Deep discovery reveals why some octopuses are wartier than others

9 Oct 2019

A warty-skinned octopus living in the deep sea. Image: ROV Jason

In the weird world of marine life, researchers have found octopuses that live deeper in the oceans have wartier skin.

Researchers have found that by taking a good look at the skin of an octopus, it can tell a lot about where it lives. A team of scientists that was led by The Field Museum of Natural History, Chicago, and also included Prof Louise Allcock from NUI Galway, discovered that octopuses from deeper in the ocean had wartier skin when compared with those from shallower depths.

Writing in the Bulletin of Marine Science, the team described how DNA sequencing showed that despite their different appearance, the octopuses were the same species. The study focused on a species referred to as the Pacific warty octopus, classified as Graneledone pacifica.

Despite being the same, researchers were puzzled as to why some were found to be covered in large warts, while others were very smooth-skinned. As part of the research, the team looked at 50 specimens of the octopus collected by a small submarine in the north-east Pacific Ocean, as well as some loaned from the University of Miami Marine Laboratory and the California Academy of Sciences.

These specimens were obtained from depths ranging from 1,100 metres to more than 2,700 metres below the ocean’s surface.

The number of warts in a line across each octopuses’ back and head was then counted, along with the number of suckers on their arms. This showed that octopuses from deeper within the oceans were smaller, had fewer suckers and, most notably, much wartier skin that those from shallower depths.

The DNA of all of the octopuses – both shallow and deep – showed minimal differences, meaning that they were all of the same species.

Very different, but the same

“We really weren’t sure what the DNA would tell us. Warty octopuses occur throughout the deep oceans of most of the world, including all the way down to the Antarctic, and there are real issues in determining the true number of species,” said Allcock, who co-authored the study.

“From many locations we only have one or two specimens, because they live in really inaccessible habitats, so we had very little experience as to how much individuals of any given species might vary.”

Lead author of the research, Janet Voight, added it’s important not to “jump the gun” and assume two animals that look different are automatically different species, highlighting dogs as an obvious example.

The team still isn’t sure why depth influences their appearance, but it does think the size difference can be attributed to the fact food is harder to come by deeper down.

“The octopus hatchlings in shallower water, only 1,100 metres, are bigger,” Voight added. “Their eggs had more yolk. As the embryos grew, they developed farther inside the egg than the ones from 2,700 metres, who were developing in smaller eggs. They had less energy to fuel their growth before they left the egg, so they made fewer suckers.”

Colm Gorey was a senior journalist with Silicon Republic