An Audacious Plan in the Twilight
The moment they fall backward through the turbulent surface, the three divers turn to look straight down—but all they see is the blue-black haze of depth. Despite the ominous view, they immediately relax their arms and legs and allow themselves to free-fall, heavier than saltwater, into the unknown below.
Careful to stick together, they glance at one another every few seconds as they drop. They look for signs of physical or psychological distress, and for equipment problems, such as bubbles escaping from the sealed “rebreathers” strapped to their backs—the equipment that will keep them alive underwater for the next five hours. The same equipment that could kill them if they fail to recognize a malfunction.
Despite the inherent risks, this is the moment they’ve been waiting for. What they’ll witness over the next 25 minutes is what it’s all about for these three. They are members of an elite, yet widely dispersed, group of scarcely a dozen scientific divers who dare to plunge into a strange, half-lit region between 200 and 500 feet below the ocean surface, a place called the “twilight zone.” It’s the chance to see things no one else on Earth has ever seen before, to discover new species, and to begin to understand what role these deep, cold, low-light, high-pressure habitats might play in the ecological health of coral reefs in general.
Bart Shepherd, 45, the expedition’s leader, spent many years and hundreds of dives teasing the edge of the twilight zone. “I did a lot of diving at the 120-foot range, and you could always look over that edge and see it kind of drops off… it’s this mystery,” he says. “I always wanted to go down there.”
But without the technology and training required to safely plunge to these depths and return to the surface, he wasn’t tempted enough to throw caution to the wind and actually go. With long, relaxed limbs and a laid-back conversational style that belie both his drive and his fastidious attention to detail, Shepherd lacks the bravado and risk-loving nature you might expect from a member of this exclusive group of deep-reef explorers. He’s not a natural risk-taker, he says. With a wife and two daughters back home—all of whom expect a daily FaceTime call when he’s away on expedition—he says he’s only willing to accept “manageable risks,” and only if the objective is worthwhile.
Which is why he and his two co-conspirators—Brazilian ichthyologist Luiz Rocha, 42, and Rocha’s tireless Ph.D. student Hudson Pinheiro, 35—now find themselves in the South Pacific, just off the coast of a tiny sliver of land known as Hat Island, accelerating past a depth of 150 feet en route to the twilight zone. They all know that both in terms of this dive and in the much larger picture of coral reef research and conservation, there’s no time to lose.
The team has traveled some 6,000 miles from their home base at the California Academy of Sciences in San Francisco to this spot near the middle of a long strand of 83 volcanic islands that make up the Pacific nation of Vanuatu. They’re here to explore the biodiversity of Vanuatu’s twilight zone reefs, which are among the least explored, least understood environments on the planet.
Also known as mesophotic—meaning “middle light”—coral ecosystems, twilight zone reefs occur nearly everywhere shallow reefs are found, as well as in some places—such as perched on undersea mountains—where they’re not. Yet most people have no idea these ecosystems exist. Even the scientists who know the most about these habitats have far more questions than answers about twilight zone reefs. “Half of what we see down there is new to science,” says Rocha.
And that half that Rocha alludes to pertains to species. Scientists know even less about the twilight zone’s connection to shallower (and deeper) habitats or its potential role as a refuge or nursery for overexploited fish populations or for corals during periodic warming events. And because they’re largely unstudied and unmapped, twilight zone ecosystems are seldom, if ever, included within marine protected areas established to preserve biodiversity in shallower ecosystems.
The poor understanding of these habitats has perpetuated a wholly incomplete understanding of what coral reefs are and how they function, says Richard Pyle, an ichthyologist at Bishop Museum in Hawaii and a pioneer in deep-reef exploration.
Most research on corals began in the 1950s, soon after scuba diving began. But because scuba technologies and techniques limited how far divers could safely descend, that research focused almost exclusively on the ocean’s upper 100 feet. As a result, Pyle says, “coral reefs as a concept were defined by this top 100 feet.” Anything deeper remained unexplored.
Diving becomes increasingly dangerous the deeper you go. Someone who has logged just a few dives can typically descend to 100 feet and come back in one piece. But venturing much beyond that depth is significantly riskier and requires divers to breathe a mix of gases different than air, and to ascend very slowly.
Navy divers and adventurers hell-bent on exploring deep shipwrecks developed these techniques and gas blends through the middle of the 20th century, but Pyle was the first to begin applying them to scientific exploration of the twilight zone, in the mid-1980s.
Over the past three decades, he has discovered dozens of probable new species of twilight zone fish—so many he’s lost count. Yet, despite the untapped opportunities for new discoveries, few scientists have taken up the mantle of this research effort. Some research groups have begun providing glimpses of twilight zone habitats using remotely operated vehicles or by dangling cameras from ships. But Pyle estimates that only 12 to 15 scientific technical divers are currently doing research at these depths.
He attributes the low numbers in part to the extreme risk and training involved in doing such dives safely. In addition, he says, many in the scientific diving community long assumed deep reefs just weren’t biologically interesting enough to bother exploring—not lifeless exactly, but not teeming with life, either.
Discoveries by two separate research groups in 2004 and 2005 changed scientists’ perception of twilight zone reefs, Pyle says. The teams documented extensive live-coral coverage down to 250 feet in the Gulf of Mexico and as deep as 400 feet off the coast of Maui. Those discoveries, Pyle says, captured people’s attention and inspired the National Oceanic and Atmospheric Administration to convene a meeting about twilight zone research, in 2008. The number of scientific papers focused on this region of depth has risen steadily ever since—from 20 publications in 2008 to 269 in 2015.
Pyle thinks deep-reef exploration is still gaining traction among marine scientists. “It’s going to require some pushes, financially and technologically,” he says. “But we’ve overcome some of the initial barriers to legitimacy and have shown that there’s important science to be done at these depths. You really can’t begin to understand it until you get down there.”
“Coral reefs as a concept were defined by the top 100 feet.”
— Richard Pyle
First up are rebreathers, closed-circuit systems that take the place of traditional scuba tanks, allowing the divers to carry less air by scrubbing carbon dioxide and recirculating the oxygen from each exhaled breath. Next come two massive bailout tanks that each diver carries in case of a rebreather failure. They also don neoprene hoods, booties, and gloves. They strap on fins, masks, and weights. They clip on wrist-mounted computers that will monitor every detail of their dive while also modeling their own physiological response to the conditions surrounding them.
Finally, they each receive a fish decompression chamber. This contraption is roughly the size and shape of a scuba tank, but is made up of valves and ports and seals designed to maintain a consistent internal pressure, to keep any organism inside alive en route from twilight zone depths back up to the surface.
As Love clips off each item to its appropriate D-ring, the divers droop further under the weight of the equipment and the heat of the tropical sun. Groaning, they wait as patiently as possible in growing anticipation of the relief they know will come when they can finally collapse backward into the 74-degree water and become weightless.
When fully outfitted, each diver weighs about 180 pounds more than he did a few minutes before. The three can scarcely sit up straight, much less stand. But finally, they’re ready. The boat motors quickly out toward the center of the channel while Love and Eroni, the boatman, search for a patch of water the deep shade of blue that promises a vertical drop of 200 feet to the bottom. When they think they’ve spotted it, Eroni puts the engine into neutral, and then briefly reverses to stop the forward momentum. At the end of Love’s quick count of three, the divers roll back over the side. Shepherd, Rocha, and Pinheiro bob to the surface briefly, and Love scrambles to place net handles into PVC scabbards at the back of their rebreathers.
Then, just like that, they all sink—visible at first as blurry patches of red, chartreuse, and black, and then gone.
High-tech on island time
The team’s base of operations is at a compound called Blacksands, on the edge of the village of Mele—a rural patchwork of thatched houses, lush gardens, livestock pens, melon vendors, and smoldering rubbish heaps. Blacksands itself is a grouping of five buildings so ramshackle Shepherd nicknamed it “the abandoned house in the woods.” It looks more like the gritty set of a low-budget psychological thriller than the command center for a scientific diving operation. Home to a bustling reef-fish export business, it’s inhabited by a band of karaoke-loving Filipino fishermen; a gaggle of laborers; five guard dogs and their roving ecosystems of fleas and ticks; and Grant Norton, the Canadian ex-pat who runs the business.
Norton’s boats, dive gear, scuba and fish-holding tanks, pumps, coolers, electrical system, and staff are the infrastructure Shepherd and his team will rely on for the expedition’s success. But pairing activities as technically complex and dangerous as deep-reef exploration and live-animal collection with the equipment conditions, safety standards, and pace of life of this developing island nation is a tricky proposition.
By the time the full team assembled at Blacksands, the situation looked grim. Many of Norton’s scuba tanks were so corroded that their valves couldn’t be used. The air compressor used to fill those tanks was pumping seven times the allowable limit of carbon monoxide. One of the two boats they’d need sat on blocks a quarter mile from its launch site. And the team’s supplies of pure oxygen and helium, ordered from Australia weeks earlier, hadn’t shown up.
At the end of their third day in Vanuatu, Shepherd and Love were seriously considering pulling the plug on the expedition. But hard-assed persistence eventually won out. After dozens of hours of hard work and patience, the team had cleaned and prepped scuba tanks and repaired the broken compressor. The boat had been carried down the road, shoved over an embankment, dragged through a rubbish pile, and pushed into Mele Bay. And the gas supplies had finally arrived.
Plenty of challenges still remained, though. Somehow they had to find a way to cram seven people and a ridiculous amount of gear into two small, underpowered boats. They had only sketchy clues about where the best dive sites would be, how the ocean currents would divert them, and what they might find when they reached the twilight zone. And the delays they’d already racked up meant they would have fewer days to explore than they’d planned.
By the morning of this, the final dive, the tally of twilight-zone fish collected so far sat at a measly 20—about a quarter of what they hoped to bring back. The pressure was on, and Shepherd was determined to make a significant dent in the current cost-per-fish figure. Everyone on the team knew there was only one way to do that.
At the final pre-dive meeting, Love could sense the pressure and urgency among the team members, not to mention the level of fatigue surrounding this last dive, and felt the need to remind the group about the diver’s worst enemy: complacency. “We’ve got a big dive today,” he said. “So pay attention to your stuff. Just because it worked yesterday doesn’t mean it’s going to work today.” The message was not unlike a football coach’s pre-game pep talk—except this game can kill you.
Less than halfway home
There’s a lot more to the risks of deep diving than the obvious distance it puts between the diver and that unlimited supply of oxygen at the surface. The pressure that goes hand in hand with depth does bizarre things to one’s physiology, particularly in relation to the air a diver breathes. Every 33 feet of depth stacks one whole atmosphere’s worth of pressure on top of a diver. That pressure compresses everything, gases in particular—so much so that a lungful of air at 300 feet contains about nine times as many molecules as the same breath at the surface. That’s nine times the number of molecules pushing into a diver’s tissues and being absorbed by the blood, which is almost exactly nine times too many.
Because of pressure, the 21-percent concentration of oxygen we breathe on land becomes toxic and seizure-inducing at depths beyond about 180 feet, more so at 300 feet. The other chief component of air, nitrogen, starts to have a narcotic effect somewhere below about 90 feet.
To prevent these potentially disastrous effects, deep divers breathe a carefully blended mix of gases in which the standard concentrations of oxygen and nitrogen are diluted significantly with helium, an inert gas that has none of the toxicity and narcotic effects of the other two elements. This gas blend is combined with the closed-circuit rebreather system that scrubs the carbon dioxide and recycles the oxygen from air the diver exhales, which allows for longer dive times. The same onboard computer that calculates TTS also monitors this mix of gases in real-time and meters out just enough oxygen to keep a diver conscious, clear-headed, and seizure-free.
But using a rebreather doesn’t change how the increased pressure at depth compresses gases inside the body. The deeper you dive, the longer you stay, and the harder you work while at depth, the more gas the tissues absorb. As a diver heads toward the surface, that gas has to go somewhere. Ascending too quickly causes dissolved nitrogen to form bubbles in tissues and blood, causing the painful and sometimes life-threatening condition known as decompression sickness, or the bends. To prevent the bends, deep divers spend hours decompressing at increasingly shallow depths in exchange for 15 minutes spent chasing fish around in the twilight zone.
Like humans, fish carry a tremendous amount of gas inside their bodies, particularly inside organs called swim bladders. These internal sacs enable the fish to maintain neutral buoyancy underwater. The deeper a fish resides, the more gas that fish’s swim bladder contains. Although fish don’t get the bends, if a diver brings a deep-water fish up too quickly, without taking any precautionary measures, its swim bladder can expand so much that it pushes the fish’s stomach and other internal organs out through its mouth.
Until recently, the only way to prevent such trauma was to poke hypodermic needles into the swim bladder to vent the gas on the way up to the surface. Shepherd was never keen on this technique. “I don’t like poking holes in fish,” he says. So he challenged his team of biologists to develop a portable decompression chamber that could be taken down to the twilight zone, sealed at depth, and brought to surface-level pressure over the course of one or two days.
“During the whole dive, you have to be thinking that the rebreather is trying to kill you. If you don’t take it that way, it will kill you.”
Dripping wet and still covered in neoprene, they scramble to get the three chambers hooked up to a system of tubes, pumps, valves, and gauges that will flush the chambers with clean, chilled seawater while maintaining the pressure inside. Yim flips the switch on the power unit and the system hums to life. Although he, Upton, and their new cargo still face a 90-minute boat ride back to Blacksands, they’ve bought themselves enough time to get to a more reliable source of power that will help them maintain conditions inside the chambers that match those where the fish came from. And that, after all, is precisely the point of this whole endeavor—to replicate a small piece of the twilight zone environment.
Beyond bragging rights
Left alone again, Shepherd, Rocha, and Pinheiro continue on to their next decompression stop at 90 feet. They will make six more stops on this dive, each one longer than the last. The final stop, at 30 feet, will last more than an hour-and-a-half. In all, they will spend more than five hours underwater on this dive.
These shallow-water decompression stops can be dreadfully boring. To combat the monotony and to stay warm, the divers try to keep busy. They spend a lot of time looking for interesting creatures to observe and photograph. Pinheiro lays out a 20-meter tape along the ocean floor and then swims the length of that transect as he meticulously counts and records in a waterproof notebook the fish species he sees.
The more pristine the habitat, the more rewarding the hours spent decompressing tend to be, because there’s so much more ocean life to see. But practically everywhere they go, the team also witnesses the negative impacts humans are having on coral reefs. They see garbage and sediment. They see corals damaged by destructive fishing practices. They generally see few large predatory fish, because these are the species that most often end up at the end of fishing lines or in a fisherman’s net. And increasingly they see signs of coral bleaching, the result of unusually warm ocean water.
These are observations they have a lot of time to make in the shallows, unlike during their too-brief visits to the twilight zone. But they, perhaps more than anyone, know that the distance is not so great between deep and shallow reefs. And they feel certain that twilight zone species are likely impacted by many of the same threats that shallow reefs are.
The thing is, no one really knows. We have no frame of reference for twilight zone ecosystems. We have no point of comparison, no way of knowing which organisms occur there naturally and which might be new arrivals. We don’t know who eats what, or how they breed, or how long they live. We don’t know which species are most threatened. And we can’t possibly know which twilight zone species may have already been lost.
The connections between deep and shallow reefs are also unknown. Do deep reefs serve as refuges or nurseries for shallow-water fish populations? Are twilight zone corals less vulnerable to warming seas and thus capable of seeding shallow reefs damaged by future bleaching events?
Again, no one knows the answers to these questions. The quest to find answers to any of them is powerful motivation for these divers. It’s what drives them to risk everything to plunge into the twilight zone, and then while away the hours decompressing in the shallows.
Scale is perhaps the biggest challenge facing this tiny research community. It’s a big ocean, and twilight zone reefs are at least as extensive as shallow reefs are. At the current rate of exploration, any insights that might influence conservation decisions are likely still decades away. But Richard Pyle, for one, seems confident that a change is underway, and that the area of research he pioneered 30 years ago is about to take off.
Much of the research thus far has been focused on discovering new species and documenting the distribution and extent of deep-water coral reefs. But Shepherd, Rocha, and Pinheiro say there is much more interesting work to be done at twilight depths—work that will clarify these habitats’ ecology and the environmental factors that influence the communities of organisms that call them home. This will require more time, greater investment, and more brave souls willing to venture into the twilight zone.
But meanwhile, Shepherd argues, although it’s critical to expand exploration and research efforts in the twilight zone, it’s not necessary to exhaustively survey every twilight zone ecosystem before taking action to preserve them.
“If we take enough snapshots in enough different places,” he says, “it will provide us with models that will help us understand the system as a whole. I don’t need to know every species that lives there to know that an ecosystem is worth protecting.”
“I don’t need to know every species that lives there to know that an ecosystem is worth protecting.”
From Vanuatu’s Twilight Zone
Some descriptions of organisms that live in the twilight zone and technical aspects of deep diving originally appeared in the author’s Scientific American blog “The Richest Reef.”
Bathymetry data used to create the 3D model of the Hat Island dive site provided by the Pacific Community (SPC); data collected, analyzed, and published by Kruger, J., Pelletier, J., Sharma, A., 2007.
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