Karasik, the largest underwater mountain in the Arctic, was meant to be dead. Volcanically, it is. But biologically, it’s home to a teeming community of creatures, surviving in an environment with barely any food, through means that no one expected.
The mountain lies 300 kilometers from the North Pole, at a ridge where the tectonic plates that hold Europe and North America are slowly drifting apart. With its base sitting 5,000 meters below the surface, and its tip 560 meters below a thick sheet of ice, Karasik had barely been explored since its discovery in 2001.
And then, in 2016, Antje Boetius, a deep-sea researcher at the Alfred Wegener Institute, in Germany, led a team to Karasik. Cutting through the ice floes in an icebreaker, the researchers towed along a special underwater camera at the end of a long steel cable. From the deck, they saw the first ever images of Karasik come into focus. “At first you see nothing, because it’s all blurry and the lights only penetrate for 10 meters or so,” Boetius told me. “But when we were five meters away, it looked like it was covered with round blobs. And then we got closer, and we all screamed, ‘Sponges!’”
Sponges don’t usually elicit such excitement. The simplest and earliest of all animals, they mostly sit in place, filtering small morsels of food from the water. But while most sponges are a few centimeters wide, those on Karasik were giants—some a whole meter in diameter. And there were just so many of them. In high-Arctic waters, “maybe every 10 meters you find a worm, every 100 meters a sea cucumber, and every kilometer a sponge,” Boetius said. “We never thought that we’d find an area close to the North Pole where we couldn’t see the seafloor because of the sponges sitting on top of each other.” And on top of them was a bustling menagerie of worms, starfish, snails, crabs, shrimp, clams, and corals. Boetius and her team watched the creatures in awe. They had initially come to Karasik because they thought it would be geologically interesting, if biologically barren. “We were completely shocked to see that it was crawling with life,” she said.
They were also deeply confused. In the ice-covered waters around Karasik, “there’s nothing to eat,” Boetius said. The region can barely sustain the algae and plankton that form the base of a normal food web. A large community of animals couldn’t possibly exist. And yet the sponges weren’t just existing, but flourishing. The team collected tissue samples from the creatures using robots designed to explore icy extraterrestrial moons; by analyzing these, the scientists calculated that, on average, the sponges were 300 years old. They first settled on Karasik while the United States was still a British colony, and are older than both Baltimore and New Orleans. And “they look absolutely healthy,” Boetius said. “They were full of baby sponges.”
By analyzing samples from the sponges and their environment, the team tried to work out how the creatures survive in such a desolate place. “We had about 10 hypotheses,” Boetius told me. All of them failed. Sponges can filter food particles from the surrounding water, but there weren’t enough of such particles around. Upwelling currents could draw nutrients from the depths, but the currents around Karasik are sluggish. Then, she and her colleagues realized that they were sitting on a crucial clue—or rather, that the sponges were.
The creatures were all resting on a dense black mat of hard, hollow tubes. These tubes were made by siboglinid worms—soft-bodied creatures that live in hard cylinders of their own creation. Mouthless and gutless, these worms survive with the help of internal microbes, which can process the methane and sulfides that spew from undersea volcanoes. Karasik used to release such chemicals, but no longer does. The worms that used to live on its summit died off, and none survive today. But their tubes, now roughly 2,000 to 3,000 years old, endure. This ancient architecture—the ruins of a fallen worm civilization—are what the sponges now eat.
Teresa Morganti and others at the Max Planck Institute for Marine Microbiology, in Germany, showed that the blend of carbon and nitrogen isotopes in the sponges was a close match for that in the tubes—a strong sign that the former were eating the latter. “The dead material is definitely a majority of the food they use,” Boetius said. Like the tube worms that lived on Karasik before them, the sponges rely on microbes, which abound in their body. The worm tubes are made of chitin and tough proteins, but the sponge microbes have just the right enzymes to break down these normally indigestible substances. The microbes digest the tubes, liberating nutrients that the sponges can then absorb. For the sponges, the extinct worm community beneath them is both foundation and food.
“Years of studying sponges have thoroughly convinced me that they are anything but simple,” Stephanie Archer, an ecologist at the Louisiana Universities Marine Consortium, told me, and each new discovery fills her “with a sense of awe.” Some sponges are so big that a human could nestle inside them. Others create delicate baskets of woven silica that human glassblowers would struggle to mimic. Yet others are carnivorous and look like harps. Sponges filter so much water that, for example, the community at Karasik can sieve almost the entire 600 meters of ocean above it every year. And though supposedly immobile, some of them can crawl.
At Karasik, some of the sponges have obvious trails behind them. Those tracks were initially confusing—were the sponges trying to mate?—but their purpose became clearer after Boetius realized what the sponges were eating. The tubes won’t last forever, and as they run out, the sponges can move—perhaps only a few centimeters every year, but enough to keep finding untouched corners of their millennia-old buffet.
Sponges have a very slow metabolism, but their food supply is finite. It’s hard to calculate how much remains, Boetius said, because nothing like this has ever been found before. She suspects that the tubes will feed the sponges for centuries, but not millennia. “I think they’re a transient community that’s probably blossoming now,” she said. Perhaps once the sponges die off, their remains will nourish other communities thousands of years in the future. For now, they’re thriving in an otherwise inhospitable world, by devouring the abandoned cities of long-extinct creatures that once ate the ocean’s farts.
Such wonders are peppered throughout the polar oceans. Another team, using the same camera and research vessel that Boetius used, recently discovered a giant breeding colony of icefish in Antarctica, with 60 million nests. “When we find something no one has ever seen before, we can assume that, 10 kilometers ahead, there’s also something new,” Boetius told me. But climate change is outpacing the rate of exploration. Both poles face an uncertain future, and “we don’t know how many mysteries we might lose as we’re speaking,” she said. The tragedy is an ironic one: The climate is changing because humans have been burning fossil fuels, exploiting the dead remains of extinct ecosystems just like Karasik’s sponges are doing. And like the sponges, we cannot keep it up forever.