Octopuses are known to sleep and change color while doing so. Now, a study published on March 25, 2021 in the journal iScience finds that these color changes are characteristic of two major alternating sleep states: an “active sleep” stage and a “quiet sleep” stage. The researchers say that the results have consequences for the development of sleep and may indicate that it is possible for octopuses to experience something similar to dreams.
Researchers used to believe that only mammals and birds had two sleep states. More recently, it was shown that some reptiles also show sleep that is not REM and REM. A REM-like sleep condition was also reported in octopus, an octopus relative of the octopus.
“It led us to wonder if we could also see evidence of two octopus sleep states,” said senior author Sidarta Ribeiro of the Brain Institute at the Federal University of Rio Grande do Norte, Brazil. “Octopuses have the most centralized nervous system of all invertebrates and are known to have a high learning capacity.”
To find out, the researchers took video recordings of octopuses in the laboratory. They found that the animals were still and quiet during ‘quiet sleep’, with pale skin and eye pupils gathered on a slit. During “active sleep” it was a different story. The animals dynamically changed their skin color and structure. They also moved their eyes as they contracted their suction and body with muscle oscillations.
“What makes it more interesting is that this ‘active sleep’ usually occurs after a long ‘quiet sleep’ – generally longer than 6 minutes – and that it has a characteristic periodicity,” says Ribeiro.
The cycle is repeated at approximately 30-40 minute intervals. To determine that these conditions actually represented sleep, the researchers measured the octopus’ arousal threshold using visual and tactile stimulation tests. The results of these tests showed that the octopuses in both ‘active’ and ‘quiet sleep’ needed a strong stimulus to elicit a behavioral response compared to the warning condition. In other words, they slept.
The results have interesting consequences for octopuses and for the development of sleep. They also raise exciting new questions.
“The shift in sleep states observed in Octopus insularis seems quite similar to ours, despite the enormous evolutionary distance between octopuses and vertebrates, with an early divergence of lines about 500 million years ago,” said first author and doctoral student Sylvia Medeiros of the Brain Institute at the Federal University of Rio Grande do Norte, Brazil.
“If two different sleep states actually developed twice independently in vertebrates and invertebrates, what is the essential evolutionary pressure that shapes this physiological process?” she asks. “The independent development in squid of an ‘active sleep’ analogous to vertebrate REM sleep may reflect an emerging trait common to centralized nervous systems that reaches a certain complexity.”
Medeiros also says that the results increase the possibility that octopuses experience something similar to dreaming. It is not possible to confirm that they are dreaming because they can not tell us, but our results suggest that the squid during “active sleep” may experience a condition analogous to REM sleep, which is the condition during which people dream the most, “she says.” If octopuses really dream, they are unlikely to experience complex symbolic plots as we do. ‘Active sleep’ in the octopus has a very short duration – usually from a few seconds to a minute. states dreams, it should be more like small videos or even gifs. ”
In future studies, researchers want to record neural data from octopuses to better understand what happens when they sleep. They are also curious about the role of sleep in animals’ metabolism, thinking and learning.
“It’s tempting to speculate that, just like in humans, dreaming in octopus can help adapt to environmental challenges and promote learning,” says Ribeiro. “Do octopuses have nightmares? Can octopus’ dreams be inscribed on their dynamic skin pattern? Can we learn to read their dreams by quantifying these changes? ”
Reference: “Cyclical change of silent and active sleep status 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, Sequerra, Sandro de Souza, Tatiana Silva Leite and Sidarta Ribeiro, 25 March 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 Higher Education Staff (CAPES), the National Council for Scientific and Technological Development (CNPq) and by the São Paulo Research Foundation (FAPESP) Center for Neuromatics.