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This image from the brochure provided by the Brain Institute of the Federal University of Rio Grande do Norte, Brazil, shows an octopus in active sleep.
WASHINGTON – An octopus named Marshmallow lies at rest at the bottom of his tank, suddenly changing color from pale green to brown to orange, as his muscles contract, the suction cups contract, and his closed eyes move around.
This moment was captured in remarkable images filmed by scientists in Brazil, who published a new study in the journal iScience on Thursday that says sophisticated cephalopods experience at least two different types of sleep.
One of these states, which they called “active sleep,” is similar to rapid eye movement (REM) in mammals, birds, and some reptiles, raising the intriguing possibility that, like humans, octopuses experience dreams.
“Octopuses are unique in terms of their complexity, both behavioral and neuronal,” lead author Sidarta Ribeiro, from the Brain Institute of the Federal University of Rio Grande do Norte, Brazil, told AFP, noting that they have the most complexes of all invertebrates.
“However, they are very different from us.”
To explore the question of octopus sleep patterns, the researchers continuously recorded four octopuses in their tanks for several days and then re-analyzed the images.
They found that during “quiet sleep,” the animals were still, with pale skin and pupils constricted into a slit. During “active sleep,” however, their skin color and texture dynamically changed, noticeably twitched and contracted, and their eyes moved.
The pattern was cyclical, with the period of silence lasting around six to seven minutes and followed by an active cycle lasting about 40 seconds.
The cycle could repeat itself, or the octopus could wake up, but it usually went back to sleep 30 to 40 minutes later. In total, your dreams take a quarter of the day.
To establish whether these states actually represented sleep, the researchers designed visual and tactile stimulation tests, the paper’s first author, graduate student Sylvia Medeiros, told AFP.
The first test consisted of playing a video of a crab on a screen that was placed next to the octopuses.
“When they are awake, because the crab is a natural prey, they try to attack,” he said. But they did not attempt to jump in the two states where they were supposed to sleep.
In the second test, they hit the octopus tanks with a rubber hammer, and the animals physically reacted and changed color when awake, but not when asleep.
– ‘Video clips, or even gifs’ –
Learning more about what makes us similar to octopuses, from whom our species diverged 500 million years ago, can shed a new perspective on our different evolutionary paths, Ribeiro said.
“If we see a similar phenomenon, in this case, a sleep cycle that comprises a calm and then active sleep, it is most likely due to a convergent evolution,” he said, meaning that our two species independently developed the same biological mechanisms.
That, in turn, sheds light on the evolutionary pressures that shaped this behavior.
For mammals, REM sleep represents a time of memory consolidation and triggers a variety of molecular mechanisms that have a restorative effect on brain health and cognition.
The authors believe that this could also be the case for octopuses.
Most human dreams also occur during REM sleep, so could this also be the case for eight-limbed mollusks?
“We cannot say for sure,” Medeiros said.
But if they do, it is unlikely to be similar to the complex narratives we may experience, given how short the active phase is for octopuses, he added. “It should be more like little video clips or even gifs.”
The color patterns that octopuses form on their skin during sleep could offer a window into their minds, as they can mirror the patterns they exhibit while awake.
For example, the “half and half” where they are black on one side and white on the other is sometimes seen during courtship, so could an octopus displaying this pattern during sleep be dreaming of romance?
Maybe, but it’s too early to tell, Ribeiro said, and the subject of future research.
Next, the team hopes to find ways to record neural data from octopuses, a tricky proposition given that they move in water, and learn more about the role sleep plays in the animals’ metabolism and cognition.
