Washington (AFP)

An octopus named Marshmallow is resting in its aquarium.

Suddenly she goes from a pale green-white to brown, then to orange, her muscles twitch, her suction cups twitch, and her eyes twitch.

This moment was captured by scientists in Brazil, who published a new study in the journal IScience on Thursday showing that these animals experience two types of sleep.

One, which the researchers called "active sleep", is similar to the REM sleep seen in mammals, birds and some reptiles - suggesting that like humans, octopuses can dream.

"Octopuses are unique in terms of both behavioral and neurological complexity," Sidarta Ribeiro, of the Federal University of Rio Grande do Norte in Brazil, and co-author of the study told AFP.

Their brains are more complex than those of all invertebrates, he notes, "but they are still very different from us."

To study their sleep, the researchers recorded four octopuses in aquariums over several days.

During the phases of "calm sleep", the animals were motionless, pale in color, and their eyes slit closed.

In times of "active sleep", on the contrary, they kept changing color, contracting, and their eyes were mobile.

The quiet phase lasted six or seven minutes, before being followed by an active cycle of about 40 seconds, the two succeeding each cycle.

In total, octopuses slept about a quarter of the day.

The researchers carried out several tests to verify that the octopuses slept well during these phases, Sylvia Medeiros, lead author of the study, told AFP.

The first test was to broadcast a video of crab near the octopus.

“When they are awake, since the crab is prey, they try to attack,” she says.

Which did not happen during the two supposed sleep phases.

Likewise, octopuses then did not react to blows given by rubber hammers on their aquarium, unlike when they were awake.

- Dreams in gifs?

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Most dreams in humans occur during the REM phase of sleep.

Can we assume the same in octopuses?

“We can't say for sure,” explains Sylvia Medeiros.

But if so, those dreams are certainly not as complex as ours, given the short duration of the "active sleep" phases observed in them.

“It's probably more like little videos, or even gifs,” she suggests.

In addition, the colors observed during their sleep could give indications of their moods, as when they are awake.

For example, when courting, they are sometimes half white and half black.

If a sleeping octopus is found with these colors, does that mean that it is dreaming of love?

Maybe, but it's too early to tell, and that will be the subject of further studies, according to Sidarta Ribeiro.

In the meantime, learning more about what is common between us and octopuses, from which our species diverged 500 million years ago, can help understand the evolutionary paths taken, according to the researcher.

"If we observe a similar phenomenon, as here a sleep cycle comprising an active then a passive phase, it is surely due to a convergent evolution", he explained, that is to say that the two species have developed the same biological mechanisms.

This in turn informs the selection processes at work to develop this behavior.

In mammals, REM sleep is a time when memory consolidates, and causes a variety of mechanisms that have a restorative effect on brain health.

According to the study authors, this could be the case for octopuses as well.

© 2021 AFP