The ocean depths are incredibly inhospitable. Pressure crushes and light becomes scarce. The lifeforms that exist in such a harsh environment are mysterious and certainly tough. There are limits, however, to how resilient organisms can be when facing these extreme environments.

Just ask the largest deep-sea octopus nursery discovered off the coast of California. They're likely Muusoctopus robustus, given their pink-purple appearance and "dinner-plate-sized" bodies and "enormous eyes." Divers found a large group — potentially more than 1,000 individuals and most of them brooding mothers— lying upside-down on rocks to protect their eggs they hid in the crannies. There are several reasons why this is odd.

Muusoctopus octopuses tend to be solitary, so finding a group — let alone a group this large — is unusual. That the octopuses had apparently set up a nursery of sorts was also different. They also have never been spotted on the western coast of the U.S. until now.

"We went down the eastern flank of this small hill, and that’s when—boom—we just started seeing pockets of dozens here, dozens there, dozens everywhere," Chad King, chief scientist on the Exploration Vessel Nautilus, told National Geographic.

King said the water was also shimmering around the octopus — suggesting that they were surrounded by warmer waters and seeking that environment to lay their eggs. However, previous research shows that these deep-sea creatures typically survive better in colder waters. More research is needed to determine exactly why this environment was selected over a warmer environment.

This isn't the first time a large number of Muusoctopus robustus have been spotted in an unusual area.

Popping up in peculiar spots

Scientists discovered another octopus nursery in April 2018 on the Dorado Outcrop, about 100 miles off the western coast of Costa Rica, by accident. They were exploring the outcrop, a bit of seafloor created by cooled flows of underwater lava, using a submersible vehicle to gather samples of the warm fluids that come from cracks in the rock. Instead, they found hundreds of octopuses sharing sorts on various rocks on the rocky patch, and nearly all of them had eggs.

"When I first saw the photos, I thought, 'They shouldn't be there! Not that deep and not that many of them!'" said Janet Voight, a zoologist at The Field Museum in Chicago and co-author of a paper on the octopuses.

The paper was published in the journal Deep Sea Research Part I.

Perhaps most troubling, the octopuses were found nearly 2 miles (3 kilometers) underwater in a spot where oxygen was low and water temperatures were high. According to The Field Museum, deep-sea octopuses prefer colder water since warmer water speeds up their metabolisms, requiring them to need more oxygen. Warmer water, however, has less oxygen. So, it doesn't make sense for these octopus to lay their eggs in this kind of an environment; it'd be suicide, scientists said. They reported that some of the octopuses showed signs of severe stress.

A possible octopus exodus

One of the Muusoctopus octopuses studied by the researchers. One of the Muusoctopus octopuses studied by the researchers. (Photo: Phil Torres, Geoff Wheat)

There were so many octopuses — researchers acknowledged that they may have double-counted some individuals — that it's possible the octopuses had fled from another location, or that their current site was better for hatching eggs when they first arrived than it is now.

If the octopuses left another location, it's possible their move was the result of overcrowding in more hospitable waters. If their site changed, it would indicate that the flow of water was cooler and more oxygen-rich when they got there. Either way, the mothers wouldn't abandon their eggs once they were laid.

However, the octopuses on the eggs might also just be the moms who got to the outcrop last. Scientists observed some octopuses living inside the outcrop, and even founds eggs inside, too. The water inside this rock is probably cooler than the water outside, with more oxygen, too.

"Octopus females produce only one clutch of eggs in their lives," Voight explained. "For this huge population to be sustained, there must be even more octopuses to replace the dying mothers and the eggs we can see.

"Odds are that this outcrop has hollow areas where other females nurture their eggs to hatching. They're analogous to boomers who have all the good jobs, while the millennials wait, seeking one little piece of cool rock."

The mystery of this particular nursery will likely go unsolved, but even an unsolved mystery can be a clue to unlocking other mysteries in the ocean's depths.

"Never would I have anticipated such a dense cluster of these animals in the deep sea," said Voight. "The numbers we see may simply be the surplus population. What else is down there we can't yet imagine?"

Editor's note: This article has been updated since it was originally published in April 2018.

Scientists stunned to find huge octopus nursery
A new species of octopus has been found laying eggs on in too-warm water with too-little oxygen. Many of them are probably doomed.