The deeper octopuses dive, the more warts they grow

Octopuses from the same species can appear remarkably different. One octopus can sport a smooth complexion, while another can be covered in warts. The differences can appear so stark that scientists often second-guess the specimen's classification.

Until now, scientists were confused by differences in skin texture, but the latest research suggests the variation follows a pattern. Octopuses living deeper beneath the ocean surface have smaller bodies and grow more warts.

Using DNA tests, scientists confirmed that octopuses with different appearances belong to the same species.

The findings, published this week in the journal Bulletin of Marine Science, serve as a reminder of the importance of large sample sizes.

"If I had only two of these animals that looked very different, I would say, 'Well, they're different species, for sure.' But variation inside animal species can sometimes fool you," lead study author Janet Voight, associate curator of zoology at the Field Museum in Chicago, said in a news release. "That's why we need to look at multiple specimens of species to see, does that first reaction based on two specimens make sense?"

For their study, scientists used a remote-controlled submersible named ALVIN to examine 50 specimens thought to belong the the species Graneledone pacifica, the Pacific warty octopus. Scientists also collected several specimens from the Northeast Pacific for study, in addition to examining several more specimens loaned from the University of Miami Marine Laboratory and the California Academy of Sciences.

Researchers surveyed specimens from up and down the northern half of the West Coast and from depths ranging from 3,660 feet to more than 9,000 feet below the ocean surface. Scientists counted the number of warts on each octopus' arms, back and head.

Specimens from deeper in the ocean had bumpier skin, but were smaller and had fewer suckers on the undersides of their arms. DNA analysis revealed only small genetic differences. All of the surveyed specimens belonged to the same species.

Researchers aren't sure why octopuses living deeper in the ocean have more warts, but their smaller size is likely to result of having to work harder for food. Octopuses capture small prey from the seafloor, and seafloor prey is sparser at greater depths.

"The octopus hatchlings in shallower water, only 3,660 feet, are bigger," Voight said. "Their eggs had more yolk. As the embryos grew, they developed farther inside the egg than the ones from 9,000 feet, who were developing in smaller eggs. They had less energy to fuel their growth before they left the egg, so they made fewer suckers."

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