Scientists observed unusual behavior in an octopus that they said looked similar to it waking up from a nightmare.
The cephalopod, named Costello, was filmed 24 hours a day in a laboratory at The Rockefeller University in New York over the course of a month.
On four occasions, the animal awoke "abruptly" before engaging "in antipredator and predatory behaviors", changing color, and flailing his arms around erratically, researchers said in a paper published last week on the website bioRxiv.
In two of the instances the octopus shot black ink into the water – a common tactic used to escape from predators – despite there being no predator present.
The behavior suggested that it was in temporary distress, which scientists said could suggest he was responding to a bad dream.
"It was really bizarre, because it looked like he was in pain; it looked like he might have been suffering, for a moment," Eric Angel Ramos, one of the researchers, told Live Science.
"And then he just got up like nothing had happened, and he resumed his day as normal."
When Costello came to the laboratory from the wild, he appeared to have suffered severe injuries, including losing the majority of two of his arms, which researchers said was likely due to a previous attack.
They noted a study that found such cases in animals "can result in long-term behavioral and neural hypersensitivity", suggesting Costello may have been responding to memories of the attack.
The study has not yet been peer-reviewed and only observed one octopus, but the findings have raised questions about the possible dream-like experiences of these intelligent creatures.
One of the study's co-authors noted that it would be difficult to study an octopus' brain activity and determine whether they actually dream.
"Where do you put electrodes on an animal that has no shape?" study co-author Marcelo Magnasco told New Scientist.
Scientists published a study in 2021 about octopuses' sleep and found evidence of a human-like sleep cycle. Researchers found that the animals' skin color fluctuated in a similar fashion to the rapid eye movement (REM) phase of sleep in humans – which is when dreams occur.
Another scientist who was not involved in observing Costello said the strange behavior could have another explanation, however.
Robyn Crook, an associate professor of biology at San Francisco State University, told Live Science that the octopus' behavior could have been due to senescence, which is when an octopus' body starts to break down before death.
Costello died shortly after these episodes, according to Live Science.
Crook said that Costello's movements in the video appeared to be due to a lack of motor control, possibly pointing to senescence.
"I don't exclude that senescence could be one of the drivers of this," Ramos told Live Science.
The study's authors noted that the results couldn't be considered conclusive until replicated. As the episodes in question were fleeting, the scientists recommended that future researchers also observe octopuses for 24 hours a day using cameras.
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