A white octopus sat on the seabed, gently waggling its short, stubby arms and peering with beady eyes into the camera of a deep-diving robot.
It was 2016, in waters off Hawaii, at a depth of 4,290 metres (2.6 miles). No one had ever seen an octopus like it, and certainly not so deep. Based on its ghostly appearance, it was nicknamed Casper.
Until then, the only cephalopods filmed at such depths were Dumbo octopuses, named after another cartoon character, seen swimming around as deep as 6,957 metres, with elegant, ear-like flaps on either side of their heads.
The sighting of Casper was a striking moment for Janet Voight, associate curator of invertebrate zoology at the Field Museum of Natural History in Chicago. “This is totally new and different,” she says, recalling the discovery.
That first glimpse of Casper threw up many tantalising mysteries. Why is it so pale? Most other octopuses have colourful chromatophores in their skin which change their appearance in an instant and act as camouflage to confuse predators.
Janet Voight, octopus expert Is this a breakthrough in octopus evolution?
Even in the deep sea, octopuses can be colourful, like the purple, warty Graneledone. Some use a cloak of dark skin pigments, seemingly to hide glowing, bioluminescent prey they grab in their arms and thus avoid alerting other predators. Voight guesses that Casper’s pallor may come down to a lack of pigments in its food.
Another puzzle is the short arms, although Casper is not alone in having a limited reach. “The shallower and more tropical you are, the longer and thinner your arms,” says Voight.
This trend towards shorter arms in deep-dwelling octopuses doesn’t have a definite explanation. Voight thinks that, rather than stretching out to grab food, they evolved an alternative tactic of twisting their bodies around so that their mouths, on the underside of their bodies, are directly over their food.
Watch: Very venomous blue-ringed octopus found living inside discarded beer can
Scientists have learned more about Casper by scouring five years of archived footage gathered on deep-sea surveys across the Pacific. They spotted dozens more like Casper perched on the seabed, from two distinct species.
“It could be that they’re fairly common,” says Voight. “It’s just an indicator of how little we know about what’s down there.”
For Voight, especially exciting were the Caspers with their arms wrapped around clutches of eggs stuck to tall sponges. Previously, she had theorised that seabed-dwelling octopuses need hard rocks to lay their eggs on. Further down, there could be fewer exposed rocks, limiting how deep they can go.
“Casper showed there are ways around that by finding a sponge stalk,” she says. “Is this a breakthrough in octopus evolution?”
The sponges themselves are attached to rocky nodules that lie scattered across swaths of abyssal plains and take millions of years to form.
If other deep-sea octopuses are anything to go by, the female Caspers probably spend a long time guarding their eggs. An octopus from another species (Graneledone boreopacifica) was seen off the coast of California, on a steep escarpment in Monterey Canyon, brooding her single clutch in the exact same spot for more than four years.
For now, the pale and mysterious Casper octopuses have not yet been officially named, because all we know of them comes from imagery; no one has been able to collect a specimen to study in detail.
“With an octopus, you really need it in your hand,” says Voight.