderful, majestic, ridiculous, hardcore and mind-blowing creatures. They reveal how much there is still to learn about the least known environment on Earth – and how much there is to protect. 

Their eye pigment allows the fish to distinguish between sunlight and bioluminescence, says Bruce Robison, deep-sea biologist at the Monterey Bay Aquarium Research Institute in California. It helps barreleyes to get a clear view of the animals trying to erase their shadows.

The barreleye’s tubular eyes are extremely sensitive and take in a lot of light, which is useful in the inky depths of the twilight zone. But Robison was initially mystified that their eyes seemed fixed upwards on the small spot of water, right above their heads.

“It always puzzled me that their eyes aimed upward, but the field of view did not include their mouths,” says Robison. Imagine trying to eat scraps of food floating in front of you, while fixing your eyes on the ceiling.

But, after years of only seeing dead, net-caught specimens, Robison and colleagues finally got a good look at a living barreleye through the high-definition cameras of a remotely operated vehicle. “Suddenly the lightbulb lit and I thought ‘A-ha, that’s what’s going on!’,” he says. “They can rotate their eyes.” This means the fish can track prey drifting down through the water until it is right in front of their mouth.

Seeing a barreleye alive in the deep, Robison saw something else that scientists had previously missed. “It had this canopy over its eyes like on a jet fighter,” he says, referring to the transparent front part of the barreleye’s body, which had been torn off all the specimens he had previously brought to the surface.

He thinks this canopy probably helps protect their eyes as they steal food from among the stinging tentacles of siphonophores – animals that float through the deep sea in long, deadly strings, like drift nets.

Barreleyes have been found with a mix of food in their stomachs, including siphonophores’ tentacles, as well as animals that siphonophores feed on, including small crustaceans called copepods. Their tactic may be to swim up to siphonophores and nibble on the small prey snagged in their tentacles, using the transparent shield to protect their green eyes from stings.

But encountering barreleyes in the wild is not easy. In his 30-year career, Robison says he has only seen these 15cm-long fish alive maybe eight times. “We do spend a lot of time exploring down there, so I can say with some confidence that they’re quite rare,” he says.

Contributor

Helen Scales

The GuardianTramp

Related Content

Article image
Discovered in the deep: the rainbow fish that’s born female and becomes male
Scientists in the Maldives were only able to reach the rose-veiled fairy wrasse by using specialised diving gear

Helen Scales

19, Oct, 2022 @5:00 AM

Article image
Discovered in the deep: the anglerfish with vampire-like sex lives
A deep trawl has brought up a potentially new species of a fish whose extreme mating methods include permanent physical fusion

Helen Scales

01, Feb, 2023 @6:00 AM

Article image
Discovered in the deep: the crustacean with eyes for a head
Shrimp-like Cystisoma are protected from predators by being virtually invisible – thanks to unique retina and a body that casts almost no shadow

Helen Scales

14, Sep, 2022 @5:30 AM

Article image
Discovered in the deep: the ghost catshark found after an egg hunt
Scientists in Australia solve puzzle of sole egg left in museum and identify new deep-sea species with unique ridged egg case

Helen Scales

04, Jul, 2023 @5:00 AM

Article image
Discovered in the deep: the worm that eats bones
Thought to have existed since the Cretaceous, more than 30 species of Osedax – ‘bone devourers’ or zombie worms – feast on whale skeletons

Helen Scales

22, Aug, 2022 @5:00 AM

Article image
Discovered in the deep: the ‘forest of the weird’
More than a mile beneath the Pacific Ocean, is a seascape of oddly shaped corals and a glass sponge named after ET

Helen Scales

11, Oct, 2022 @5:00 AM

Article image
Discovered in the deep: meet Casper the ghostly octopus
First spotted in Hawaii in 2016 more than two miles beneath the surface, scientists are slowly learning the cephalopod’s secrets

Helen Scales

08, Aug, 2022 @5:00 AM

Article image
Discovered in the deep: the ‘Elvis worms’ that sparkle in the darkness
In 2020, scientists found sparkling Peinaleopolynoe on hydothermal vents in the eastern Pacific – and were irresistibly reminded of the king of rock’n’roll

Helen Scales

07, Sep, 2022 @5:00 AM

Article image
Discovered in the deep: is this the world’s longest animal?
A submersible off the coast of Western Australia chanced upon a 45-metre-long deep-sea siphonophore arranged in a feeding spiral, trailing its deadly tentacles

Helen Scales

30, Nov, 2022 @6:00 AM

Article image
Discovered in the deep: the sharks that glow in the dark
Measuring up to 1.8 metres, kitefin sharks are by far the biggest bioluminescent sharks to have been found

Helen Scales

09, Nov, 2022 @6:00 AM