These creatures of the 'twilight zone' are vital to our oceans

On the rolling deck of a 56-foot-long research vessel in California’s Monterey Bay, Karen Osborn peers into a cooler filled with sloshing seawater and a galaxy of twitching life-forms. Moments earlier, this living constellation emerged from a net that had been slowly towed around 1,500 feet down, through an inky realm of near-total darkness. “It’s a good catch,” she says.

Most intriguing is a hand-size squid that gleams ruby red. Strawberry squid, as they’re known, are well adapted to their habitat. Their red color, when absorbed in the sunless deep, fades into a brownish black, blending them into their surroundings. Occasional flashes of bioluminescent light that shimmer across their bodies startle intruders. And their mismatched eyes look in two directions at once: One, huge and yellow, gazes upward, detecting silhouettes passing overhead. The other, smaller and blue, stares down, watching for glowing prey in the darkness. This specimen is surprisingly pristine. “Usually they’re all scraped up,” says Osborn. The strawberry squid likely got caught right before the net was carefully pulled to the surface.

(Octopuses and squids switch camouflage mode to stay invisible in the twilight zone.)

Osborn, an invertebrate zoologist at the Smithsonian Institution’s National Museum of Natural History in Washington, D.C., is no stranger to the fantastic beasts that inhabit this so-called twilight zone, a dusky, horizontal layer of the ocean at depths of between 660 and 3,300 feet. Over the past 25 years, she’s studied it remotely with camera-clad robots and been there herself in deep-diving submersibles. Her co-discoveries include how twilight zone fish make their skin intensely black and how the bodies of crustaceans called Cystisoma are so transparent they are almost completely invisible. “Every time we go out, we still see something new that we haven’t seen before,” she says.

By its very nature, the twilight zone is obviously difficult and expensive to access and study. Also known as the mesopelagic, it makes up a fifth of the ocean’s total volume, and much of it remains largely unexplored. The zone begins at a depth where photosynthesis fails and continues down until the last remnants of sunlight taper out. To a human inside a submersible, this realm appears pitch-black, but animals there have evolved all sorts of tricks to navigate the lack of light while at the same time avoiding predators in the open ocean. “We see all these cool shapes and sizes: transparent animals, mirrored animals, red animals, and ultrablack animals,” says Osborn. “They’re solving the same problem in a bunch of different ways.”

This phantasmal region holds a particular lure for Osborn—not simply to uncover its hidden biodiversity but also to find out how living things can survive such extreme conditions. What she and other researchers have discovered, however, is that while many of the twilight zone’s inhabitants might at first appear otherworldly, they’re very much earthlings with vital roles in the health of the entire ocean and the balance of our planet’s climate.

(Meet the creatures of the deep, dark sea.)

“It’s really important to understand what’s going on there and who’s living there, what they’re doing, what they’re eating, how much they’re pooping, where they’re dying,” says Osborn. And it’s becoming increasingly apparent that even this remote part of the ocean is not out of the reach of humans, making a better understanding of how the whole ecosystem works more urgent than ever.

Arriving onshore, Osborn’s twilight zone catch is transferred to the laboratory at the Monterey Bay Aquarium Research Institute, or MBARI, in Moss Landing, where she and her colleagues begin sorting the mix of minute animals.

“There’s a giant Paraphronima,” says Osborn, clearly delighted. “It’s huge!” Although smaller than a pinkie fingernail, it’s a substantial specimen for this group of crustaceans, distant relatives of sand hoppers called hyperiid amphipods that can be flea-size or even tinier. In the twilight zone, amphipods have evolved a variety of unique and elaborate eyes, to catch any snatches of light that make it through to the depths. Glassy eyes take up Paraphronima’s entire head; another species in the Streetsia genus has a single, cone-shaped eye. Osborn wants to find out why so many highly specialized eyes have evolved among twilight zone amphipods. “This doesn’t happen anywhere else,” she says, as most animals that live in darkness have reduced eyes or no eyes at all. “Not in caves, not on the deep seafloor.”

In the lab, using dessert spoons with the handles bent, Osborn and the other scientists delicately scoop up individual amphipods and place them in jars covered by aluminum foil. When their eyes are once again adapted to the dark, the animals will be sent to other parts of the laboratory to test various aspects of their vision. Some get passed to Jake Manger, a Ph.D. candidate at the University of Western Australia, who will release them into a virtual reality version of their habitat—a small tank of seawater surrounded by digital screens—to see how the animals respond to shapes of different sizes and brightness. Manger plans to build computer simulations of their brains so he can see the twilight zone as amphipods do.

(These deep-sea animals are new to science—and already at risk.)

Meanwhile, off Monterey Bay, a dramatic nocturnal shift is getting under way. Most days around sunset, throngs of twilight zone animals embark on a mass commute to the surface. Trillions of fish, shrimps, amphipods, jellyfish, and squid rise to feed, using the cover of darkness to hide from predators. “It’s by far the biggest movement of any animals on the planet, and it happens every day, all over the ocean,” says Osborn.

As a senior scientist at MBARI, Bruce Robison has witnessed this migration firsthand. Years ago, hundreds of feet down, he drove a submersible through a shoal of lanternfish so big and dense, it was impossible for the sub’s sonar to gauge its size. “It was pretty exciting,” Robison says of being surrounded by the countless silvery bodies. “It was almost like it should tickle.”

(These primitive, deep-sea fish live to 100, surprising scientists.)

People first noticed the massive scale of the ocean’s daily vertical migration in the mid-20th century, when U.S. Navy sonar seemed to show the seabed rising toward the surface at night. The deep scattering layer, as it came to be known, is in fact created by sound waves bouncing off the gas-filled swim bladders of twilight zone fish and the bodies of other migrating animals, such as the relatives of jellyfish called siphonophores.

Now scientists are studying the role this phenomenon plays in regulating the global climate. Migrating animals retreat to the deep, usually before dawn, with their bellies full of food, including carbon harnessed from the atmosphere by phytoplankton. Waves of migrants then release much of that carbon down deep, in their feces and through their gills. “Vertical migration is this rapid elevator or conveyor belt connecting the surface ocean to the deep sea,” says Kelly Benoit-Bird, a marine acoustician at MBARI.

Approximately a quarter of carbon dioxide emissions from fossil fuel burning and other human-made sources get absorbed by ocean life, a process called the biological carbon pump. Scientific models have tended to focus on processes such as sinking dead plankton and their feces, but more recently attention is turning to living animals. Studies suggest migrating twilight zone animals may move as much as 50 percent of the pump’s carbon load into the deep where it’s stored, away from the atmosphere, for hundreds or thousands of years.

(Greenhouse gases lurk in the oceans, and could make warming far worse.)

To decipher more precisely how much carbon is shuttled around, Benoit-Bird and colleagues are using echo sounders to uncover the migration’s finer details. For two years, one such device a half mile down in Monterey Bay has been sending a sound pulse upward every two and a half seconds. Underwater transducers detect the patterns of echoes bouncing back, which are converted into charts, called echograms, that give a vertical view of where things are in the water column.

So far, the data show that daily migrations can cease and start within the course of a day or stop altogether for weeks at a time. The presence of predators, such as Risso’s dolphins, may also influence the animals’ movements.

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Despite their best efforts at evasion, many animals still get eaten, playing a key part in the ocean’s food web. “They support a lot of fishery species and things that we care about,” says Ilysa Iglesias, a graduate student researcher at the University of California, Santa Cruz. She led a 2023 study revealing how twilight zone fish are commonly found in the diets of dolphins, sea lions, swordfish, sharks, tuna, and sometimes salmon. Some of those are nocturnal hunters that pick off the fish when they rise to the surface, while others are deep-diving foragers that pursue them during the day. 

Yet there may be far greater threats to twilight zone animals on the horizon.

Though previous attempts to establish industrial-scale fisheries in this ultradeep water zone have come up against the high costs of operating far below the surface, advances in harvesting technologies could make twilight zone fisheries viable. Millions of dollars are being spent on research and trial fisheries targeting abundant pelagic fish, such as lanternfish and bristlemouths, in Europe, particularly Norway. And factory ships could continuously pump up catches from huge trawlnets, similar to how krill are extracted from the upper reaches of the twilight zone around Antarctica.

(Proposed deep-sea mining would kill animals not yet discovered.)

With their high concentrations of indigestible oils and waxes, these fish are not suitable for human consumption. As with Antarctic krill, they would be mashed into meal and oils to be used as animal feed, especially for farmed fish. But because so much remains unknown about these animals’ lives, scientists are concerned about trawling. “How old are they when they spawn? How old do they get? Where do they reproduce?” says Iglesias, who also co-leads the Mesopelagic Fisheries Working Group at the Deep-Ocean Stewardship Initiative. “Really basic life history is missing.”

As Benoit-Bird at MBARI frames it, the race is now on for researchers to establish a baseline for what this healthy ecosystem should look like before it disappears. At the same time, plans are accelerating to mine metal-rich rocks from as far as three miles underwater, which would likely cause long-term damage to fragile seabed ecosystems and the species living there. Silty seawater containing rock fragments, toxic heavy metals, and radioactive isotopes could be discharged from ships processing the ores and pumped thousands of feet down, where they would choke delicate life-forms and contaminate food webs. Noise pollution from mining operations would add to the problems, masking the calls of whales and likely changing their behavior.

Some corporations that plan to mine the deep seabed are considering how to minimize their impacts—but developing new wastewater-handling methods and installing environmental monitoring to prove they work will likely take several years. In the meantime, marine heat waves at the surface can also influence what floats beneath. When a massive one, nicknamed the Blob, struck the U.S. West Coast from 2014 to 2016, it killed off swaths of shallow sea life, and the daily migrations of twilight zone animals shifted about 330 feet deeper.

(Do we know enough about the deep sea to mine it?)

For now, the twilight zone remains one of the least spoiled parts of the planet, and there are growing efforts to keep it that way. Since 2022, dozens of governments and corporations have backed proposals to halt deep-sea mining until the environmental risks are better known. The United States has introduced a precautionary ban on some twilight zone fisheries in its Pacific waters. And a global treaty for the high seas, which nations began signing in 2023, could help protect more of the region from mining and fishing.

Back in the Monterey Bay, in Karen Osborn’s lab, the strawberry squid hasn’t made it through the night, but it won’t go to waste. Its genetic makeup will be sequenced and the unscathed body preserved for future observation. That’s a win for science since these animals are inspiring solutions for human-world challenges. From cameras that work in the dark to miniature surgical robots that zip through blood vessels, there are countless ideas to adopt from the living inhabitants of the twilight zone.

“It’s the most exciting place in the universe, in my opinion,” Osborn says. “It’s interesting what’s in black holes and what’s out there on Mars, but there’s so many cool things right here that we don’t know anything about that we’ve gotta get out there and see.”

(Why this deep-sea explorer thinks diversity is so important for science.)