Animals

The deeper this purple octopus lives, the bumpier its skin

Smooth and bumpy octopuses deep in the Pacific have long confused scientists: Are they the same species? Now we finally have an answer.Wednesday, 9 October 2019

By Liz Langley
Graneledone verrucosa, the Atlantic cousin of the Pacific warty octopus, is seen off the northeastern U.S. coast in 2013. All Graneledone species have warts, but some are smoother than others.

Deep in the Pacific Ocean, pale purple octopuses with giant cartoon eyes roam the seafloor. Some are covered with pronounced bumps, and others look like they have nearly smooth skin, a puzzle that has long confused scientists. Are these animals with different appearances actually the same species?

The answer is yes, according to a new study—which also revealed that the wartier the octopus, the deeper it lives, says study leader Janet Voight, associate curator of invertebrate zoology at the Field Museum in Chicago.

Not only that, but the team also found that the bumpiest Pacific warty octopuses (Graneledone pacifica), which can live up to 1.7 miles deep, are about the size of a desktop keyboard—much smaller than the smoother-skinned ones, which are about a yard long and dwell about 0.7 mile deep. 

That’s surprising, especially because it doesn’t fit with a biological phenomenon called deep-sea gigantism, in which living at greater depths yields larger animals. As the theory goes, colder water temperatures increase cell size and cell life span, which results in a bigger body size. Chilly waters also slow the metabolism of some species, like giant isopods, so that an environment with minimal food isn’t as problematic.

But for these octopuses, it's possible their smaller size at depth could be related to a scarcity of food, Voight says. Little is known about their diet, but based on dirt she’s found on specimens, Voight thinks they may just “put their little suckers right under the sediment and pick up little snails, worms, and clams and just pass it to their mouth.” (See more denizens of the deep.)

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It’s also possible octopus females with limited diets produce smaller eggs, which in turn leads to more petite adults.

It's also unknown why they're warty, Voight says—illustrating how much is left to discover in the oceans, which make up 71 percent of our planet yet remain little studied.

Location, location, location

For the research, Voight collected eight octopuses off the U.S. West Coast using remotely operated vehicles and ALVIN, a human-occupied submersible vehicle. Preserved specimens, including from the Field Museum, brought the total to 50 individuals. Voight and colleagues then studied the animals' physical structures, including their arm suckers, warts, and tubercles, or little bumps inside the warts that give them texture.

Once the animals' body structures were catalogued, sophisticated 3-D computer analysis concluded that depth is what determines whether the octopuses are more smooth or more warty. Further DNA testing confirmed that both types of octopuses are indeed the same species, according to the study, published today in the Bulletin of Marine Science.

Jennifer Mather, an octopus expert at Lethbridge University in Alberta, was impressed by the number of specimens studied. Octopuses are famously good at hiding, and finding them requires complicated equipment, particularly in the deep sea. (Read about the world’s largest deep-sea octopus nursery.)

A particularly warty example of the related Atlantic species, Graneledone verrucosa.

Dedication helps, too. “Dr. Voight’s patience was very much rewarded,” says Mather, who wasn’t involved in the research.

She adds that some shallow-dwelling octopuses can form temporary bumps if camouflaging their bodies to a bumpy surface, which some thought was the case with Pacific warty octopuses. But by studying live and dead animals, Voight confirmed their “skin sculpture” is defined at birth.

Warty ancestors?

The purpose of the warts are unknown, though all species of the genus Graneledone have warts. It’s possible that wartiness is a vestigial trait, "so all members of the group have it, regardless of whether it provides a current benefit," Voight says.

Why the invertebrates get smaller and wartier with depth is Voight’s next mission. For instance, she wants to look inside the warts for clues: The “what” may lead to the “why,” she says.

Overall, her study is part of an ongoing effort to decipher how life can thrive so far down in the dark, food-scarce ocean, she adds.

“How do they live in the deep sea? I think that's what all deep-sea biologists really want to know,” says Voight.

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