Atop an Underwater Hot Spring, an ‘Octopus Garden’ Thrives

In 2018, Amanda Kahn, an invertebrate biologist at San Jose State University, joined an ocean expedition to scout the base of Davidson Seamount, an inactive underwater volcano off the coast of central California. She came for the sponges and corals.

But she and her colleagues stumbled across something much more astounding. As their remotely operated vehicle, which was probing the seafloor and streaming video back to their ship, rose from behind a rock, the crew gasped. In shimmering waters, they saw scores of upside-down octopuses nestled in rocky crevices with their arms clutched around their frames. A closer look revealed that they were protecting eggs, similar to the way that birds brood in a nest.

“Sometimes you recognize immediately the magnitude of something special that you’ve found,” Dr. Kahn said. “And I think that was one of those really special moments.”

When James Barry, a marine ecologist at the Monterey Bay Aquarium Research Institute, got a glimpse of on a later expedition, he instantly wondered why so many octopuses were here. “And so we set about to figure out,” he said.

Dr. Barry assembled a team of ecologists, biologists, geologists and engineers who, for the next three years, studied what they dubbed the “Octopus Garden” — the world’s largest known aggregation of these eight-legged creatures. It turned out that the sheen of the water was a clue: The nursery sat atop a hydrothermal spring; the shimmering was caused by heat emanating from the seabed. The team’s findings, detailed in a new paper published Wednesday in Science Advances, suggest that this hot spot makes the octopuses’ eggs hatch faster, which improves reproductive success.

The researchers used remote cameras to study the behavior of the grapefruit-size animals, and temperature and oxygen probes to understand the surrounding environment. They witnessed some very intimate moments in the octopuses’ lives: brooding mothers swatting away males seeking to mate, scavengers trying to steal the eggs, and hatchlings “swimming through the gauntlet” of shrimp and anemones waiting to attack, Dr. Kahn said. (The crew cheered whenever a newborn octopus successfully swam off into the dark, she added.)

Using a mosaic of underwater photos spanning a region the size of a few football fields, the team counted 6,000 octopuses in the garden. “And that’s only part of the area,” Dr. Barry said. They estimated that the total population is around 20,000. More than 80 percent of the octopuses at the site were nesting females, identified by their unique, protective postures.

Nudging them aside with probes, the scientists measured how the water around their eggs differed from the ambient environment. They found that temperatures reached up to 52 degrees Fahrenheit at the sites where females chose to brood, compared to only 35 degrees in the surrounding waters.

“That’s a big deal for these eggs, because in the deep sea, one of the really big challenges is that it’s cold,” Dr. Barry said. Chilly temperatures slow down the metabolism of coldblooded animals, including rates of embryonic growth. For this species of octopus, it could have taken anywhere from five to 10 years for the eggs to fully develop in ambient waters — but in this nursery, the scientists found that they were hatching in less than two years on average.

The earlier the better, the team reasoned, when it comes to reproductive success. Less time spent as an embryo reduces the risks of being eaten by predators, or suffering infections or injuries that lead to death. Because octopuses don’t eat while brooding — and die after reproducing — they also suspect that quicker egg hatchings might make for a higher chance of survival, since the mother is less likely to lose the energy needed to sustain them.

It’s the mothers’ last hurrah, Dr. Kahn said: “They go all out in protecting those eggs.” She added that brooding near a hot spring helps ensure the mothers’ final acts are a success.

The findings make sense to Michael Vecchione, a deep-sea cephalopod biologist at the National Oceanic and Atmospheric Administration who was not involved in the study. Dr. Vecchione, who had seen the discovery of the garden back in 2018, had also speculated that the octopuses were using the heat to speed up embryo growth. “I’m not surprised that the warm temperature was beneficial to them,” he said. “And apparently, it’s starting to look like it’s a pretty widespread phenomenon, even though nobody had ever seen it until just a few years ago.”

Dr. Vecchione is referring to a similar aggregation of octopuses, found in 2013 by a different group of researchers, off the coast of Costa Rica. (At the time, however, scientists weren’t sure that the waters surrounding a hydrothermal spring could be hospitable for developing eggs. It was confirmed as an active nursery earlier this year.) And Dr. Barry’s team has already discovered another reproductive hot spot, which they named the “Octocone,” five miles northeast of the Octopus Garden.

Curious about how common thermal spring nurseries are, Dr. Barry plans to organize more expeditions along other areas of the coast. There’s still so much more to learn about these ecosystems, Dr. Kahn said, including what attracts the octopuses to the garden, whether, like sea turtles, they migrate back to reproduce at the same spot they are born, and how the mothers regulate their energy during brooding.

“Until now, we always thought that octopods were pretty solitary,” Dr. Vecchione said. “But the fact that this is showing up more and more indicates that, at least for these deep-sea octopods, this is an important life cycle that we didn’t know about.”

For Dr. Barry, studying these ecosystems is important for both resource conservation and understanding the planet we live on. “We depend upon the ocean in ways that most of us do not understand,” he said, noting that it plays a role in climate and biodiversity as well as carbon cycling and storage. Learning about what lies under the sea — and how to protect it — “is worth the investment,” he added. “Because it can make our lives better, too.”