Warty octopus dives deep for change

 

We like to think there’s lots of variety when it comes to human skin – think colour, oiliness, hairiness, wrinkliness – but the truth is it’s essentially the same large external organ.

And it certainly seems dull once you become acquainted with the surprisingly cute Pacific warty octopus, Graneledone pacifica, a species that takes skin texture variations to a whole new level.

Some G. pacifica individuals have outrageous warts, while others appear nearly smooth-skinned. They’re so varied that scientists weren’t sure if these octopuses were even members of the same species. 

But a study just published in the Bulletin of Marine Science has gone better than skin deep: it turns out that the deeper in the ocean the octopuses live, the bumpier their skin and the smaller their bodies become.

DNA studies revealed that even though the octopuses looked different, they were indeed the same species.

“If I had only two of these animals that looked very different, I would say, ‘Well, they’re different species, for sure’,” says Janet Voight, associate curator of zoology at the Field Museum, Chicago, USA, and the study’s lead author. 

“But variation inside animal species can sometimes fool you. That’s why we need to look at multiple specimens of species to see, does that first reaction based on two specimens make sense?”

The research team examined 50 specimens of G. pacifica, most collected from Pacific waters off the US west coast. The animals inhabited ocean depths ranging from 1115 to more than 2740 metres. 

The researchers counted the number of warts in a line across each individual’s back and head and the number of suckers on their arms. The deep-sea specimens were smaller, with fewer arm suckers, and, most noticeably, bumpier skin than those from shallower depths. 

Comparing the octopuses’ DNA sequences revealed only minor differences, supporting the idea that they were all the same species, despite looking so different.

Scientists aren’t sure why the skin texture variations occur with depth, but they do have a hunch about the size difference.

“There’s less food as you get deeper in the ocean,” says Voight. “So these animals have to work harder to find food to eat. And that means at the end of their lives, they’ll be smaller than animals who have more food.

“If you’re a female who’s going to lay eggs at the end of your life, maybe your eggs will be smaller,” she adds. 

Smaller eggs mean smaller hatchlings. 

Support for this hypothesis comes from the number of suckers on the males’ arm that transfers sperm packets to females.

Voight’s earlier research found that male hatchlings have a full-formed arm with all its suckers in place. The researchers documented that the number of suckers on this arm was way smaller in males from greater depth, and Voight hypothesises it relates to egg size.

 “The octopus hatchlings in shallower water… are bigger,” she says. “Their eggs had more yolk. As the embryos grew, they developed farther inside the egg than the ones from deeper water, who were developing in smaller eggs. They had less energy to fuel their growth before they left the egg, so they made fewer suckers.” 

Voight says this study could help researchers in the future trying to identify life forms in the deep sea. Remotely operated vehicles collect video footage of the ocean floor, which can be used to estimate the number of species present – provided we know what they look like.

This is why it’s so important to examine specimens in person and use characteristics you can’t see on video to identify species boundaries, according to Voight.

 “There’s still just so much we don’t know about the deep sea,” she says. “We need to be able to understand the information that’s becoming available from [remote vehicle] footage. And we can only do it by knowing what the animals look like.”