Turning On the Lights within the Ocean’s Twilight Zone


Many fathoms below the surface of the sea, mysteries still abound—mostly invisible to human eyes. Sunlight disappears and water pressure mounts, making the deep sea one of the least explored and understood environments on the planet. Even today.

Still, humans have found ways to sate their curiosity, from diving bells (first envisioned by Aristotle) to scuba gear to remotely operated vehicles, and the 4K video cameras they now carry. The latter technology has enabled a group of Australian scientists to share their exploration of the continent’s largest marine sanctuary, the Coral Sea Marine Park, via livestream with colleagues sheltered in place around the world during the global pandemic.

The deep did not disappoint. Coral reefs were plentiful even where light was faint or entirely absent; colorful sponges and a species of fish native to Hawaiʻi put in appearances. A fanciful octopus with ear-like fins resembling Dumbo the elephant cruised by at 3,300 feet, and chambered nautiluses—particularly ancient creatures—bobbed about somewhere around 2,000 feet.

The purplish glow of the ocean’s mesophotic (“middle light”) zone gives rise to its nickname: the twilight zone. Sandwiched between the sunlit shallows and pitch-black deep, this layer sits roughly 100 to 500 feet underwater. This is a “lights off” view of the South Diamond Islet Reef.
The purplish glow of the ocean’s mesophotic (“middle light”) zone gives rise to its nickname: the twilight zone. Sandwiched between the sunlit shallows and pitch-black deep, this layer sits roughly 100 to 500 feet underwater. This is a “lights off” view of the South Diamond Islet Reef. Courtesy Schmidt Ocean Institute
Once the lights go on, colors emerge. Most of this bed of hard coral belongs to the genus <em>Leptoseris</em>, a group abundant in the deeper waters of Hawaiʻi, but observed here for the first time in the Coral Sea. The pink and reddish hues belong to photosynthetic algae pulling energy from the dim light.
Once the lights go on, colors emerge. Most of this bed of hard coral belongs to the genus Leptoseris, a group abundant in the deeper waters of Hawaiʻi, but observed here for the first time in the Coral Sea. The pink and reddish hues belong to photosynthetic algae pulling energy from the dim light. Courtesy Schmidt Ocean Institute

For six weeks the Australian scientists, led by marine geologist Robin Beaman of James Cook University in Cairns, worked from home with the crew of Falkor, a research ship provided by the Schmidt Ocean Institute, a philanthropic nonprofit cofounded by Eric Schmidt, former Google CEO, and his wife Wendy. The expedition mapped more than 13,000 square miles of the ocean floor east of Australia (an area about the size of the island of Taiwan), in addition to capturing captivating glimpses of life brimming up to 5,300 feet below.

Although these dives were look-only (“It’s a bit like a kid going into a toy shop but being told you can’t touch anything,” says coral specialist Tom Bridge, a co–principal investigator), another expedition is planned—and collecting specimens is part of the plan. Bridge’s wish list includes samples of the corals he can currently only gaze at from afar. By firmly identifying each species and learning how they compare genetically to others in the Pacific, he says, we might have a better window on life’s evolution in the sea.

Fan corals known as gorgonians spread wide to maximize their surface area for feeding. While corals in shallow reefs reap abundant energy from their symbiotic algae photosynthesizing in the sun, in deeper, darker water some corals feed on small plankton or organic particles floating by. For this reason, fan corals can grow quite big—up to 10 or 12 feet across, says Tom Bridge, senior curator of corals at the Queensland Museum—and favor areas with speedy currents.
Fan corals known as gorgonians spread wide to maximize their surface area for feeding. While corals in shallow reefs reap abundant energy from their symbiotic algae photosynthesizing in the sun, in deeper, darker water some corals feed on small plankton or organic particles floating by. For this reason, fan corals can grow quite big—up to 10 or 12 feet across, says Tom Bridge, senior curator of corals at the Queensland Museum—and favor areas with speedy currents. Courtesy Schmidt Ocean Institute
A spindly brittle star hitches onto an octocoral, so named for its eight-tentacled polyps, which deploy their appendages to feed on drifting organic material. Trawlers of the sea floor, brittle stars also cling to corals and sponges to get in on a feast. Says Bridge: “It’s a good place to sit and collect food if you’re mobile.”
A spindly brittle star hitches onto an octocoral, so named for its eight-tentacled polyps, which deploy their appendages to feed on drifting organic material. Trawlers of the sea floor, brittle stars also cling to corals and sponges to get in on a feast. Says Bridge: “It’s a good place to sit and collect food if you’re mobile.” Courtesy Schmidt Ocean Institute
Impossibly cute and whimsical though they may appear, dumbo octopuses live at extreme depths of up to 13,000 feet. Named for the appendages that resemble elephant ears and propel them through the water, they are the deepest known octopuses and the only ones that don’t use jet propulsion to move. Webbed arms provide steering.
Impossibly cute and whimsical though they may appear, dumbo octopuses live at extreme depths of up to 13,000 feet. Named for the appendages that resemble elephant ears and propel them through the water, they are the deepest known octopuses and the only ones that don’t use jet propulsion to move. Webbed arms provide steering. Courtesy Schmidt Ocean Institute
A living relic of ancestors that predate the dinosaurs, chambered nautiluses are buoyed by a shell that delicately balances the volume and density of the fluid in its chambers. These cephalopods dwell in lower depths and migrate hundreds of feet upward to hunt, using up to 90 retractable tentacles.
A living relic of ancestors that predate the dinosaurs, chambered nautiluses are buoyed by a shell that delicately balances the volume and density of the fluid in its chambers. These cephalopods dwell in lower depths and migrate hundreds of feet upward to hunt, using up to 90 retractable tentacles. Courtesy Schmidt Ocean Institute





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