Giant group of octopus moms discovered in the deep sea

We know more about the surface of the moon than we do about the bottom of the ocean. The seafloor is an alien landscape, with crushing pressures, near-total darkness and fluids wafting from cracks in the Earth's crust.

It's also home to weird animals that scientists are only just getting to know. Now, two deep-sea expeditions have revealed a giant group of octopuses and their eggs in a place where they shouldn't be able to survive.

Where no octopus had gone before?

"When I first saw the photos, I thought, 'They shouldn't be there! Not that deep and not that many of them!'" says Janet Voight, a zoologist at the Field Museum in Chicago and co-author of a paper on the octopuses published this month in the journal Deep Sea Research Part I.

Nearly two miles deep in the Pacific Ocean and 100 miles off the coast of Costa Rica, scientists on two oceanographic cruises used subsea vehicles to explore the Dorado Outcrop, a rocky patch of seafloor formed of cooled and hardened lava from an underwater volcano.

Geochemists explored the outcrop in a submersible, hoping to collect samples of the warm fluids that emerge from cracks in the rocks. They didn't count on finding dozens of octopuses huddled around those openings.

The octopuses are an unknown species of the genus Muusoctopus -- pink, dinner-plate-sized creatures with enormous eyes. Up to 100 of them occupied every available rock in the area.

That in itself was strange -- Muusoctopus are normally loners. Stranger still was that nearly all the octopuses seemed to be mothers, each guarding a clutch of eggs. And this nursery was situated alongside the warm fluids issuing from the cracks in the outcrop.

Serendipitous discovery

"These surprising observations show us how a deep-sea animal reproduces," says Barbara Ransom, a program director in the National Science Foundation's Division of Ocean Sciences, which funded the research. "The findings were serendipitous. The researchers saw something unusual and stopped to find out what it was. Unexpected discoveries like this can dramatically change our understanding of how the oceans work."

Deep-sea octopuses usually live in cold waters. Exposure to higher temperatures jump-starts their metabolism, fueling a need for more oxygen than warm water can provide. So it doesn't make sense for deep-sea octopuses to brood eggs in warm water, scientists say: That's usually suicide.

Indeed, the octopuses the scientists observed showed evidence of severe stress. The researchers could only guess that the 186 eggs that were attached to the rocks faced the same challenges. None had any sign of a developing embryo.

Dorado Outcrop is not a great place to start an octopus family.

Dozens of octopuses

However, the sheer number of what the scientists think were doomed octopuses and their eggs suggests that there's a better habitat nearby.

The team suspects there must be more octopuses living inside crevices in rocks where the water is cool and rich in oxygen. These crevices could be such a good octopus environment that the booming population is forced to spill over into the dangerously warm region outside.

"Octopus females produce only one clutch of eggs in their lives," says Voight. "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."

Voight notes that the scientists observed octopus arms emerging from cracks in the rock, evidence of a larger population.

Biologists meet geologists meet octopuses

"This project involved scientists from different research backgrounds coming together to investigate a fascinating observation," says Anne Hartwell, the paper's lead author and an oceanographer affiliated with the University of Akron and the University of Alaska Fairbanks (UAF).

The focus of the expeditions to Dorado Outcrop was to study a cool hydrothermal system, says Geoff Wheat, a geochemist at UAF. "In doing so, we discovered this fascinating congregation of brooding octopuses.

"To maximize the scientific return, we shared the results with deep-sea biologists, which led to this publication. This is only the third hydrothermal system of its type that has been sampled."

Adds Voight, "To my knowledge, there had been no reports of octopuses at this or comparable depths between Southern California and Peru. Never would I have anticipated such a dense cluster of these animals in the deep sea. The numbers we see may simply be the surplus population. What else is down there we can't yet imagine?"