Octopuses Get Wartier The Deeper They Live In The Ocean, But We're Not Sure Why

A deep-sea octopus with very warty skin. Field Museum, John Weinstein

Some octopuses found in some of the deepest parts of the Pacific Ocean have warty, textured skin that only a mother – or cephalopod expert – could love.

Janet Voight, the lead researcher on a new study published in the Bulletin of Marine Science, teamed up with a diverse team of researchers to understand why some Pacific warty octopuses (Graneledone pacifica) have warts while others have silky smooth skin. To do so, they examined 50 species both loaned by research institutions and collected by submersible vehicles in the Northeast Pacific Ocean, along the west coast of North America from Washington to central California.

For each individual octopus, researchers counted the number of warts in a line across the back and head, as well as the number of suckers found on their arms. Experts similarly conducted DNA analysis and found that octopuses of varying wartiness were not multiple species as was previously thought but belonged to the same species. What’s more, the octopuses that lived in the deeper ocean were smaller with bumpier skin and fewer arm suckers than their shallower counterparts.

But that’s about all researchers know.

“At this point, we don’t even know what the warts are made of, but we tend to think that they are cartilaginous,” Voight told IFLScience in an interview. “I am at a loss to come up with a benefit for these octopuses’ warts.  They live too deep in the ocean for light to impact them, so it doesn’t seem that it could be for camouflage and it’s hard to envision what else they could do for the animals.”

An octopus with comparatively smooth skin, living 5,000 feet below the ocean's surface. Ocean Networks Canada/ CSSF-ROPOS

Are their unique characteristics a result of their deep-sea environment? Do octopuses with warts have a competitive advantage to those without? The verdict is still out.

“I think that recognizing that the characters that are most obvious to us aren’t necessarily the most important thing for the animals in life is a good take-home message,” explained Voight. “Octopuses of this genus occur pretty much globally between 8,000- and 3,000-meter depths; the more we learn about within-species variation will help us with the others.” 

Researchers have one working theory. As depths increase potential prey possibilities decline, making the hunt for food more difficult and energy-consuming. It could be that over generations, octopuses who ate less due to prey availability evolved to be smaller, produced smaller eggs that, in turn, hatched smaller offspring. Understanding how they adapt to such extreme pressure could help them serve as a “canary in the coal mine” as CO2 is projected to increase in the world’s oceans.

“If these deep individuals are near living in what to them is an extreme habitat, their presence might give us a clue to the health of the deep ocean,” said Voight. “The deep sea is the largest habitat on the planet, covering about 68% of its surface, but it is by far the least known. The deep sea supports a surprisingly diverse array of animals that delights us all. We need to learn more about the animals that live in the biggest habitat on the planet.” 

A warty-skinned octopus living about 9,000 feet below the surface of the ocean. (c) ROV Jason

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