Up from the Depths

The mass nighttime movement of life from deep sea to surface is Earth’s largest wildlife migration—a vertical feast that helps fuel the planet

Richard Conniff
Animals
Nov 30, 2017

Schools of hungry sardines rise to devour red clouds of krill seeking smaller fare at the surface of the sea off California’s coast. Across the world’s oceans, a Seussian menagerie of beasts rises and falls on an endless hunt for food. Aglow with bioluminescent light, lanternfish (below) are one of the world’s most abundant deep-sea prey species.

EVERY EVENING as sunset rolls across the Earth, vast numbers of ocean creatures emerge from the depths to flutter and fin their way toward the surface. Most of them are small, translucent crustaceans called copepods. But trillions of shrimp and krill, bioluminescent jellyfish, tentacled squid, stringlike salps and other ocean oddities also join the voyage, often rising thousands of feet from the depths in numbers beyond imagining.

Just one animal on the ascent—the bioluminescent lanternfish, only six inches long—is abundant enough to outweigh the entire planet’s annual fisheries catch. In all, some 5 billion metric tons of animals move to the surface each night, according to researchers aboard the 2010-2011 Malaspina Expedition, a global survey of biodiversity in the world’s oceans. To put that number in perspective, it’s roughly equivalent in weight to 17 million 747s on the move at once. This is the biggest animal migration on the planet, and it’s vertical.

What’s the driver? Food—and a considerable dose of fear. Penetrated by sunlight, the upper few hundred feet of ocean are home to microscopic algae and other drifting plant forms that flourish in the light. Small marine creatures called zooplankton want to flock there to graze on these products of photosynthesis, but they visit only in the darker hours to avoid predators that hunt by sight. Heading back to deeper waters before dawn may mean going hungry but staying alive, a move some oceanographers dub “better unfed than dead.”

This global phenomenon of feeding by dark and retreating from the light is known as the “diel vertical migration,” with diel referring to events that happen on a 24-hour cycle. For scientists, this migration now looks like one of the great forces shaping the diversity and productivity of the oceans, and the health of the Earth itself.

“It overshadows all the migrations on the planet,” says marine ecologist Bruce Robison of the Monterey Bay Aquarium Research Institute (MBARI). “But this one is out of sight, and as a consequence, most people are unaware of it. It is a huge factor in the ecosystem of the ocean, but we’re still groping to figure it out.”

New research from MBARI and elsewhere across the globe is now shedding light on this mass migration, opening up a world we only began to recognize for the first time just 75 years ago.

Finding a “False Floor”

In 1942, a U.S. Navy research vessel, the USS Jasper, was testing new sonar technology off California’s coast when it reported sound waves being deflected (or scattered) from a mysterious layer more than 1,000 feet below the surface. Incredibly, this dense layer stretched 300 miles long, leading researchers to think at first that it might be the sea floor itself. Other sonar pioneers soon found similar layers all across the Pacific, the Atlantic and even in lakes worldwide. Yet the cloudlike layers remained an enigma—and a peril for Navy sonar operators, for whom the layers might hide an enemy submarine.

Three years later, in 1945, Martin Johnson of the Scripps Institution of Oceanography used crude plankton nets to conduct nighttime surveys of marine life at various depths and thus became the first to report that the dense clouds were masses of living creatures that moved up and down nightly.

The masses of life in what’s called the “deep scattering layer” (DSL) can be hundreds of feet thick and extend for hundreds of miles at various depths across the world’s oceans. In 2017, using a sonar-equipped underwater robot to probe the DSL off California, a team of researchers discovered that it contains distinct schools of animals such as shrimp, krill and squid rather than a random mix of creatures. Remarkably, animals within the schools maintain consistent spacing among individuals, moving closer as predators approach. Squid, for instance, clump together, apparently to confuse predatory dolphins, which target one animal at a time. “Schooling was previously thought to occur mostly in surface waters,” reports lead researcher Kelly Benoit-Bird of MBARI. “This cooperation among animals within a single layer suggests there must be strong survival benefits in both maintaining a continuous horizontal layer and distinct groups within the layer.”

WAVE OF LIFE. As night falls, trillions of krill, copepods, lanternfish and squid join other creatures that rise from black depths below 1,000 feet to feed in the nutrient-rich epipelagic zone, the only ocean zone sunlight penetrates. They then descend to the dark as light returns. Large predators such as tuna, sharks, elephant seals, salmon and dolphins follow the feast, a migration largely hidden from human eyes.

Other-Worldly Wonders

Thanks to such studies, advanced acoustic sensors and other new technologies, creatures of the vertical migration have gradually come into focus. En masse, they resemble an almost endless cloud of drifting snow, yet they are spectacularly varied. Comb jellies resemble translucent baubles, with rows of paddlelike cilia undulating along their flanks. Snipe eels twist like whips, their thin jaws curved at the tips. And lanternfish gleam from bioluminescence, which they can manipulate for either communication or concealment.

Interactions among such creatures are equally varied. Robison and other researchers recently documented that deep-sea, juvenile swordtail squid brilliantly mimic the physical form and behavior of the Nanomia siphonophore, a gelatinous chain of tiny animals akin to jellyfish. Like most siphonophores, it has stinging cells called nematocysts and therefore makes a nasty meal for hungry predators, so the squid’s mimicry buys it a degree of protection.

Different species may prefer to hang out at different depths by day and night, or at different temperature and salinity gradients. For some smaller creatures, such as copepods, seawater can seem dense and viscous, making the migration “like a nightly marathon through honey,” says researcher Deborah Steinberg of the Virginia Institute of Marine Science. Many bony fish species, by contrast, have swim bladders they can inflate for ballooning to the surface and deflate again for the descent. Some animals may thus travel only a few dozen feet on their migration, while others travel several thousand feet. The result, researchers say, is less like a coordinated mass movement from the depths to the surface and back and more like overlapping ladders of migration.

From jewels of bioluminescent light to tentacled and toothy hunters, the creatures of the oceans’ vast vertical migration fuel a massive exchange of nutrients, oxygen and carbon that sustains life both above and below the waves. This vivid gallery only hints at the scope of sea life in motion.

A nutrient “bucket brigade”

Some species in the deep scattering layer don’t bother to migrate at all. Instead, they wait and eat other creatures returning with full bellies to the supposed safety of the depths. Or, they simply eat the detritus raining down from their more energetic neighbors. In effect, according to one theory, the migration becomes a sort of bucket brigade—for poop.

One zooplankton consumes nutrients at the surface, heads back down, and excretes a fecal pellet, which another individual slightly lower down consumes and excretes, and so on into the depths. The collective effect can move nutrients down from the surface as much as 53 percent faster than would happen by gravity alone, according to one recent study. It’s a biological pump, and an important driver of nutrient cycling in the ocean. Whales, tuna, elephant seals and other megafauna take advantage of the windfall by diving down to target smaller players in the brigade, inhaling krill, lanternfish and squid.

Beyond moving nutrients, the migration pumps carbon to the deep, making it a critical player in carbon sequestration—and thus a boon to the climate. Like trees on land, microscopic plants at the ocean surface convert atmospheric carbon dioxide into organic matter. When migrating zooplankton consume this plant matter and carry it down, they sequester carbon in the depths, where it may remain for hundreds or thousands of years.

Remarkably, the vertical migration occurs even in the endless darkness of winter at the North Pole. In a recent study, researchers used acoustic devices moored to the sea bottom across the Arctic and found that zooplankton flee to the depths to escape the faint light of the rising moon. “It’s absolutely amazing to think it can happen under several meters of ice that is covered with snow,” says Kim Last of the Scottish Association for Marine Science, lead author of the study. The researchers were so doubtful that such minimal light could drive the migration that they implanted an electrode in the optic nerve of a krill to measure the amount of light needed to elicit a response. “What few photons get through are enough for organisms adapted for living in this environment,” says Last, who likens predators that hunt by the light of the Arctic moon to werewolves.

A warming climate could of course change this pattern if the Arctic becomes ice free or if ice melt causes the water to become more stratified, altering the pattern of nutrient flow. “It’s something that scientists are really interested in now,” says Last. “At this stage, the jury is out on whether the Arctic Ocean is going to become more productive or less productive.”

Moon Jellyfish on the Yıldızkoy bay underwater trail

Drifting in surface light off Turkey’s Gökçeada Island, moon jellyfish graze on zooplankton. Unlike these gleaming snacks for visual predators, creatures of the vertical migration retreat by day to the relative safety of black depths.

An unsustainable harvest?

Scientists worry about the future of the vertical migration in other oceans, too. As commercial fishing decimates populations of larger fish, the tendency is to move down the food chain. Commercial products from the migration already include krill paste and lanternfish protein concentrate, mainly as feed for fish farms. “It’s not something that any of us think is a good idea,” says Robison. “If we take away all the food of whales, tuna or salmon, we are certainly ensuring that those charismatic animals are going to be starved out.” Targeting the migration also risks jeopardizing the nutrient and carbon cycles of the oceans, with consequences we cannot foresee. Plant life at the ocean surface, for instance, produces about 20 percent of the Earth’s oxygen—one in every five breaths we breathe.

For now, the migration persists, following its ancient rhythms out of our sight. “It’s a rolling wave that moves across the world as the Earth spins and the sun comes up and down,” says biologist Peter Wiebe of the Woods Hole Oceanographic Institution. Our own lives, he says, also move in a wave across the planet. Along the Atlantic from Labrador down to Patagonia, everyone wakes more or less in unison with the sunrise. Likewise, 12 hours later in the Western Pacific, they rise from Siberia to Tasmania. Even so, the great vertical migration of the oceans may remain beyond our understanding because its movements are so contrary to our own. As we rise to the sun, the animals of the migration retreat to the frigid depths, and as we sleep, they rise again to the comforting return of darkness.

Humpback Whale feeding with Herring Gulls waiting for leftovers

NWF PRIORITY

Helping ocean wildlife thrive

A humpback whale vies with gulls to feed on forage fish off Massachusetts—one of many marine areas the National Wildlife Federation and its affiliates help protect. In 2016, NWF helped gain establishment of the Northeast Canyons and Seamounts Marine National Monument, the first such monument in the Atlantic. NWF is also leading a campaign to sustain populations of Atlantic forage fish, and working as a neutral voice to help recreational fishing interests and government agencies negotiate inclusive conservation approaches for ocean resources.

Connecticut-based writer Richard Conniff is author of The Species Seekers and other books.