Octopuses may be the subject of many mariners’ nightmares but new research has added to growing evidence the cephalopods may themselves dream while asleep.
Octopuses are thought to undergo two different stages of sleep: “quiet sleep” and “active sleep”, the latter of which involves twitching body parts and rapid changes in the texture and patterning of the skin.
Researchers at the Okinawa Institute of Science and Technology (OIST) in Japan have said they have not only confirmed octopuses are asleep during this active stage, but that the stage shows close similarities with REM sleep seen in vertebrates including humans – a phase of slumber associated with dreaming.
Prof Sam Reiter, the senior author of the research, said the team’s results were consistent with the idea, but did not prove, that octopuses dream.
“We can associate certain skin patterns during wakefulness to certain situations: hunting, social displays, threat displays, camouflage to different sorts of environments. We show that these patterns reappear during active sleep,” he said.
“So, if we are looking at something like dream, and I repeat this is a possibility we do not prove in this study, they would resemble a pseudo random walk over different types of waking experiences.”
However, Reiter noted there were other possible explanations, for example the octopuses might be refining their camouflage patterns while asleep.
Writing in the journal Nature, the team reported how they studied 29 octopuses of the nocturnal species Octopus laqueus, revealing how during daylight the creatures closed their eyes and adopted a resting posture associated with sleep.
Every 60 minutes or so, the animals underwent rapid changes of skin colour lasting about one minute, together with changes in breathing rate, as well as body and eye movements.
When the team tapped on the tanks and watched how the animals responded, they found the octopuses showed different reactions depending on whether they were awake, in the quiet stage of sleep, or in the active stage.
What’s more, if the octopuses were deprived of sleep for two days by being tickled with a paintbrush, they subsequently showed an increased rate of active sleep and entered the stage sooner, suggesting a self-regulating, or homeostatic, control – one of the key criteria of sleep.
The researchers said the findings confirmed that octopuses were indeed asleep during the active stage.
They then inserted probes to make recordings of the octopuses’ neural activity, finding a strong correlation between a brain region’s activity while awake and during active sleep. During active sleep, octopuses also underwent rapid transitions through skin patterns that matched those shown when awake.
The findings supported previous suggestions, revealing that active sleep in octopuses resembled REM sleep in vertebrates.
It was not the first time sleeping octopuses had caused a stir: last month, a preprint by researchers in the US found a male Octopus insularis occasionally detached itself from its sleep position, apparently mid-slumber, and began showing behaviours associated with responding to an attack by a predator.
While only looking at one octopus, and cautioning no conclusions could be drawn, the US team suggested the behaviour might be down to the cephalopod “responding to a negative episodic memory”. In other words, it might have been having a nightmare.
