A coral in the mesophotic zoneLuiz Rocha / California Academy of Sciences

It’s faintly absurd to be in one of the most obscure parts of the planet—a creepy zone of perpetual gloom and imminent danger, where no other humans have ventured—and have a cartoonishly squeaky voice. That’s what Luiz Rocha and his team repeatedly experienced in their attempts to study the world’s deep coral reefs.

Picture a coral reef and you’ll likely imagine a sun-drenched world lying just below the ocean’s surface. But reefs also exist beyond these shallow waters, in the so-called mesophotic zone, from 100 to 500 feet down. To study the unfamiliar animals that live in this dim world, normal scuba skills won’t cut it. Divers need special training and equipment—including larger gas tanks, rebreathers that recycle the air that divers exhale, and special gas mixes that include helium. And the helium means that anyone who enters the mesophotic zone ends up with a high-pitched squeak when they try to communicate through their rebreathers.

Why bother? Because coral reefs are among the most endangered ecosystems on the planet. Rising temperatures, acidifying waters, and emerging diseases have turned once resplendent reefs into ghostly dead zones; in just the past two years, half the corals in Australia’s mighty Great Barrier Reef have died. As the calamity worsens, some scientists have looked to mesophotic reefs for hope. If shallow species also live farther down, where the harms inflicted by humans are presumably weaker, then these deep waters could act as refuges for the shallow species—sanctuaries where corals could wait out the perils that threaten them up above. “That’s something everyone assumed is the case,” Rocha says.

But according to his work, that assumption is ill-founded. In one of the first thorough studies of mesophotic reefs, Rocha and his colleagues have shown that these habitats have very different communities of fish and corals than their shallower counterparts. “To me, saying that deep reefs are a refuge for shallow reefs is like saying that the savannah is a refuge for the rainforest,” Rocha says.

Luiz Rocha / California Academy of Sciences

“That may not be the case everywhere, and it’s too early to state definitively about whether mesophotic ecosystems can or cannot serve as a refuge for shallow-reef species as we are still learning about them,” says Kimberly Puglise, a program manager from the National Oceanic and Atmospheric Administration who co-authored a recent report on mesophotic reefs and was not involved in the new study. Still, she added, these reefs “tend to be forgotten and every time we have a new study that brings us new information, it’s a plus.”

A lot of the evidence behind the refuge idea comes from historical data, which suggested that shallow-water corals and fish species both persist to great depths. But those records aren’t reliable, Rocha says. “You could have collected something from [300 feet] down 100 years ago in some crazy situation, and the record is there, but it doesn’t mean that particular species actually occurs at those depths.”

To actually understand what life in the mesophotic is like, Rocha needed to see it for himself. Submersibles are of little help; their off-putting noises and lights make it impossible to do a proper census of local fish. Instead, Rocha needed to dive.

He and his colleagues did so in Bermuda, the Bahamas, Curaçao, the Philippines, and Micronesia. At each site, teams of five would descend dozens of times, surveying the fauna along the way. It was grueling and risky work. To explore the deepest waters without getting the bends, the team could only spend 15 minutes at the bottom before they had to make a slow, six-hour ascent, voices squeaking along the way. “It gets very dark and mysterious,” Rocha says. “You always have this feeling that this is the first time anyone has looked at this habitat.”

Their work showed that even though a quarter of shallow-reef fish do descend into this world, they are far less common there—more like visitors than residents. The two zones are distinct, and even within the mesophotic there are differences in the species that live at, say, 200 feet compared with those that live at 300 feet. “If you sent me a picture taken anywhere in the Pacific, I could probably tell you what depth it was taken to within 15 to 30 feet,” Rocha says.

The only thing that these reefs share is their threats. The team saw everything that endangers shallow reefs in the deeper ones, too. They found invasive lionfish, plastic junk, signs of bleached overheated corals, and evidence of destruction by anchors. “There’s one reef in the Philippines where there were so many fishing lines 450 feet down that they were a tangling hazard for us,” Rocha says. “And that was a place that nobody other than us has ever seen before.”

Luiz Rocha / California Academy of Sciences

Natural disasters strike these deep reefs, too. Just four days after Hurricane Matthew passed over the Bahamas in 2016, Rocha’s team visited a 450-foot-deep reef within the storm’s path. The corals there were well beyond the pounding reach of surface waves, but they had nonetheless been completely buried by a blanket of sediment, and an overlying cloud of suspended muck. “When we got to mesophotic depths, we couldn’t even see the reef,” Rocha says.

Deep reefs are yet another world that humans have imperiled. And because they have long been assumed to be safe, they aren’t subject to the same conservation efforts that are directed at shallow corals. “Because nobody goes there, everyone assumed that they were out of reach for humans,” Rocha says. They clearly are not.

Indeed, the team found that the only deep reefs that consistently showed few signs of damage or pollution were those that were far from human settlements. The same thing can be said for shallow reefs. Depth is no protection; only distance is. And even these far-away reefs can’t escape the global nature of climate change.

“Sometimes it seems that it’s hopeless,” Rocha says. “But I think we have to get our stuff together and protect what we have. The mesophotic reefs aren’t refuges, but they are important for a different reason: They’re unique, and just as threatened as the shallow reefs.”

“Climate change poses an imminent threat to the world’s shallow-water coral reefs,” says Julia Baum from the University of Victoria. “As comforting as it is to imagine that they will be rescued by their deeper counterparts, this new study clearly shows that such hopes are unrealistic. There’s no quick fix to saving the world’s shallow-water coral reefs.”

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