Why ‘Hot Springs’ Draw the World’s Largest Gathering of Deep-Sea Octopuses

Some 20,000 octopuses congregate near an inactive underwater volcano off California’s coast, using heat from thermal springs to hatch their eggs faster

white octopuses, some upside-down with their tentacles out, on the seafloor with some anemones and other creatures. A mechanical arm is in the middle
A remotely operated vehicle measured environmental conditions around the octopus nest site, including temperature and oxygen levels. © 2020 MBARI

Shimmering water intrigued scientists at a spot off the coast of California in 2018, where they made a staggering discovery: Far below the surface, a legion of an estimated 20,000 deep-sea octopuses had gathered, the largest congregation of the cephalopods ever found.

The grapefruit-sized creatures, called Muusoctopus robustus, are usually solitary. But at this spot near the Davidson Seamount, an extinct volcano two miles below the Pacific Ocean’s surface, they had come together in a massive group—and about 80 percent of them were female, identifiable by their special, upside-down brooding posture.

The reason for this “octopus garden,” as scientists have deemed it, remained a mystery until now. As it turned out, that shimmering water served as a clue—it glistened because the ambient seawater was mixing with warmth from deep-sea hydrothermal springs.

In a new study published last week in Science Advances, researchers suggest the eight-tentacled animals flocked to the site because the heat from the springs makes it an ideal spot to brood eggs, confirming their initial hypothesis. In cold and deep waters, octopus eggs might ordinarily take several years to hatch—in 2014, for example, scientists announced they had observed a mother deep-sea octopus brood her eggs for 4.5 years. But with the hot springs at this nesting site, embryos develop in a mere 1.8 years, per the new paper. With a shorter brooding period, the eggs are exposed to potential predators like snails and shrimp for far less time, increasing the hatchlings’ chances of survival.

“That’s a big deal for these eggs, because in the deep sea, one of the really big challenges is that it’s cold,” Jim Barry, a benthic ecologist at the Monterey Bay Aquarium Research Institute and lead author of the study, tells the New York Times’ Katrina Miller.

In this dark “midnight zone,” of the ocean, the only natural light comes from the bioluminescence of deep-sea critters, and the temperature of the water is normally near freezing, hovering around 35 degrees Fahrenheit. But close to the hydrothermal springs, the water is a balmy 51 degrees Fahrenheit.

“Usually, colder water slows down metabolism and embryonic development and extends life span in the deep sea,” says Adi Khen, a marine biologist at the Scripps Institution of Oceanography who was not involved in the study, to Christina Larson of the Associated Press (AP). “But here in this spot, warmth appears to speed things up.”

Scientists solve mystery of why thousands of octopus migrate to deep-sea thermal springs

When scientists made the initial shocking discovery of the “octopus garden,” located within the Monterey Bay National Marine Sanctuary, they had been using a remotely controlled vehicle to explore previously unstudied hills. They counted roughly 6,000 of the octopuses in just a small, 6.2-acre area. But the entire 823-acre seamount could contain at least 20,000 pearl octopuses, so nicknamed by the researchers for their resemblance to pearls strewn across the seafloor.

“It was completely incredible,” Andrew DeVogelaere, a study co-author and marine biologist with the National Oceanic and Atmospheric Administration, tells the AP. “We suddenly saw thousands of pearly-colored octopus, all upside down, with their legs up in the air and moving around. They were pushing away potential predators and turning over their eggs,” to let water and oxygen flow evenly across them.

Since then, the scientists have returned more than a dozen times to better understand the aggregation, one of four deep-sea octopus nurseries that have been discovered across the world.

By setting up time-lapse cameras and visiting 31 specific nests, the team revealed more about the ins and outs of the octopus life cycle, reports Helen Scales for the Guardian. Mothers lay a clutch of about 60 eggs and affix them to the rock, tending them constantly. New males and females arrive throughout the year, and nesting females sometimes shoo away males who want to mate. Scientists did not see any evidence of octopuses feeding, or any mid-size individuals at the site, suggesting it is used only by adults for breeding. Hungry shrimp and anemones waited to pounce on the fresh hatchlings.

The threat of predators may act as a critical incentive for octopus mothers to speed up their eggs’ development by placing them near a thermal spring. Brooding females do not eat, so a shorter brooding time means a lower chance they run out of energy to defend their eggs.

After the hatchlings finally emerge and swim off into the darkness of the sea, the mothers, as many female octopuses, may self-destruct. Males typically die after mating.

But the octopuses’ lives and deaths support an ecosystem around the thermal springs, seemingly playing a crucial role in the survival of other organisms, such as crustaceans and snails, Janet Voight, an expert on deep-sea octopuses at the Field Museum of Natural History in Chicago, tells NPR’s Nell Greenfieldboyce. “These are all parts of an ecosystem that’s sustained by the octopus being there and dying in there,” she says to the publication.

The newfound importance of heat for octopus reproduction at the Davidson Seamount site may extend to other nurseries as well, as Mike Vecchione, a zoologist with the Smithsonian’s National Museum of Natural History who was not involved in the study, tells the New York Times. These so-called gardens “may be widespread and really important in the deep sea, and we just previously knew very little about them,” he tells the AP. “There’s still so much to discover in the deep sea.”

Editor’s Note, September 1, 2023: This story has been updated to more accurately reflect the brood time of a deep-sea octopus.

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