Octopuses are known to sleep and change color in the process. Now a study that was published in the journal on March 25, 2021 iScience notes that these color changes are characteristic of two main alternating states: an “active sleep” stage and a “calm sleep” stage. The researchers say the results have implications for the development of sleep and could suggest that octopuses can experience something similar to dreaming.
Scientists used to think that only mammals and birds had two sleep states. More recently, it has been shown that some reptiles also exhibit non-REM and REM sleep. A REM-like sleep state has also been reported in cuttlefish, a cephalopod relative of the octopus.
“That asked us whether we might also see evidence of two sleep conditions in octopuses,” says senior author Sidarta Ribeiro of the Brain Institute at the Federal University of Rio Grande do Norte, Brazil. “Octopuses have the most central nervous system of all invertebrates and are known to have high learning abilities.”
To find out, the researchers recorded video footage of squids in the laboratory. They found that during their “peaceful sleep” the animals were still and calm, with pale skin and eye pupils contracting into a slit. During “active sleep” it was a different story. The animals dynamically changed their skin color and texture. They also moved their eyes as they tightened their suckers and body with muscular twitches.
“What makes it more interesting is that this ‘active sleep’ mostly occurs after a long ‘quiet sleep’ – generally longer than 6 minutes – and that it has a characteristic periodicity,” says Ribeiro.
The cycle would repeat at approximately 30 to 40 minute intervals. To determine that these states actually represented sleep, the researchers measured the squid’s arousal threshold using visual and tactile stimulation tests. The results of these tests showed that the squids needed a strong stimulus in both active and quiet sleep states in order to evoke a behavioral response compared to the alarm state. In other words, they slept.
The results have interesting implications for squid and sleep development. They also raise interesting new questions.
“The alternation of sleep states observed in the Octopus insularis seems to be quite similar to ours despite the enormous evolutionary distance between cephalopods and vertebrates with an early divergence of the lineages about 500 million years ago,” says first author and doctoral student Sylvia Medeiros from the Brain Institute of the Federal University of Rio Grande do Norte, Brazil.
“If in fact two different sleep states in vertebrates and invertebrates developed twice independently of each other, what are the main evolutionary pressures that shape this physiological process?” She asks. “The independent development of ‘active sleep’ in cephalopods, analogous to REM sleep in vertebrates, may reflect an emerging property that is common to centralized nervous systems and of a certain complexity.”
Medeiros also says the results open up the possibility that octopuses may experience something similar to dreaming. “It is impossible to say that they are dreaming because they cannot tell us, but our results suggest that the squid may experience a state during ‘active sleep’ that is similar to REM sleep in humans dream most. “She says.” If octopuses do dream, they are unlikely to experience complex symbolic actions like us. ‘Active sleep’ in the octopus has a very short duration – usually from a few seconds to a minute. When dreaming in this state , it should be more like small video clips or even gifs. “
In future studies, the researchers want to record neural data from cephalopods to better understand what happens when they sleep. They are also curious about the role of sleep in animals’ metabolism, thinking and learning.
“It is tempting to speculate that dreaming in the octopus, just like in humans, can help adapt to environmental problems and encourage learning,” says Ribeiro. “Do octopuses have nightmares? Could the dreams of octopuses be inscribed into their dynamic skin patterns? Could we learn to read their dreams by quantifying these changes? “
Reference: “Cyclical alternation of calm and active sleep states in the octopus” by Sylvia Lima de Souza Medeiros, Mizziara Marlen Matias de Paiva, Paulo Henrique Lopes, Wilfredo Blanco, Françoise Dantas de Lima, Jaime Bruno Cirne de Oliveira, Inácio Gomes Medeiros, Eduardo Bouth Sequerra, Sandro de Souza, Tatiana Silva Leite and Sidarta Ribeiro, March 25, 2021, iScience.
DOI: 10.1016 / j.isci.2021.102223
This work was supported by the State University of Rio Grande do Norte (UERN), the Coordination for the Improvement of University Staff (CAPES), the National Council for Scientific and Technological Development (CNPq) and the São Paulo Research Foundation (FAPESP) Center for Neuromathematics.