Massive Yet Misunderstood, What Is the Ocean’s Midwater?

Discover what lives in the largest and least understood ecosystem on Earth

Composite photo of seven different colorful and partially transparent deep-sea animals on black background
Contrary to popular belief, the open ocean is not just empty space with a few animals here and there. The cold, dark waters between the surface and the seafloor are filled with billions of creatures with untold importance to the entire marine ecosystem. K.J. Osborn, Smithsonian

Imagine climbing through a remote rain forest in the dead of night with nothing but a flashlight to guide your way. There are no trails, no landmarks and no destinations. As you clamber through the trees, the creatures you encounter are bizarre and infinitely better at navigating the darkness, though most of them flee from your light’s bright beam before you can catch a glimpse.

This is what it’s like to explore the ocean’s midwater — the largest and least understood ecosystem on Earth. With more than a billion cubic kilometers of living space, this section of the ocean between the surface and the seafloor holds more species, animal biomass and individual organisms than anywhere else on the planet.

“There are millions of animals down there, no matter where you go,” said Karen Osborn, a zoologist and curator of marine worms and crustaceans at the Smithsonian’s National Museum of Natural History. “It's not a couple of jellies here and there, it's millions and billions of animals, and they're all just as interesting as anything we have up here.”

Unlike rain forests, the deep sea is extremely difficult for humans to study, let alone travel to. Few people — most of them scientists like Osborn — have directly witnessed the midwater’s menagerie through the windows of deep-diving submersibles, making it difficult to garner support for conservation and research.

And yet, each time we descend into the depths of the ocean, we discover new species, new medical and tech applications and ecological connections beyond our imagination.

“If we want the ocean to keep functioning and providing us with essential environmental services, there needs to be a diverse community of animals down there, whether we understand them or not,” said Osborn.

As full as it is deep

The midwater is home to billions upon billions of organisms, contrary to the popular belief that most of the open ocean is empty. For example, the fish living in the midwater, like this dragonfish, are estimated to be ten times more abundant than all commercially fished species combined. K.J. Osborn, Smithsonian

Beneath the sunlight zone — or the top 600 feet of the ocean where we trawl for fish, SCUBA dive and conduct most marine research — blue quickly fades to black and the midwater begins.

With our human senses, surviving down here is unfathomable. Even in the twilight zone, the midwater’s top layer, the surrounding pressure is crushing. There’s nothing to hide behind and most animals here will never touch a solid surface. It is intensely cold, averaging 4 degrees Celsius, and only 1% of the sun’s light makes it down here.

“When we send cameras down there, the lights are almost always turned on, so we forget how dark it really is,” said Allen Collins, a zoologist at NOAA Fisheries and curator of gelatinous zooplankton at the Museum. “It's amazing how much of the livable space on this planet is in total darkness.”

Twilight dims to midnight when every photon of sunlight disappears around 3,000 feet below the surface. The midnight zone — though deeper and darker than what lies above — still harbors millions of organisms, many of them gelatinous.

“We might drive our ROV [remote operated vehicle] around for an hour or two without seeing any of the target animals we're looking for,” said Osborn. “But if we're looking for siphonophores and other jellies, we're happy as clams.”

Gelatinous animals like siphonophores and medusae are extremely abundant in the open waters of the deep sea. Though many of them are small, they play an outsized role in transferring carbon and other nutrients from the ocean’s upper layers to the seafloor. K.J. Osborn, Smithsonian

Far outnumbering all of the fish in the sea, most jellies are voracious predators, eating anything they can catch. By releasing massive amounts of mucus and waste, or sinking to the bottom themselves, jellies alone sequester an estimated 2 billion metric tons of carbon to the ocean’s depths each year, nearly equivalent to the amount of carbon produced by one third of U.S. citizens in the same time frame.

“Jellies are only becoming more appreciated as a huge part of the carbon cycle,” said Collins.

As jellies and other nutrients sink below the midnight zone, they enter the abyss. A truly crushing environment, the abyssal zone stretches from 13,000 feet down to the seafloor — usually around 20,000 feet. Though the abyss is less dense with life than the upper midwater zones, many animals including anglerfish, octopus and worms still manage to eke out a living here.

Looking down to move forward

Throughout the midwater, nearly everything is trying to eat everything else. With no sun to illuminate their prey, most creatures here find food by creating their own light, known as bioluminescence. To avoid being caught in someone else’s headlights, lots of deep-sea creatures have evolved ways to appear invisible. One such strategy is to blend into the darkness with skin as black as black can be.

In 2020, Osborn and a team of researchers found one of the blackest materials known to science while photographing fish hauled up from the dark depths of the Pacific Ocean. Some species, like Oneirodes anglerfishes, only reflected 0.04% of the light that hit their skin, thanks to unique microscopic structures that absorb and scatter the light.

These ultra-black fish are one example of the many deep-sea discoveries that were initially driven by curiosity but could yield far-reaching applications. Osborn’s finding might help U.S. Navy scientists create better camouflage, while astronomers could line their telescopes with a similar material to peer deeper into space.

“We will find more things like this if we keep exploring,” said Osborn. “Certainly not everything is going to have a direct application like that, but we won’t know if we don’t keep exploring.”

Although it’s pitch black in the midwater, some animals can produce their own light using bioluminescence. To avoid getting caught in another animal’s "headlights," many fish, like this fangtooth, are completely black. Some species have special skin cells that can absorb over 99% of the light that hits them, making them virtually invisible underwater. Allen Shimada, NOAA/NMFS/OST

Even closer to the surface, new findings are everywhere. In the twilight zone, a never-ending, slow-motion blizzard of organic matter sinks from the sunlit waters above. This flurry of leftovers, dead creatures and poop is called marine snow, and it nourishes an incredibly rich community of fish, squid, crustaceans and jellies that congregate in the darkness between 1,000 to 2,000 feet below the surface.

“In some places, this layer of animals is so dense that when it moves up and down each night, it may actually stir the ocean,” said Mike Vecchione, a research zoologist at NOAA and the Museum’s curator of cephalopods. The vertical movement he's referring to is the largest animal migration on Earth. It happens every night as billions of fishes, shrimps, squids, siphonophores, jellies and other deep-sea animals rise toward the surface at dusk and return to the depths at dawn. 

Explore the midnight zone with deep-sea biologist Bruce Robison

The explanation for this massive nocturnal trek is complex. Some scientists believe the animals are searching for food under cover of darkness to avoid predators. Others believe the animals are saving energy by catching food near the surface and digesting it later in colder waters. Vecchione, who studies octopods, squids and other mollusks that move throughout the deep, said it’s probably both, adding that each species has its own suite of reasons, so we have lots to learn.

Whatever the rationale for their journey, the abundant animals of the midwater are a cornerstone to a functioning ocean ecosystem — and one that’s increasingly under threat. In 2010, the Deepwater Horizon oil spill dumped more than 3 million barrels of oil into the Gulf of Mexico’s deep waters. Vecchione has spent the last decade surveying the health of the region’s midwater community as it recovers.

Most studies from the region, he said, have primarily focused on tiny creatures like plankton and large animals like whales, which have mostly recovered and returned to the area. But for medium-sized midwater residents like squids, fishes and shrimps, Vecchione and his colleagues recently concluded that right after the spill, there was no detectable impact — but their numbers have been decreasing ever since. “As far as we know, there’s still no sign of them coming back,” he said.

Deep-sea squids make up a significant portion of the midwater community. They provide food for many of the deep-diving species we care about, including seals, whales and dolphins.  Courtesy of Linda Ianniello

A worthy investment strategy

Since daily deep-sea safaris have yet to gain as much popularity as rainforest tours or sending people to space, the midwater has yet to win the hearts of policymakers and conservationists. An orphan ecosystem of sorts, the midwater mostly exists outside national boundaries and jurisdictions, leaving roughly 90% of Earth’s habitable space unprotected from activities like mining, trawling and dumping.

The remoteness, extreme pressures and chilling darkness of the midwater shouldn’t keep us from investigating and protecting the hidden treasures within. Indeed, those same limitations never stopped us from exploring outer space, and space has a far worse track record when it comes to discovering new life.

“We might not know who all these creatures are, we might not know how they all function and we can't possibly guess which ones are holding up critical ocean processes for us,” said Osborn. “But just like in finance where you need a diverse stock portfolio, if we can maintain the diversity of the deep sea, it will be more resilient to change.”

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