On the ocean floor near the Galapagos Islands, a submersible controlled by scientists came across a mysterious octopus as blue as the ocean and no bigger than a golf ball.

"He's tiny! It's blue!" one excited scientist was recorded as saying when she first caught sight of the cerulean cephalopod on footage transmitted from the sub.

The team from the Charles Darwin Foundation had just discovered a new species of octopus nearly 1,800 meters (5,900 feet) below the water's surface, according to research published Monday.

"Right away, I knew it was something really special," said octopus expert Janet Voight, who was asked to identify the strange species.

At first, the curator at the Field Museum of Natural History in Chicago had to make do with photos of the animal.

Then she received its preserved body in the mail.

"When it arrived, I was like 'Oh! My goodness! It's beautiful,'" Voight told Agence France-Presse (AFP).

She was immediately interested because the closest known octopus of that shape lives off the coast of Uruguay – in a different ocean on the other side of South America.

Normally, to describe a new octopus species, a specimen needs to be cut open so that its mouth, beak, teeth and other parts can be examined.

"We only had the one specimen, so I didn't want to take it apart," Voight said.

Instead, the team at the Field Museum used CT scans to take thousands of X-ray images, then compiled them for a 3D model of the octopus, revealing its insides.

"There's nothing like spending the day looking at something no other human has ever seen," the Field Museum's X-ray lab head Stephanie Smith said in a statement.

'Deep purple'

The new species, named Microeledone galapagensis, stands out for reasons other than its blue hue, which is believed to be the rarest color in nature.

The octopus appears to be the runt of the Megaleledonidae family, whose members are normally much larger and live in the Southern Ocean surrounding Antarctica.

"Its stubby little arms with only one row of suckers set it apart from most octopus we are familiar with," Voight said.

Even among "other species with short little arms and a single sucker row, its colouration and smooth skin on the back surface separate it," she added.

While the octopus is light blue on its back, underneath it is a "very deep purple," Voight said.

"We think this color pattern helps keep it safe. If the octopus grabs a prey item that emits light, that light may attract predators that might then eat the octopus," she explained.

"So the octopus puts its dark-colored web over the prey item, keeping itself safe."

Surprisingly, it is not uncommon to find new species of octopus in the deep sea, particularly in areas that have not been well explored, which is a massive portion of the ocean floor.

"If you took all the land on Earth and pieced it together, you would not cover the Pacific Ocean," Voight pointed out.

She added that she had last seen a new octopus in 2023, off the coast of Costa Rica.