The ocean never seems to run out of mystery.
In between the sun-soaked reefs of the shallows and the weird depths of the deep sea lies another marine world full of secrets: the mesophotic zone.
This ecosystem of twilight reefs and species unknown to science lives at the limit of the sun’s reach. Here, corals still use photosynthesis to soak up the shards of light that barely reach its 60m depths.
It’s a difficult place to explore — too deep and dangerous for scuba diving and too delicate for submersibles. But in recent years, marine biologists have begun using special equipment called rebreathers, which don’t exhale bubbles. The devices recycle a diver’s breaths, which still contain oxygen. It also adds more oxygen and filters out carbon dioxide.
While hardly new, rebreathers have vastly improved in the 21st century, allowing biologists to start exploring a world largely unknown to marine science.
“It’s so quiet,” Kylie Lev, assistant curator at the Steinhart Aquarium at the California Academy of Sciences in San Francisco, told The Guardian. “I think it’s the only time in my life that I feel like there’s just an absence of noise.”
A dive off the coast of Brazil in 2020 at a depth of 110m. Photo: Mauritius Bell/Mesophotic
How rebreathers work
So if mesophotic reefs harbor such diverse wildlife, why haven’t we studied them sooner?
The answer is simply the difficulty of getting there, according to a 2019 study from Colombian biologists. At depths of 30-150m, the zone falls between the territory of conventional scuba diving and the heavy water that submersibles ply. (On top of that, subs also tend to scare away marine animals.)
“However, because of the great interest that has emerged in understanding deep ecosystems, techniques and procedures have been adapted for their exploration,” the study said.
To bring up specimens, scientists use a special decompression chamber, seen here in a dive in French Polynesia. Photo: Luiz Rocha/Mesophotic
Rebreathers can keep a diver oxygenated for up to eight hours, though few dives last that long. Most of the dive requires a slow ascent back to the surface to avoid decompression sickness — colloquially known as the bends. That’s when gas bubbles form in the body.
To survive at such depths, the rebreathers provide a mix of gases because too much oxygen becomes toxic 60m below the surface, according to LiveScience. Breathing too much nitrogen can also become a problem. So the rebreathers mix in some helium to reduce oxygen and nitrogen toxicity at depth.
While rebreathing tech goes all the way back to the 1870s, the computers that control gas levels have become more reliable and therefore safer for divers.
“The most important thing when you’re diving on a rebreather is knowing how much oxygen you’re breathing at all times,” Luiz Rocha, the curator of fishes at California Academy of Sciences explained. “We use a computer that sits right in front of our eyes.”
At a depth of nearly 64m off the coast of Hawaii, nearly every fish in this image exists only in this environment. Photo: Greg McFall
A marine world full of new species
Mesophotic reefs exist, largely unexplored, all over the world.
In 2016, a team of scientists investigated the mesophotic reefs in the Hawaiian Archipelago. Its research concluded that these deep-reef habitats are home to many species not found on shallow reefs.
In fact, these twilight reefs might have a higher number of unique species than any other marine environment on earth. Many adaptations of colorful fish, meadows of algae, and corals have never been seen before.
“These are some of the most extensive and densely populated coral reefs in Hawaii,” said Anthony Montgomery, a U.S. Fish and Wildlife biologist and co-author of the study. “It’s amazing to find such rich coral communities down so deep.”
