What Can We Learn From A Sleeping Octopus?
In recent years, an accumulation of research has made very clear how important getting enough sleep is for human health. There’s a wider biological corollary to this — sleep seems to be pretty much universal in animals. For all its importance though, it’s still not 100% clear what sleep is actually for.
In vertebrates like humans, the mystery is compounded by the fact that there are two types of sleep: a more quiescent slow-wave type, as well as a more active REM (rapid-eye-movement) type of sleep. Most other animals appear to have only a quiescent phase.
The REM sleep state is paradoxical in some ways. It involves higher metabolic activity, and brainwave patterns more similar to wakefulness. At the same time, most skeletal muscles become relaxed (eye muscles being an exception).
The fact that most animals don’t have anything like REM sleep has been a hindrance in understanding it. For this reason, reports that cephalopods (a group that includes octopus, cuttlefish, and squid) might have an active REM-like sleep phase caught the attention of a group of researchers in Japan. They decided to study sleep behaviors in a local nocturnal cephalopod called Octopus laqueus.
These small cephalopods have the useful property of sleeping during the day, allowing researchers to study them then. And like other cephalopods, they have the ability to control their skin coloration dynamically. This they can use to camouflage themselves and to make displays for others of their species to see. This video, made by the group, shows some of what Octopus laqueus can do, and at the end includes a bit of one individual sleeping.
It had been previously reported that when sleeping, cephalopods go through periods of quiescent sleep which are interspersed with short (minutes long) periods of a more active type. This active sleep involves movements of the arms as well as changes in skin coloration.
In a recent paper, the team in Japan picked up this work with Octopus laqueus. They demonstrated that the active phase is a true sleep state, meeting the commonly accepted criteria for sleep. (A sleeping animal has a higher threshold for arousal, the sleep state is easily reversible, and there is a ‘need’ for sleep — if you deprive an animal of it, they will sleep more later to make up for it).
This active phase of sleep has some interesting similarities to REM sleep in vertebrates. In active phase sleep, the brain of the octopus shows higher levels of activity, more like those of the awake octopus brain. This is also the case with REM sleep in vertebrates.
Two things make this especially interesting. First cephalopods and vertebrates are distantly related. Their common ancestor was a small animal with simple behavior. For this reason, similarities in their sleep (and many other things) are almost certainly independently evolved. Second, cephalopods are similar to vertebrates in having big brains and complex behavior.
One interesting possibility is that the presence of an active phase of sleep is somehow connected with having a big brain. Maybe once the brain is big enough, and behavior complex enough, it becomes necessary to have an active phase of sleep. It is known that sleep in vertebrates helps with motor learning and memory consolidation. It will be interesting to explore whether it also serves these functions in cephalopods
