Octopuses have alternating periods of "quiet" and "active" sleep that make their rest similar to that of mammals, despite being separated by more than 500 million years of evolution.
During their active periods of sleep, octopuses' skin color changes and their bodies twitch, according to a report in the journal iScience, and they might even have short dreams.
"If they are dreaming, they are dreaming for up to a minute," says Sidarta Ribeiro, a neuroscientist at the Brain Institute at the Federal University of Rio Grande do Norte in Brazil.
It's been clear for awhile that octopuses can change color as they sleep; videos of this have even gone viral. And previously, other groups had suggested that cuttlefish have an active sleep state similar to the rapid eye movement (REM) sleep seen in birds, mammals and some reptiles.
But those researchers did not check to see if the creatures in that state would respond to something like prey, says Ribeiro, which left open the possibility that the animals were actually in a state of quiet alertness.
Ribeiro and some colleagues decided to video-record four adult octopuses in the lab to monitor their sleep. And to make sure the animals were genuinely sleeping, the researchers checked to see if they would respond to a video of a swimming crab, a favorite food item, or to a vibration made by a hammer tapping on the tank.
The octopuses remained indifferent and difficult to arouse while sleeping, instead of reacting as they normally would when awake. The scientists found that the octopuses had periods of quiet sleep, when they were pale and still, followed by short bursts of active sleep. This cycle repeated every 30 to 40 minutes.
The active sleep periods were brief but obvious; the octopuses' skin darkened and their bodies and suckers contracted.
"For around 40 seconds, they dramatically change their color and their skin texture. Their eyes are also moving," says Sylvia Medeiros, a graduate student at the Brain Institute of the Federal University of Rio Grande do Norte. "All of this happens very conspicuously."
Their dreams, if they have them, can't be terribly complex or symbolic, given how short these active phases are, says Medeiros. Octopuses are known to be problem solvers, however, so maybe dreams help their impressive, unusual brains consolidate memories and improve on tasks.
Carrie Albertin, who studies octopuses at the Marine Biological Laboratory in Woods Hole, Mass., says these creatures spend a lot of time in their dens and it certainly can look like they're sleeping and even dreaming.
"I think as you watch these animals, it's really hard to deny that something is going on, but it's really important to actually quantify it and do the study and set up the study so that you can characterize it in a rigorous way," says Albertin. "And that's exactly what this group has done."
She thinks this work is a good first step in characterizing the different stages of octopus sleep.
"I think it's really important to study these sorts of questions in animals like octopuses and cuttlefish," says Albertin, "because they are a separate example of the evolution of large brains. And so they are telling us something fundamental about what it is to have a large brain and what you need as part of that."
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