Scientists uncover the secret of the deep-sea ‘octopus garden’

About 80 miles from the coast of central California, thousands of octopuses gather on rocks two miles below the surface of the ocean. This location, dubbed “octopus garden,” is the largest known aggregation of these mollusks in the world, though it doesn’t attract much else of equal size. Scientists have been puzzled why so many octopuses plant themselves in these abysmal, colder waters – until now.

After three years of monitoring the area, researchers found the site is a popular mating and nesting ground for pearl octopuses, where hot springs help embryos develop at twice the rate expected at this depth. The faster development increases a hatchling’s likelihood of survival through the brooding period, according to a study released Wednesday in the journal Science Advances.

The findings, which investigate an unexplored corner of the ocean, help researchers better understand how coldblooded animals adapt in a variety of environments.

“We’re supposed to be managing these areas and protecting them for future generations, and we didn’t even know that this habitat was down there [and] what kind of impact it was having,” said Andrew DeVogelaere, research ecologist and study co-author. “You’re the first human eyes in all of history to see this part of the planet.”

The discovery of the octopus garden back in 2018 was lucky, said DeVogelaere, who runs the research program for NOAA’s Monterey Bay National Marine Sanctuary.

For more than two decades, researchers have been studying an extinct underwater volcano called the Davidson Seamount. The seamount, located around 80 miles southwest of Monterey, California, is one of the largest seamounts in the world and known for its beautiful deep sea corals. It is also one of the most well-studied seamounts in the world, so when DeVogelaere wanted to explore it further, others were skeptical he would find anything new.

“We said, ‘Don’t waste your time going there,’” said Jim Barry, senior scientist at the Monterey Bay Aquarium Research Institute and lead author of the new study. “We’ve been all over it, you’re not going to discover anything that we haven’t already discovered, or very likely not.”

Nevertheless, Barry and his colleagues said there is an unexplored area on the foothills of the seamount that could be interesting to investigate. In 2018, researchers sent a ship to study the area.

DeVogelaere, who was following along on a video feed, said it was at first pretty dull, just a lot of mud. Then suddenly, they spotted a hoard of balls, like opalescent pearls, on the seafloor.

They had found the “octopus garden,” home to at least 6,000 nesting, Muusoctopus robustus – commonly known as pearl octopuses. The researchers estimate there could be 20,000 in this nursery.

For three years, the team monitored the population through cameras and dives with remotely operated vehicles, measuring water temperature and oxygen. They found only adult males and female octopus, developing eggs, and hatchlings at the location, indicating the site was used exclusively for mating and nesting.

They also noticed shimmering water, which was indicative of a thermal spring – something “totally new in this area,” Barry said. He explained the shimmering water appears when warm water (such as from a thermal spring) and cool waters mix. Water temperature from these thermal springs can reach nearly 51 degrees Fahrenheit, while ambient water temperature is around 35 degrees. The warm water is only bathing a small area about a foot above the seabed, but scientists found many female octopuses brooding in the warm pool.

The team reasoned that the warmer water helps accelerate the development of the embryo, speeding up the metabolic processes. Typically at near-freezing temperatures in the deep ocean, researchers expected pearl octopus eggs at least five years to hatch. These hatched in just two years.

“As you get into colder waters, whether it’s polar waters or deeper waters, colder water slows down metabolism, slows down metabolic rates, slows down growth and embryonic development rates,” Barry said. “We think that the warm water is providing some sort of reproductive benefit for them.”

Geomicrobiologist Beth Orcutt, who was not involved in the study, said the “most exciting aspect of this new study” is the confirmation that the warmer water does actually shorten brooding times.

“This had been hypothesized but not documented until now,” said Orcutt, vice president for research at the Bigelow Laboratory for Ocean Sciences in Maine.

The octopuses are the biggest and most abundant animals in the garden, but their presence does attract other predators. The shorter brooding time probably reduces the embryo’s risk to predators skulking around to eat the eggs, such as shrimp. Scavengers also roam the area to feed on the mother octopus, who dies after the baby hatches.

“The deeper you go, the less food there is … it has already been chewed up by the animals that live in the water column, including microbes, fish, whatever,” said Barry. The octopus carcasses are important because they “become part of the food web” at lower depths.

The team thinks the octopus garden is just one of many deep sea octopus environments out there. Just five miles away from the garden, researchers found another octopus nursery near thermal springs. In June, another group of researchers discovered a nursery near thermal springs in Costa Rica.

Orcutt, who helped lead the Costa Rica expedition, said both of these discoveries show that these small outcrops of rock on the seafloor, which are often passed over for study to focus on larger seamounts, can be incredibly rich and productive sites of life.

“I don’t think it’s a one-off curiosity,” DeVogelaere said. “Most of the deep sea hasn’t been looked at.”