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Here's why 6,000 octopuses like to be under the sea at an 'octopus garden'

STEVE INSKEEP, HOST:

Octopuses tend to live alone. Kind of sad. Who do you hug with all of those arms? But the largest congregation of deep-sea octopuses who live together, the largest one ever discovered, is thriving off California. NPR's Nell Greenfieldboyce reports on the secrets of a real-life octopus garden.

NELL GREENFIELDBOYCE, BYLINE: Beneath the Pacific Ocean is a mountain, an extinct volcano called the Davidson Seamount.

JIM BARRY: And we've been all over the top of it.

GREENFIELDBOYCE: Jim Barry is with the Monterey Bay Aquarium Research Institute. He says not too long ago, a submersible vehicle explored some hills at the bottom of this mountain.

BARRY: And so as they're going through the dark, basically, you see what's in the headlights.

GREENFIELDBOYCE: The headlights revealed a few purple octopuses huddling over nests of eggs and then, suddenly, a lot more octopuses, a huge gathering.

BARRY: Which turns out to be many thousands of octopuses. And it's just a startling discovery.

GREENFIELDBOYCE: The explorers noticed something else, too, a kind of shimmering in the water.

BARRY: And they realized this is a thermal spring. This is warm water coming out.

GREENFIELDBOYCE: Barry and some colleagues recently spent months trying to understand if the warm water is what drew the octopuses to this place. They mapped an area the size of a couple football fields.

BARRY: We found 6,000 animals in just a small area. So we're estimating - I don't know - up to 20,000 animals may be there.

GREENFIELDBOYCE: They monitored the development of eggs and octopus nests and used probes to check the water temperatures.

BARRY: These animals that are nesting - the females only nest in warm sites, as far as we can tell. We could find none in cold waters.

GREENFIELDBOYCE: The warmth likely speeds up the maturation of eggs. In the journal Science Advances, the research team says octopuses there spend nearly two years guarding nests of eggs before they finally hatch. That sounds like a long time to brood, but Beth Orcutt says it's actually fast.

BETH ORCUTT: Because the other deep-sea octopus that we know have brooding times of at least four years.

GREENFIELDBOYCE: Orcutt is with the Bigelow Laboratory for Ocean Sciences. She studied smaller octopus congregations around warm water seeps off Costa Rica. She says it's great to have such detailed observations of the California site.

ORCUTT: It shows that there's an advantage to these octopus of brooding their eggs in the warmer water.

GREENFIELDBOYCE: And it shows the potential importance of small, cozy spots on the ocean floor, isolated patches of warmth that haven't gotten a lot of attention, even though they're probably vital for creatures trying to eke out a living in the frigid darkness.

Nell Greenfieldboyce, NPR News. Transcript provided by NPR, Copyright NPR.

NPR transcripts are created on a rush deadline by an NPR contractor. This text may not be in its final form and may be updated or revised in the future. Accuracy and availability may vary. The authoritative record of NPR’s programming is the audio record.

Nell Greenfieldboyce is a NPR science correspondent.