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The wooden fishing boat chugs across Comau Fjord, a finger of dark water wedged between the snowy peaks of Chilean Patagonia. A Chilean flag flaps in the wind above a hand-painted black cormorant on the bow. Inside the boat’s cabin, Boris Hernández, a fisherman who is among the few settlers in this remote fjord, peers through foggy windows. With one hand on the throttle, he steers to a spot fixed in his mind but otherwise indistinguishable from its surroundings. He cuts the engine, pulls his yellow beanie over his ears, and steps out onto the deck. A sheer cliff looms ahead.

“Right here,” Hernández says in Spanish. He points to the watery depths. In this very spot, Hernández has pulled up fishing nets tangled with the white, knobby branches of dead coral. Their skeletons call to mind the architecture of some distant tropical reef, but Desmophyllum dianthus isn’t your average warm-water coral. Instead of forming shallow reefs full of charismatic fish, D. dianthus thrives in total darkness and near-freezing temperatures, at depths of up to 2,000 meters. Instead of partnering with photosynthetic algae within its cells for food, it catches and kills plankton and tiny shrimp with stinging tentacles. And while D. dianthus and other cold-water corals live throughout the world’s oceans, including the Antarctic—providing habitat at least as critical as that of tropical coral reefs—the deep, inhospitable conditions they prefer make them difficult to study.

Only here, in the icy waters of Comau and two other nearby fjords, does D. dianthus live close enough to the surface that open-water scuba divers can reach it without any special equipment. Just 30 meters below us, vertical walls dense with thickets of D. dianthus and several other coral species stretch straight down into the inky gloom.

D. dianthus usually lives in deep, inhospitable conditions that make the species difficult to study. Photo by Ignacia Acevedo-Romo

From Hernández’s boat, though, there’s nothing to see but water so dark it appears more black than blue. That water is the reason why D. dianthus survives so close to the surface. Sediment-rich fresh water from rainfall and melting glaciers floats atop the salt water, forming a light-blocking layer that darkens the environment below. Thanks to this quirk, scientists in Comau Fjord can study the basic biological processes of D. dianthus in ways that would be impossible in its normal deep-sea habitat. And these days, “more people are interested in learning about [the corals],” Hernández says. He’s become something of a scientific guide, ferrying researchers, a few tourists, and even the rare intrepid journalist to Comau’s coral gardens.

In June 2024, an international team of scientists dove into these waters to collect samples. Now, the coral—and the scientists—wait in a dark, refrigerated lab borrowed from a government fisheries agency on Chiloé Island, Chile, 90 kilometers across the Gulf of Ancud. At any moment, the scientists are hoping to witness something no one has ever seen before—D. dianthus spawning and making new coral babies in a lab.

The three WhatsApp messages arrive in all caps and in quick succession: BABIES. ARE. COMING. The messages have the intended effect: Rhian Waller sits bolt upright in bed, checks the time—11:00 p.m.—and pulls on her coat. A marine biologist at the University of Gothenburg in Sweden, Waller has been waiting for this moment for days; the coral her team had carefully extracted from Comau Fjord a month earlier and watched 24/7 in shifts ever since is finally ready to spawn.

Waller races beneath the frosty, star-filled sky of a winter night to the equally cold lab, where she joins PhD candidate Diego Moreno Morán, originally from Mexico, and Chilean graduate student Ignacia Acevedo-Romo. The cold doesn’t bother Waller—she’s spent her career in it, chasing winters from hemisphere to hemisphere, Sweden to Chile, Alaska to Antarctica, trying to catch cold-water corals during their spawning season. She’s one of only a handful of people worldwide who study cold-water coral reproduction and is renowned for her dedication to getting samples by any means necessary—from century-old museum collections and on ship and submarine expeditions at depths of up to 5,000 meters, even in stormy winter weather.

At a borrowed government lab in Chilean Patagonia, graduate students Ignacia Acevedo-Romo (left) and Diego Moreno Morán (centre) assist marine biologist Rhian Waller, who uses a flashlight to illuminate tiny coral larvae in a flask. Photo by Christian Elliott

Despite such efforts, scientists still know nothing about the reproductive behaviors of 96 percent of cold-water coral species, including D. dianthus, one of the most widespread and common cold-water corals. With such corals under increasing threat from climate change and deep-sea trawling, mining, and drilling, better understanding D. dianthus could help ensure its survival worldwide. In particular, figuring out how and when it spawns may be critical to burgeoning efforts to restore deep-sea coral ecosystems around the globe that have been damaged by industry.

Waller arrives at the lab just in time. Nearly as soon as she gets there, the coral—dangling upside down like tiny red, orange, pink, and white tentacled palm trees—puts on a dramatic show. The females inflate, their semi-translucent tentacles filling with hundreds of tiny orange eggs released in a steady, delicate stream through their open mouths. Not to be outdone, the males spurt clouds of milky-white sperm into the water, then deflate, limp and exhausted. The scientists literally ooh and ah as they watch.

“Any hesitation you have because you’re really tired kind of goes away as soon as you enter this room,” Waller says. “There’s so few people on the planet that have ever seen this happen, especially in deep-sea species.”

The researchers work for hours, hands in the icy water, gripping glass pipettes and collecting sperm and eggs, filling test tubes, pairing off the gametes, and filling refrigerated incubators with vials. The sun will rise before it’s time for them to warm frozen hands, cracked from exposure to seawater, and finally rest. Six or so hours later, they’ll know if their efforts were worth it.

Cold-water corals have been known to exist for centuries. In the mid-1700s, apothecaries in Bergen, Norway, sold coral amulets from fragments pulled up in fishing nets to ward off the plague. Contemporaneous natural historians like Bishop Erich Pontoppidan drew beautiful diagrams, theorizing that the corals were rocks or maybe plants. Research came in fits and starts following new technological developments. In the 1860s, with dredges and steam-powered winches, natural historian Charles Wyville Thomson pulled up great masses of living coral from the seabed off the Scottish coast, disproving the azoic theory of the day that the deep sea was a dead zone. (Pressure changes caused Wyville Thomson’s specimens to quickly die.) Then, in the 1950s, military sonar made seabed mapping possible. Everywhere scientists looked, they found massive rocky ridges and mountains which, in the 1970s when research submersibles became available, they found to be covered in reef-building coral species. On closer inspection, some of these underwater formations consisted entirely of coral skeletons—built from the ground up over millennia, teeming with forms of life that hadn’t yet been defined.

Marine biologist J. Murray Roberts, of the University of Edinburgh in Scotland, calls these coral mounds the “cities of the deep sea.” The corals that build them may grow mere millimeters each year, but some reach 300 meters, tall enough to engineer currents to drive organic material—food—from the surface to themselves. Cold-water corals have persisted this way for millions of years, thriving during interglacial periods and spreading to the poles, then dying off and retreating equatorially during ice ages or when food supplies changed. Some coral seamounts are so old that geochemists use them as climate archives, drilling through alternating layers of coral and glacial deposits to unravel the ancient ocean chemistry locked into the coral’s bony skeletons. One, found some 365 meters deep off the Hawaiian coast, is the oldest known continuously living marine organism on Earth, at 4,265 years old.

Natural historian Erich Pontoppidan drew cold-water corals for his book The Natural History of Norway, published in 1755—though he called them “sea-vegetables.” Image courtesy of the Internet Archive

“It blows my mind,” says Michelle Taylor, a marine biologist at Essex University in England. “When there were people in Egypt considering making pyramids, [the larvae] landed. And then through all of the human time that’s happened since then, [the reef’s] just been getting a little bit bigger, getting another polyp, a little branch.”

Cold-water coral science took off in the 1990s, around the time that Roberts and Waller first joined the field. Commercial fishers were increasingly trawling the deep sea and bulldozing reefs scientists hadn’t even discovered, especially in international waters beyond the jurisdiction of individual nations. Today, scientists estimate trawling impacts nearly 14 percent of all continental shelves and slopes shallower than 1,000 meters. With modern gear, trawlers can scrape thousand-year-old coral gardens off the tops of seamounts in minutes to capture the fish hiding within them. Erik Cordes, a deep-sea ecologist at Pennsylvania’s Temple University, says that some sites were already flat fields of coral rubble when scientists first found them. Before the trawlers came through, these “were probably big cold-water coral reefs,” he adds. “We just never got to see them.”

Residents of these reefs—from cod to sponges, tube worms to brachiopods—rival the diversity of tropical reefs. They play a key role in the global carbon and nitrogen cycles, provide spawning grounds for commercial fish species, and they likely contain undiscovered pharmacological compounds. But because most people can’t see them, they receive only a fraction of the attention of tropical coral reefs.

To find and save the cold-water reefs that remain, Roberts and other scientists began using high-resolution tools, such as echosounders that can measure bathymetry and determine the shape and hardness of the seabed, along with remotely operated underwater vehicles with cameras. In the absence of oceanwide maps, they defined the niche where cold-water corals can live and fed that data into models to predict where corals could be. In places where corals were actually found, the United States, Australia, and New Zealand closed large areas to fishing. Meanwhile, in 2006, the United Nations required member states to protect vulnerable ecosystems like cold-water coral reefs from destructive fishing practices. Despite these protections, enforcement remains difficult, especially in international waters, and cold-water corals still face anthropogenic threats like trawling, deep-sea mining, and oil exploration.

D. dianthus grows in cold water throughout the world’s oceans, including the Antarctic, providing habitat for a breadth of species. Photo by Thomas Heran

Today, some 4,200 cold-water coral species have been discovered—more than double the number of known tropical corals. Every expedition turns up a couple of new species and often a new reef. Scientists estimate cold-water reefs cover at least twice the area that tropical reefs do, though their exact extent remains unknown.

These ecosystems and the diversity they contain, says Taylor, represent an “untold wealth. We should probably be allowed to investigate that before destroying it.”

With shallow-water corals increasingly falling victim to hotter ocean temperatures and bleaching, cold-water corals may stand a better chance of surviving in the future. Yet, since they grow incredibly slowly—especially in the coldest, deepest parts of the ocean—reefs decimated by trawling haven’t recovered over the last 20 years. It may take centuries for meaningful recovery.

That’s why scientists are testing out active restoration—sinking live coral fragments into destroyed reefs to give new life a better chance of taking root. Of the 16 ongoing or upcoming restoration projects Taylor knows of, most are small scale, like a Spanish effort using fishermen to return corals they accidentally pull up in their nets to the sea. But some have ambitious goals. Researchers in Norway, Sweden, France, and Spain are planning large-scale restorations in the Mediterranean Sea and on the Mid-Atlantic and Arctic ridges, using artificial reef structures seeded with baby corals, while Roberts co-leads a multinational project to develop more pilots that can eventually scale up restoration work.

“So it’s very new. And one of the biggest hurdles for restoration is understanding reproduction,” says Taylor. “If you want to replenish an area, you need to understand how animals breed, or else you’re doing it quite blind.” It’s one thing to place adult corals on the seafloor. To know how far apart to install new reefs so that corals can “cross-pollinate,” as well as to understand whether the resulting larvae will be able to reach other populations further afield, requires a fundamental understanding of how corals reproduce—and each species is different.

Because ship-based expeditions to collect samples are incredibly expensive and time-consuming—not to mention dangerous, since most corals spawn during winter storm season—progress has been slow. “You have to be quite cussed and awkward to stick with deep sea corals,” says Roberts. “It’s really difficult. And the simplest question requires all of this kit.”

Which is why, when Waller learned that a pair of German biologists had found D. dianthus in relatively shallow Patagonian fjords, she raced to Chile. On the sheer cliff walls of the fjord, D. dianthus grows as a vertical marine animal forest—hundreds of thousands of individuals reaching out to dine on the plankton that rise to the surface every day and fall to the fjord’s bed every evening. While deep-sea D. dianthus grows short and stubby, in groups of two or three, and has a wide mouth, the fjord variety is long and slender, forming dense groves of individuals that reach into the water column to catch as much food as possible.

Rainwater and glacial melt help keep the water of Comau Fjord, in Chilean Patagonia, dark and cold enough to support species that otherwise live only in the deep ocean. Photo by Thomas Heran

Her first dive in Comau Fjord some 15 years ago was like a “night dive in the middle of the day,” Waller says. She didn’t care that visibility was low even with her headlamp, or that her fingers were numb and clumsy, or that she had only a few minutes of air to spend 30 meters below the surface. “The first time I saw these [kinds of] deep-sea corals that I’ve only ever seen before through a submersible window,” she says, “it was really amazing. You know, I could stick my head in the coral, I could poke the coral, I could take the sample. That’s something you just can’t do with a robot or on a TV screen.” And accessible D. dianthus samples provided a unique opportunity—bringing live specimens back to the lab to watch them spawn.

Back in the borrowed government fisheries lab on Chiloé Island, 28 D. dianthus specimens hang suspended in tanks within a larger tank, which cools them with a constant bath of filtered seawater, like a frigid bain-marie. For the month of July while the scientists live here, losing track of time in the dark and cold, Chiloé is Love Island for the coral contestants. Unlike most tropical corals, where entire reefs are made up of genetically identical polyps, D. dianthus is a solitary coral—each polyp is genetically distinct. That means the sex of a given specimen plucked from the fjord remains a mystery until it’s ready to reveal itself with a stream of eggs or spurt of sperm. “It would be wonderful if they color-coded themselves or something, but they don’t,” says Waller. “So we just have to watch and wait for them to really show us who they are.”

Unlike most tropical corals, cold-water corals are carnivorous, catching and killing plankton and tiny shrimp with stinging tentacles. Photo by Thomas Heran

They may lack brains, but each individual coral seems to have a mind of its own, and after weeks the team has gotten used to the corals’ distinct personalities. Some tolerate the light of a flashlight or the slam of a door; others curl their tentacles at the slightest disturbance, preferring privacy—they’ll only do it with the lights off. The researchers running this show have their patience tested coaxing the coral contestants to cooperate. “They all do their own thing,” says Waller. “If they’re happy, they’ll spawn well; if they’re unhappy, then they just close up.”

Tonight, both males and females have chosen to spawn. After the team collects sperm—a frantic endeavor, as it dilutes quickly into the water—Waller and Acevedo-Romo set to work pipetting up eggs that have settled to the bottom of the tanks. Meanwhile, Morán crouches over a microscope, painstakingly counting individual sperm to determine concentrations and how far they can travel in the water to successfully reach and fertilize eggs.

Everything about the process—done in the cold, in comparative obscurity, and with methods that must be developed from scratch—is just a bit harder than studying tropical coral spawning. Tropical coral scientists snorkel through warm reefs on the one beautiful night when corals spawn all together under a full moon, collecting eggs and sperm by the bucket to take back to their labs. Sperm concentrations are so dense they’re measured by the color of the water. In cold-water reefs, on the other hand, only a few corals spawn at the same time, though each male produces a lot of sperm. And unlike tropical eggs, evolved to mix with sperm in surface waves, cold-water coral eggs aren’t adapted to touch the surface, so they must be gently pipetted one by one to prevent them from exploding.

Marine biologist Rhian Waller, one of the few people in the world studying cold-water coral reproduction, pipettes larvae from one beaker to another. Photo by Christian Elliott

“This is a question that I get a lot: why do you, from Mexico, having tropical corals in your country, why you’re suffering in the cold?” Morán asks as he works. His answer is nonchalant: “Someone needs to do it.”

With sperm and eggs counted and sorted, the team sets to work mixing different concentrations of each to learn the minimum needed for fertilization. The initial results are surprising enough that the scientists wonder if they’ve messed something up—D. dianthus needs shockingly low concentrations of sperm: roughly 500 per milliliter of water, compared to up to one million per milliliter for tropical corals. It’s a good evolutionary strategy for a coral that lives in a high-current environment that quickly spreads out eggs and sperm, Waller explains. Sperm can disperse over vast distances and still manage to fertilize eggs, which means D. dianthus can create new reefs throughout the world’s oceans, far from parent colonies.

The researchers combine eggs and sperm from various females and males to see which pairings are more compatible, and swap newly declared male and female coral contestants around in tanks to try to generate more larvae. In between, they clean glassware, take hot showers, guzzle tea, moisturize cracked hands, and nap. During the spawning window of a few weeks, each coral can spawn once or multiple times, rarely coordinating with its neighbors. No one knows when the next spawning will happen; the team has to be ready to go at any time. “It’s a constant cycle,” says Waller. “Everything’s on their timeline … We have no control, and that’s okay.”

To illustrate just how little scientists know about cold-water corals’ critical first life stage, Waller points to Primnoella chilensis, a whip coral that grows on horizontal steps in Comau Fjord. Waller’s team snipped specimens on a whim while collecting D. dianthus; now, in a separate tank in the lab, P. chilensis sway in the slight current. Up close, they look like orange pipe cleaners—twiggy stalks covered in whirls of tiny polyps.

Primnoella chilensis, another cold-water coral that lives in Comau Fjord, has its own unique reproductive strategies. Photo by Christian Elliott

Waller expected P. chilensis to be a broadcast spawner like D. dianthus, but her team found that instead, the females brood their young, taking in packets of sperm to fertilize eggs in the nooks and crannies of their long bodies and then dropping fully formed larvae that crawl around the tank’s bottom. That finding has implications for possible future restorations and connectivity models, because crawling larvae can’t travel nearly as far as swimmers that hitch rides on ocean currents.

Cordes, the deep-sea ecologist, is keeping a close eye on this kind of work. Since 2010, he’s studied how the Deepwater Horizon oil spill in the Gulf of Mexico damaged cold-water reefs. Now, with the National Oceanic and Atmospheric Administration (NOAA), his team (which includes one of Waller’s former students) has cast replica reef skeletons out of coral sand and concrete and installed an initial test structure with eight coral fragments attached. It’s the world’s first truly deep attempt at cold-water coral restoration, at a depth of around 1,000 meters; last year, a NOAA team conducted a similar trial in shallower water. It’s too soon to tell whether either effort will be successful, but the hope is to restore the physical structure and habitat function of damaged reefs without having to wait centuries.

“The deep ocean is a very big place. We can put these artificial reefs down everywhere we want, but only if there’s a good source of larvae are they going to actually turn into real cold-water coral reefs,” Cordes says. And the only way his team can ensure larvae make it to the artificial reefs is if they know how larvae move and how their movement matches up with complex ocean currents. “Without that information, we’re just putting a bunch of rocks down in the deep sea, and they’re just going to sit there.”

About six hours after mixing sperm and eggs in the Chiloé Island lab, the team returns, triumphant, to vials of D. dianthus embryos. Under the microscope, one cell becomes two, two become four, and four become eight. The proto-coral babies look, Morán quips, like tiny yellow cloudberries, “the best berry” back in Sweden, where he studies under Waller. After a few more divisions in this “berry phase,” the offspring become official larvae: the first of their species ever bred in a lab. The team moves them to larger flasks of filtered seawater flush with oxygen, where the larvae will have more room to grow.

Larvae from D. dianthus corals bred in the Chiloé Island lab swim beneath a microscope. Photo by Ignacia Acevedo-Romo

A few days later, the babies are visible to the naked eye—they look like flecks of gold dust or glitter slowly swirling around, or yeast right after it’s stirred into warm water but before it dissolves. Acevedo-Romo extracts a few into a petri dish. Under a microscope, their little yellowish bodies, about twice the width of a human hair, are covered in tiny hairs of their own that they use to swim. The hairs grow on one side first, she explains, so the larvae swim in circles until they grow more and learn to steer, swimming up higher and higher in the flask.

After a few more days, the larvae are fully developed. In the wild, these delicate deep-sea dandelion seeds would be ready to drift for hundreds of kilometers, riding ocean currents for weeks or more before deciding on a place to settle, attaching to rocks, elongating their bodies, and ultimately transforming into coral polyps to live out the rest of their lives across centuries. Scientists hypothesize that D. dianthus first arrived in Comau Fjord from the deep sea in this way, propelled by currents over an unusually short continental shelf and into coral utopia—plenty of food, free real estate in bare rock walls, and relatively stable temperatures and currents.

Yet, even as Waller’s team answers fundamental questions about coral reproduction to inform faraway restorations, the Patagonian fjord they pull their specimens from is changing rapidly. Corals now spawn a full month earlier than they did when Waller first dove there in 2008, perhaps because of warming temperatures. “You’re trying to study them and trying to get this baseline, and it’s like, is it baseline at this point, when there’s so much change already happening?” Waller asks. Since there aren’t consistent temperature records for the fjord, it’s impossible to know.

Chilean graduate student Ignacia Acevedo-Romo and PhD candidate Diego Moreno Morán use a microscope to examine larvae from the first D. dianthus corals bred in a lab. Photo by Christian Elliott

And despite Comau’s status as a marine protected area, salmon farms are increasingly moving in, spewing organic pollution from feces and leftover food. In 2012, scientists discovered a mass coral die-off at a sampling site that just weeks before had hundreds of thousands of healthy corals. Within two months, most specimens were dead.

Jürgen Laudien, an ecologist at Germany’s Alfred Wegener Institute for Polar and Marine Research, determined that nutrient pollution from nearby aquaculture farms had helped fuel a bloom of algae that ultimately killed the corals. Another massive algal bloom in 2021 killed millions of salmon and more coral, staining the water and releasing a pungent smell that lasted for days.

Still, as creatures that have survived thousands of years’ worth of climactic and geologic upheavals, cold-water corals may have built-in resilience to changes in temperature and acidity. Comau Fjord is a natural laboratory for time travel, Laudien says. Its depths are already as acidic as the entire ocean is expected to be by the turn of the century, so it offers a glimpse into the future that cold-water corals face worldwide. Some scientists fear that rising ocean acidity will dissolve the bases of many of the planet’s massive seamounts; by the year 2100, some 73 percent of cold-water reefs are predicted to be in water too acidic for corals to calcify and form skeletons. Corals that Waller studies in Antarctica are already suffering—sometimes when she rubs them underwater, her hand comes away covered in chalky, dissolved skeleton.

But some research suggests D. dianthus in the fjord is doing better than expected—the coral is able to raise the pH in the space between its living polyp tissue and its skeletons enough to make calcification possible, even in acidic conditions. Raising the internal pH comes at a significant energy cost, which corals can afford in the fjord where food is abundant, but may be less able to afford in the nutrient-poor deep sea. And no one knows whether larvae can survive temperature, acidity, or salinity changes.

Despite recent advances, the reproductive strategies of most cold-water corals are still a mystery, making restoration efforts difficult. Photo by Thomas Heran

Ultimately, the long-term fate of cold-water corals is simply one more mystery in a field riddled with the unknown. But given that Comau Fjord gives scientists an opportunity to study the secrets of the deep seas, the answers may lurk just below the surface of fisherman Boris Hernández’s boat.

Back in Comau Fjord, Hernández ties up his boat to the dock and walks back to the cabin that he built by hand and lives in year-round, with a view of a flat-topped peak called El Tambor. It’s a cloudless, sunny winter day, the kind that would have been remarkable 20 years ago but now comes increasingly often. The forest is lush and snowless. A small, rotund, orange bird, a chucao, hops across the path to the cabin. Way across the fjord, we can see the tops of salmon farm cages, just visible above the water.

Inside the cabin, framed photos of D. dianthus hang on wood-paneled walls beside a portrait of Hernández’s children. Books about Comau’s biodiversity sit in an organized stack on an end table; they’re written in English, which Hernández can’t read, but he keeps them anyway. Carefully, the fisherman lifts a dry, white coral skeleton from the windowsill and points to a piece of monofilament fishing line running through its trunk. The coral grew around it for years before dying.

Fisherman Boris Hernández, in front of his hand-built cabin in Chilean Patagonia, holds the skeleton of a D. dianthus coral he pulled up in his nets from the nearby fjord. Photo by Christian Elliott

For decades, Hernández knew little about the ecosystems this coral came from, and even less about the coral itself. Today, he understands more—that the places corals live are also the best for fishing, for example, because of the habitat they create. He’s glad to have played a small role in helping humans better understand the intricacies of cold-water corals. Sitting by the crackling fire in the wood stove, Hernández reflects on the value that this information may play in the future of reefs, both outside his door and globally. “The worst mistakes,” he says, “are made by not knowing.”