The twilight zone
Barely explored deep-sea organisms appear to play important role in ecosystem
Between the ocean’s bright blue surface and its blackest depths — 660 to 3,300 feet below — is a mysterious, dark span of water. Welcome to the twilight zone.
Recent evidence suggests there are more animals here by weight than in all of the world’s fisheries combined. But what lives here, and in what quantities?
Since August, a group of Massachusetts scientists has been using new technology to better understand the twilight zone’s strange inhabitants. They hope their findings will lead to a more sustainable approach before the fishing industry tries to harvest some of its abundant life as fisheries closer to the surface are diminished.
“The time is right to get this knowledge before it’s too late,” said Heidi Sosik, a biologist at the Woods Hole Oceanographic Institution who is leading the Ocean Twilight Zone project. “This twilight zone region of the ocean is really, very barely explored, but the more we learn, the more interesting and more important it seems to be in playing a role in the whole ecosystem.”
Here are highlights of what the scientists pulled up on their first cruise:
Tracking with sound
Each animal in the ocean has its own auditory signature that ships usually detect by sending out sound waves that bounce or scatter off their bodies. It’s how whale-watching cruises often find humpbacks for guests to view.
But the acoustic fingerprints of twilight zone animals are still mysterious because shipboard sonar doesn’t have the bandwidth to distinguish among the many organisms living far below the surface in what’s called the deep scattering layer. It’s an area so dense with life that people once thought it was the seafloor.
Around 250 different species of myctophids, or lantern fish, make up much of this dense layer. Though abundant enough to trick sonar, individually they are no bigger than an index finger.
During a 10-day expedition in the Atlantic, the
team towed an instrument called Deep-see from the Henry B. Bigelow, a ship from the National Oceanic and Atmospheric Administration. Inside that deep layer, it detected a range of frequencies that the scientists hope to match with pictures and DNA to create an auditory dictionary of twilight zone inhabitants.
Tiny bodies, small appetites
The common bristlemouth is the most abundant vertebrate on the planet.
“Some of them look like crazy monsters,” Sosik said.
But, she pointed out, “most of the animals that live in the twilight zone are really small.”
This size adaptation is vital where food is scarce, because small things don’t need as much of it.
Though small, they’re fierce
One fang-tooth monster, the Sloane’s viperfish (Chauliodus sloani), could fit inside your hand.
“This guy would be considered one of the big, bad predators down there,” said Paul Caiger, a biologist at Woods Hole and the cruise photographer.
It has one of the biggest teeth-to-body-size ratios in the animal kingdom. Its teeth are too large for its mouth, so it closes them like a cage, imprisoning its prey before it eats.
“These fish might not see prey for a few days, and when they see something, they don’t want to lose it,” he said.
Making light in the depths
The belly of a hatchetfish contains light organs or photophores. Many animals in the twilight zone make their own light through a chemical reaction inside photophores of various shapes, sizes and arrangements.
By turning on these lights and regulating their intensity, hatchetfish blend in with light flowing down from the surface, becoming invisible to predators looking up for silhouettes from below.
This hiding technique, called counterillumination, is the most common use of bioluminescence in the twilight zone.
Bumping in the dark
For sensing a dimly lit environment, animals like the lantern fish may turn their eyes into giant lenses for low light.
Or like the common fangtooth, they opt for other senses. “It relies on kind of just bumping into things in the middle of the night,” Caiger said.
The ridge along its body has pressure sensors that can detect objects, current or animals moving in the water a few body lengths away. A mucus coating enhances the fish’s ability to detect chemicals around it.
An even weirder adaptation
Instead of squirting ink to escape predators, the
fist-size glass squid hides in plain sight, pulling its tentacles and head inside its spike-covered body cavity. Then it fills its orb with ink, becoming “an unpalatable, turgid ball,” Caiger said.
The journey of a living wave
When the Navy started using active acoustics to monitor the sea in the 1930s, it noticed lots of sound bouncing off a layer it thought was the bottom.
This “false bottom” turned out to be a mass of animals that journey hundreds to thousands of feet from the depths to the surface nightly in a living wave that wraps around the planet.
Evidence shows that during the day, surface dwellers like sharks, tuna and swordfish dive down to the depths to eat.
“The layers are not distinct,” Sosik said. “The organisms that live in one layer are influenced by the organisms that live in another.”
Scientists working to untangle this multilayered food chain think it might play a major role in regulating climate by keeping carbon in the ocean. That’s part of why it’s so important to study these watery depths that we ordinarily cannot see.
“If you imagined erasing the life in the twilight zone, it wouldn’t affect just that layer,” Sosik said. “It would affect the ocean and the whole planet.”