Every evening around the world trillions of zooplankton, many smaller than a grain of rice, hover hundreds of feet below the surface of the sea, waiting for their signal. Scientists long considered these tiny animals to be drifters, passive specks suspended in the ocean, moved by the whims of tides and currents. And yet, just before the sun disappears, the swarms begin to rise on a clandestine journey to the surface.

As they climb, clusters of other zooplankton join in: copepods, salps, krill and fish larvae. The multitudes remain near the surface at night, but just as the first beams of morning light begin to cascade across the sea, they are already turning back down to the deep. As sunset and sunrise slide from east to west every 24 hours—across the Pacific Ocean, then the Indian, the Southern and the Atlantic—swarm after swarm make the same upward journey, retreating as daylight returns.

Humans are mostly unaware of this daily aquatic movement, known as diel vertical migration, but it's the largest routine migration of life on Earth. Current estimates indicate some 10 billion tons of animals make these excursions every day. Some of them ascend from more than 3,000 feet below. It's an astonishing feat. For a quarter-inch fish larva, making a one-way vertical trip of 1,000 feet is the equivalent of a human swimming more than 50 miles—in just an hour or so. During the trip these animals pass through zones of ocean where the conditions are wildly different. At 1,000 feet the water is roughly 39 degrees Fahrenheit—maybe 20 degrees colder than the surface—and the pressure is about 460 pounds per square inch, more than 30 times what it is up top. Why would huge numbers of tiny animals make such an arduous trip every day?

The short answer is to eat—and to avoid being eaten. During the day vulnerable zooplankton hide from predators such as squid and fish in the dark depths. When night begins to fall, they rush to the surface to feed on phytoplankton—the microscopic aquatic plants that live in the top few hundred feet of water—under cover of night.

But this is just the prevailing wind of vertical migration. There are all kinds of crosscurrents and eddies. Now, with increasingly sophisticated sonar, underwater autonomous vehicles and advances in DNA sequencing, researchers are starting to understand those details. The specifics will help answer questions that have implications for the oceanic food web, the global carbon budget and the very nature of life on Earth.

Dances of the Deep

Early recordings of diel migration date to World War II, when ships and submarines using sonar to sweep the oceans for enemy subs detected something odd—parts of the seafloor seemed to be moving up and down, creating a deep “scattering layer” that reflected the sonar signals. The layer fluctuated twice a day by as much as 3,000 feet—shifts that seemed to defy logic. In 1945 oceanographer Martin Johnson embarked on a research ship to sample plankton at various times and depths over 24 hours. “From these preliminary observations there appears to be some direct correlation of the planktonic animals with the scattering layer,” Johnson wrote. The proposal that the layer was composed of living creatures raised more questions than it answered, however.

Answering those questions proved difficult. The animals involved are tiny, their passage happens in the dark and the deep ocean is tough to access. Tracking swarms of flea-size organisms through the lightless depths is trickier than following migrating whales across hemispheres. By the 1990s researchers had learned enough to describe the diel migration as a cloud of organisms rising and falling in unison. Higher-resolution sonar picked up individual clusters of animals and more subtle movements up and down. Even today, though, sonar-based surveys can't distinguish which tiny animals are on the move. Sampling the zooplankton, as Johnson did, can haul up the organisms for identification, but it blurs the nuances of time and location that could indicate where each animal was in its journey.

Despite these challenges, new research is revealing hidden intricacies of the mass migration. For one thing, the process is intimately tied to what's happening in the skies. When the sun is absent for weeks at a time during polar winters, some of these animals realign their migrations with cycles of the moon. Solar eclipses can cue them to start swimming toward the surface. Zooplankton living below 1,000 feet, where light intensity is just 0.012 percent of what it is at the surface, may shift their vertical position by as much as 200 feet as passing clouds change the trace amounts of light reaching them, explains Deborah Steinberg, chair of biological sciences at the Virginia Institute of Marine Science. She realized this during a research cruise, even though the light changes at the surface were not apparent to her or her colleagues. “From our perspective on the ship, every day of the cruise was overcast, gray and drizzly,” she and her colleagues noted in a 2021 paper. But the zooplankton somehow registered the subtle changes in light far underwater.

Autonomous vehicles equipped with cameras and collection devices that allow them to pair images with chemical signatures from the water column have begun to offer new, animal's-eye views of migration. For example, Kelly J. Benoit-Bird of the Monterey Bay Aquarium Research Institute (MBARI) in California and Mark Moline of the University of Delaware sent an autonomous underwater vehicle 1,000 feet down into the Catalina Basin off southern California to take sonar measurements of vertically migrating zooplankton. The echoes it returned were stunning: they revealed that the zooplankton were organized in well-defined clusters, tightly assembled by kind and migrating together in carefully timed ascents.

“We need to start thinking about this not just as a bulk process but as an individual and species-by-species sort of thing,” Benoit-Bird says of vertical migration. And the adventurous zooplankton are not alone in the nightly commute. “So many animals use this as a strategy,” Benoit-Bird says. Octopus, lanternfish, siphonophores and other motley deep-sea creatures also make the nightly trek to avoid their own predators and to find food—in their case, the other migrators.

Plants on the Move

Animals might not be the only ones making routine vertical migration. Kai Wirtz is a professor and ecosystem modeler at Helmholtz-Zentrum Hereon's Institute of Coastal Systems in Germany. In 2016 Wirtz and his colleagues were looking to describe how the distribution of different phytoplankton matched up with different ocean environments. But he noticed that the circulation of ocean water alone wouldn't bring enough nitrogen and phosphorus from the depths to feed the ocean's vast and essential blanket of phytoplankton at the surface.

Scientists had known for decades that many species of phytoplankton can move—some by changing their buoyancy by shedding fats or changing their dimensions and others by whipping their tail-like flagella. Wirtz mulled this over as he looked more broadly at the oceans' profile: the top is filled with sun but few nutrients. The bottom does not get enough sunlight for photosynthesizers to live on, yet it harbors an abundance of nutrients. So, he thought, why wouldn't these plants use their evolved locomotive abilities to commute between the two spaces? In fact, he says, “there is not an easy other explanation.”

By Wirtz's estimates, it's possible that half of marine phytoplankton species undertake a regular vertical migration of dozens to 100 feet, shuttling nutrients from below and solar energy from above. These microscopic organisms might take hours, days or even weeks to complete the journey, some reproducing along the way, thereby allowing their descendants to carry on the mission. This idea presents a radical change in how scientists might think of phytoplankton, which they often consider as more of a chemical compound than individual living organisms with varied behaviors.

Laboratory work confirms not only that marine plants move vertically but also that their behavior is more sophisticated than we had thought. One team at Washington State University set up 6.5-foot-tall saltwater tanks with dinoflagellate phytoplankton, then introduced predatory copepods to one of the tanks. When the scientists replicated typical day-night light cycles, they saw the hungry copepods making the traditional nighttime ascent and daytime descent. The phytoplankton in both tanks did the opposite—swimming up during the “sunlit” day and down at night, probably to maximize their sunlight exposure and minimize their risk of being eaten by the night-feeding zooplankton.

To the researchers' amazement, though, they saw that the single-celled plants in the tower with the copepods routinely retreated even deeper than usual at night, putting more distance between themselves and the enemies above. No one knows how the phytoplankton sense their predators' behavior. But as the researchers noted in their paper in Marine Ecology Progress Series, “This newly reported behavioral response … could have important ecological consequences.”

The zooplankton menagerie comprises a wild array of tiny animals. Clockwise from top left: A crab larva known as a megalopa from the Atlantic Ocean; a blue and orange plankton from the Canary Islands; a sea butterfly also from the Canary Islands; an arrow worm, found in all oceans, from the surface to the deep; a blue bioluminescent plankton from the Arctic Ocean; and a buglike copepod from the Atlantic deep sea, common in tropical regions. Credit: Solvin Zankl/Minden Pictures; Sergio Hanquet/Minden Pictures; Sergio Hanquet/Minden Pictures (top row, left to right); Solvin Zankl/Minden Pictures; Flip Nicklin/Minden Pictures; Sinclair Stammers/Minden Pictures (bottom row, left to right)

Altering the Carbon Budget

One consequence of phytoplankton migration is the extent of climate change. In 1995 Steinberg and other scientists were trying to piece together the global carbon budget—the amount of carbon dioxide emitted into the atmosphere and the amount pulled from it, in part by marine ecosystems. The numbers weren't adding up; more carbon was disappearing from the ocean surface than they could account for. Then Steinberg got a look into the darkness.

As part of her research, done at the Bermuda Institute of Ocean Sciences, Steinberg would often dive during the daytime, and she became well versed in the local fauna. But then she got to take a night dive. She plunged off the side of a small boat above 13,000 feet of dark water and soon found “it was a totally different community. I was in the water with animals of every single kind,” she recalls, her voice still ringing with excitement more than a quarter of a century later. That night was her cue to change direction and start studying diel migration. And she had an inkling that it might hold part of the carbon answer.

On the ocean's surface, phytoplankton suck an enormous amount of carbon dioxide from the atmosphere, but they release much of it right back into the air, often within days. As migrating zooplankton swim up at night and eat these marine plants, they become a kind of biological conveyor, transporting carbon down into the deep sea, where it can get sequestered for hundreds or thousands of years.

To study this crucial movement of carbon, Michael Stukel, a plankton and marine biogeochemistry researcher at Florida State University, spends a lot of time peering through a microscope at zooplankton's fecal pellets. Individual excretions are small, but when they happen on such an enormous scale, they take on global biogeochemical significance.

Fecal pellets from vertical migrators, rich in carbon, descend through the water column. They are joined by other sinking biological particles, creating “marine snow” that slowly drops to the deep seafloor. Together with the swimming zooplankton carrying their carbon-loaded dinners back down with them, this global sequestration of carbon means the planet is “not as hot as it otherwise would be,” Stukel says.

Estimates of the amount of carbon sequestered by migrating organisms vary widely because so much about the diel migration remains a mystery. Better data will improve climate models, which in turn will improve understanding of how climate change will alter these organisms' behaviors—and, subsequently, the climate again. “You run into these big questions for humanity, for climate, that we can't answer, and a fair number of them relate to these migrators,” says Ken Buesseler, a senior scientist at the Woods Hole Oceanographic Institution.

Balancing Act

Answers to the remaining big questions about these migrators are likely to come from work such as that happening in Kakani Katija's lab at MBARI. There she's adding stereoscopic cameras and vision algorithms to autonomous vehicles so they can carefully track the movements of specific migrators. She can now train a vehicle and turn it loose to locate an animal and trail it for hours.

Katija's team is training the technology on gelatinous creatures such as siphonophores, which look like ghostly worms. Because these animals have semitransparent tissue and move quickly and unpredictably, siphonophores are hard for an autonomous vehicle to keep sight of, but that's what Katija wants: “We're trying to understand how to make these systems more robust,” she says. To capture usable images and video, the team needs a robot that can swim and produce light—both of which could easily interfere with their subjects' behavior. “That's a huge concern,” Katija acknowledges. One stealthy strategy is to use red light, which most of these creatures can't see, and a cruising mode that minimizes turbulence. Researchers are also turning to satellites in space that can observe the density of animals that come up to feed at night without the risk of disturbing them. Equipped with lidar—laser-based remote sensing technology—they can peer into the water as far down as 65 feet.

To pinpoint which species are moving when and where, scientists are also combing the water column for the genetic traces of transitory organisms. One team dropped large seawater-sampling bottles at various depths from its research ship as it drifted in the Gulf of Mexico. At the same time, the researchers were taking sonar readings of the life below. From the samples, they sequenced strands of DNA to deduce what organisms had been where—and when. The results, published in 2020, revealed poorly resolved spots in the concurrent sonar readings. Although sonar data suggested fish and other relatively large targets accounted for much of the moving biomass, the DNA indicated that copepods and gelatinous zooplankton had a much greater presence.

What researchers need most, they agree, is a global network of ocean monitors that can watch these processes day in and day out to more fully understand the ocean's systems before humans further disrupt them. For example, large-scale fishing has been done almost exclusively in the ocean's surface layer, augmented more recently by bottom trawling. But now some countries, including Norway and Pakistan, are issuing commercial fishing permits for the middle swath of ocean, in part to suck in the diel migrators and process them into food for farmed fish and for fish oil.

Expanding dead zones and rising oxygen-minimum zones in ocean water are also squeezing these animals out of livable daytime habitats. And climate change is decreasing the mixing of water layers in the open ocean, bringing fewer nutrients to phytoplankton. Fewer phytoplankton means less food for migrating zooplankton. All of which means the scientists studying these animals are under growing pressure. “It's not often that we have the chance in history to understand a system before it's exploited,” Benoit-Bird says. “I feel like we're kind of racing against the clock.”

To better understand the movements of trillions of copepods, krill and other elusive migrators, this summer Benoit-Bird and her colleagues will return to sea. She hopes their expedition with underwater robots, sonar, imaging and environmental DNA can help them learn how these tiny animals self-organize during the day—rising and falling, tightening and loosening in swarms to stay connected with networks of other species.

In the meantime, the sun will continue to rise and set. As it does, an untold number of animals will follow the underwater tides of darkness and light, eating, excreting and modulating the very balance of elements on our planet.