“It’s deep. It’s dark. It’s elusive.” The ocean’s twilight zone is full of wonders.

Amanda Northrop/Vox

Scientists are probing its depths. So are fisheries.

Let’s dive down into the ocean. Start by imagining you’re floating on the surface, basking in the sun of a hot day. Next, dive.

First 50 meters.

Then 100 meters.

Then 150 meters.

(In this imaginary exercise, you have amazing, inhuman lungs.)

At the beginning of the dive, you’re in the ocean’s epipelagic, or sunlight zone: the shallow waters where light still penetrates and photosynthetic organisms live. But as you dive deeper and deeper, the sunlight above you fades. The ocean around you gets darker and darker, colder and colder.

Amanda Northrop/Vox

At 200 meters, you enter a new division of the ocean. It is “the middle ground between light and shadow,” an area which scientists call ... the twilight zone. (Or when they’re being technical, the mesopelagic zone.)

And if the televised Twilight Zone lies somewhere between “the pit of man’s ignorance and the sum of his knowledge,” so does the one in the ocean.

“It’s almost easier to define it [the ocean’s twilight zone] by what we don’t know than what we do know,” says Andone Lavery, an acoustician at the Woods Hole Oceanographic Institution who has been probing the mesoplagic’s depths for years. The twilight zone extends across all oceans on the globe. It’s so big and difficult to access, it does not lend itself easily to being studied. “It’s remote. It’s deep. It’s dark. It’s elusive. It’s temperamental,” she says.

As a result, the twilight zone is one of the more poorly understood ecosystems of the Earth. It’s important that we try to learn more, because this ecosystem may be critical. Andone and her fellow researchers suspect that it plays a crucial role in regulating the climate. But the scientists who are studying the twilight zone are working against the clock, because fisheries have also taken an interest in the region.

On this episode of the Unexplainable podcast, we ask: Can they learn enough about the twilight zone to help fisheries fish it sustainably ... before it’s too late?

The twilight zone is so mysterious we don’t even know how many things live there

Let’s start off with what is known about the ocean’s twilight zone. For one, it’s home to a lot of life.

In 2014, a paper came out suggesting that there might be 10 times as many fish living in the twilight zone as was previously suspected. That would mean that there were potentially more fish in the mesopelagic than the rest of the ocean combined. This new number is just an estimate, however, and could have its own measurement problems. Lavery laughs, explaining that the one thing she can say for certain is, “there’s a lot of fish” living down there.

Another thing we know: Every night, some number of those fish, along with many other non-fish organisms, rise up from the twilight zone to the surface of the ocean in order to feed. Then, they sink back down to their dark ocean home in the depths.

“It’s been called the greatest migration on Earth,” says Annette Govindarajan, a WHOI oceanographer who also does twilight zone research.

This process was originally discovered back in the 1940s, when navy-affiliated researchers were testing out different methods for detecting submarines. They were using SONAR — sending out pulses of sound and measuring their reflections. Normally, the strongest reflection should come from the very bottom of the ocean. But their measurements kept picking up a “false bottom,” much higher up than expected. This was a layer of life in the twilight zone, so dense it appeared like the floor of the ocean. And that layer was rising and falling each day.

Simon Thorrold, an ecologist who studies the phenomenon, describes the motion of all this rising and falling life as like a kind of breathing. “Like a diaphragm,” he says, “driving the very breath that the world is taking every day.”

The twilight zone may play a huge role in storing carbon from the air deep in the sea

The breathing analogy is especially apt because this migration may play a big role in our atmosphere’s carbon cycle. Scientists don’t have a concrete number, but it’s possible that migrating creatures help take 2 to 6 billion metric tons of carbon out of the atmosphere every year. That’s a big number. For context, road vehicles account for about 3.6 billion metric tons of carbon dioxide every year.

How could these creatures do this? The current thinking is that carbon dioxide dissolves in the surface waters. Some of it is taken up by tiny plankton. Then, when these migrating denizens of the deep come up to the surface, it’s thought that they gobble up that plankton, and then poop deep in the ocean later on. The carbon could therefore be sequestered in the poop.

“Once it’s down deep, it’s trapped,” Thorrold says, “So it doesn’t tend to come back into surface waters.”

Scientists at WHOI like Thorrold, Lavery, and Govindarajan are collaborating to uncover the specifics of how this all works. But before they can figure out the exact role the twilight zone life plays in storing carbon, they need to learn a lot more about the life itself.

The questions are big. Govindarajan lists a few: “How much biomass is there? And what are the species that are making up that biomass?” What is the life history of all these creatures? What’s their behavior, their general ecology? Scientists have some information, but so much still remains to be uncovered. “I think it’s really exciting,” Govindarajan says.

The life scientists have discovered so far in the twilight zone is wonderful ...

Researchers still don’t know everything about life in the twilight zone. But they have made some weird, wonderful discoveries. For example, the zone may not be lit up by the sun, but it has its own source of light: a dazzling array of bioluminescent creatures that sparkle in the darkness.

Paul Caiger/Woods Hole Oceanographic Institution

A close-up view of siphonophore tentacles.

There are siphonophores, living colonies of jellyfish-like clones all strung together in long chains. They can grow to incredible lengths — the longest found so far was estimated to be 150 feet. That’s longer than a blue whale, the largest animal on Earth.

Paul Caiger / Woods Hole Oceanographic Institution

Female anglerfish hunt using an enticing blue-green light called an “esca.”

There are anglerfish, familiar to some as the scariest thing that happened in Finding Nemo. Anglerfish females dangle a flashing blue-green light, also known as an “esca,” in front of their faces, which beguiles prey animals like a will-o-wisp until they come close enough to be devoured. (Also, some anglerfish can retract their teeth, and anglerfish stomachs can expand and their bones can flex, so they can eat organisms twice their size. Have a good nightmare!)

Paul Caiger/Woods Hole Oceanographic Institution

Hygophum hygomii is one of many species of lanternfish, so named because of their bioluminescence.

And then, there are the chameleons of light: the fish and organisms that use countershading, a form of camouflage. At 500 meters, Thorrold explains, everything would seem pitch black to us. But there is still some measurable light from the sun trickling down, or “downwelling.” If you’re a predator, swimming below a prey fish, Thorrold says, “You look up, and you’ve got very sensitive eyes, so you will see that silhouette of that fish.”

So the prey fish use bioluminescence to hide, to blend in with the faint sunlight. Their bellies are studded with photophores — organs that produce light — and they use those organs to match the light downwelling from the surface exactly, disguising themselves.

Paul Caiger/Woods Hole Oceanographic Institution

The photophores (light-producing organs) on a lanternfish allow it to blend in with light from above.

“It’s hard enough to match that intensity so well. That’s cool in itself,” Thorrold says. But what’s even more remarkable is that these fish also match the color of the light filtering down, which can change slightly depending on depth.

And these chameleons don’t stop there. Once they’ve done the work, they may drift up and down so they don’t have to start over again.

“If the sun goes behind a cloud,” Thorrold explains, “we think that these fish will actually move up in the water column as the cloud is passing in front of the sun to make sure that they maintain that exact match of light as that cloud goes past.”

... And a little bit scary ...

Paul Caiger/Woods Hole Oceanographic Institution

Pram bugs (Phornima) burrow into their gelatinous prey.

This is the “pram bug.” It has an innocent-sounding name, but its behavior is something out of a horror movie. When these twilight zone crustaceans catch their prey, they slice them open with their claws, chew out their insides, and move in.

But wait, it gets scarier.

The females of the species lay their eggs in the hollowed-out remains of their prey, then clammer back out again and push the husk full of eggs around like a baby pram.

... And attractive to predators

Jeff Gritchen/Digital First Media/Orange County Register via Getty Images

Blue sharks don’t live in the twilight zone, but use eddies to descend into its depths and feed.

Thorrold got interested in the twilight zone because he was researching big predator species, like sharks, swordfish, and tuna. He found that some of those species were diving deep into the mesopelagic to feed.

Swordfish and some species of sharks, he says, have evolved ways to keep parts of their whole bodies, or even specific parts of their bodies, warmer than their environment. That means that when they dive deep, “they’re warmer than their prey, which means that their brain is functioning better, their eyes are functioning better. And so they’re adapted to having a temperature advantage over their prey that allows them to feed.”

Other species don’t have these adaptations and instead go for joy rides. Blue sharks, for example, take advantage of eddies that are warm at their center, which will draw them down into the twilight zone, where they can feed without getting a chill.

This beautiful, mysterious place may be in danger

The twilight zone is a fascinating, alien part of our ocean. But it’s also a rich potential resource. As early as 1970, people were already wondering if the fish there could be eaten.

Andone Lavery doesn’t think people will be eating many siphonophores and anglerfish.

“It’s not like I anticipate, in 10 years’ time, someone putting a plate of mesopelagic fishes in front of me at a restaurant,” she says, laughing.

But she does think we may see increased commercial fishing of twilight zone organisms for fish meal, for aquaculture, or for chicken feed, say. There have already been a few attempts to catch mesopelagic fish in oceans across the world, though they’re small scale so far.

If the fishing scales up, that’s not necessarily a problem — there are many examples of fisheries that are well managed and scientifically informed. But if the twilight zone and the organisms that migrate into and out of it each day are playing a vital role in taking carbon out of the atmosphere and sequestering it deep beneath the waves, we need to understand that process as fully as possible so we can do that kind of fishing sustainably.

“If we fish out a lot of the organisms that are shuttling that carbon, then that’s going to have a huge economic cost,” Thorrold says. “So are we sure that we want to do that?”

To know the answers, we need to unlock as many of the region’s secrets as we can. And, as Annette Govindarajan puts it, “We want to get this information before it’s too late.”

Further reading

• The Woods Hole Oceanographic Institution pulled together a great primer on all things twilight zone, as well as a fantastic “creature features” page.
• For more on Andone Lavery’s work exploring the twilight zone using acoustics, check out this documentary. Or learn more about “eDNA,” Annette Govindarajan’s work, which involves studying traces of DNA left behind in the water.
• If you want to explore the future of fishing in the twilight zone, the Institute for Sustainable Development and International Relations pulled together a detailed report.
• In our podcast episode, we speak to Harriet Harden-Davies, a research fellow who focuses on questions about ocean policy. She’s the co-author of a great piece about the relationship between ocean science and ocean policy.