Nothing about the hundreds of octopus moms on the video feed from the submersible Alvin looked right.
"Those octopus shouldn't be there," Janet Voight, a marine biologist at the Field Museum in Chicago, said when she saw the footage.
The research vessel was exploring a rocky outcrop about 1.2 miles deep and 150 miles off Costa Rica's Pacific coast—far deeper than any other known octopus nursery, and (in theory) too warm.
"It was jaw-dropping," says Anne Hartwell, a research assistant at the University of Akron in Ohio, who led a new study on the 2014 footage of the octomoms guarding their precious eggs. (See our most beautiful octopus pictures.)
With Voight's help, Hartwell identified the plentiful cephalopods as part of the genus Muusoctopus—and possibly even a species new to science. These octopuses are solitary and occasionally cannibalistic.
But first, she had to figure out why they were there—and why had so many of them gathered in such a small area.
Rocky Real Estate
It's likely because the Dorado Outcrop is valuable egg-laying real estate, Hartwell discovered.
Most of the ocean bottom is covered with soft muck called marine snow, a collection of organic debris that drifts down to the bottom. Octopi can't brood in those conditions.
Although much of the ocean water around the outcrop is colder than normal, hot hydrothermal vents crisscross the region—and it's these warmer spots that attracted the octopi. (Read about another deep-sea creature that lays its eggs on hydrothermal vents.)
"There were so many of them there that at first I thought they were all really happy," Hartwell says.
As Hartwell and Voight scrutinized the Alvin footage, however, they noticed something strange. None of the eggs were developing, and all the mothers showed signs of severe stress.
According to octopus expert Jennifer Mather, who wasn't involved in the study, the water in the crevices is likely just too hot for the deep-sea invertebrates.
But with so few places to lay their eggs, the mothers have little choice but to endure the sweltering conditions, Hartwell and colleagues conclude in their study, published recently in the journal Deep-Sea Research Part I.
"This study reminds us that the deep sea is not a uniform environment. There are lots of different microhabitats for life to exploit," says Mather, of Alberta's University of Lethbridge.
It's also possibly not that surprising, considering making sacrifices is nothing new to octopuses. (Read about fierce animal moms that go to extremes for their babies.)
In 2011, researchers began keeping tabs on a Graneledone boreopacifica female off central California that guarded her eggs for an astonishing 4.5 years. That's the longest known developmental period for any animal.