CEB Committe on Evolutionary Biology

Voight: Enigma: Huge group of octopuses and their eggs where they likely can’t survive

Enigma: Huge group of octopuses and their eggs where they likely can’t survive.

"When I first saw the photos, I was like 'No, they shouldn't be there! Not that deep and not that many of them," says Janet Voight, associate curator of zoology at the Field Museum and an author of a new study on the octopuses published in Deep Sea Research Part I.

Nearly two miles deep in the ocean, a hundred miles off the Pacific coast of Costa Rica, scientists set out on two cruises a year apart to use subsea vehicles to explore the Dorado Outcrop. This rocky seamount made of cooled, hardened lava from an underwater volcano leaks warm fluid from under the earth’s crust. Geochemists went, hoping to sample the warm fluids that emerge from cracks in the rocks; they didn't count on finding dozens of octopus huddled around the cracks near 3000 m depth.

The octopus were an unknown species of the genus Muusoctopus--pink, dinner-plate-sized creatures with enormous eyes. Up to a hundred of them seemed to occupy every available rock in a small area. That in itself was strange--Muuscoctopus are normally loners. Stranger still was that nearly all the octopus seemed to be mothers, each guarded a clutch of eggs. And this nursery was centered in the area of warm fluid flow from cracks in the basaltic outcrop.

It doesn't make sense for deep-sea octopuses to brood eggs in warm water: it's suicide. Deep-sea octopus live in cold, nearly invariant temperatures. Exposure to higher temperatures jump-starts their metabolism, making them need more oxygen than the warm water carries. The octopus that the scientists observed (both in-person and via hours of video footage from an ROV) showed evidence of severe stress; attached to the rocks leaking warm, low oxygen fluid, the eggs had it worse. None of them had any sign of a developing embryo. 

However, the sheer number of what the scientist think are doomed octopus and their eggs suggest that there's a better, healthier habitat nearby. The team suspects there must be more octopus living inside tubes in the basalt, where the water is cool and rich in oxygen. 

"Octopus females only produce one clutch of eggs in their lives. In order for this huge population to be sustained, there must be even more octopus to replace the dying mothers and eggs that we can see," says Voight. "My coauthors, Geoff Wheat and Anne Hartwell, know about basalt and how an outcrop like this is made. Odds are it has hollow areas where other females nurture their eggs to hatching." Voight notes that there's evidence for the unseen population: the scientists observed octopus arms emerging once in a while from cracks in the rock.

The study doesn't just shed light on deep-sea biology; it also illustrates the collaborative nature of science. "This project was a cohesive dynamic of three scientists from different research backgrounds coming together to investigate a fascinating observation," says Hartwell, the paper's lead author and oceanographer who is now a PhD student at University of New Hampshire Center for Coastal & Ocean Mapping.

"The focus of [our] expeditions to Dorado Outcrop was to study a cool hydrothermal system. In doing so, we discovered this fascinating congregation of brooding octopuses," says Wheat, a geochemist at the University of Alaska Fairbanks. "To maximize the scientific return of the expeditions, we shared the video with deep-sea biologists, whose research led to this publication. This is only the third hydrothermal system of its type that has been sampled, yet millions of similar environments exist in the deep sea. What other remarkable discoveries are waiting for us?"

That was in April 2018.  In October 2018, Nautilus Live live-streamed ROV footage of an estimated 1000 octopus brooding eggs at near 3000 m depth on Davidson Seamount (off the Californian coast):

What other surprises does the deep sea offer?