Do Jellyfish Dream of Electric Sheep?
we're intimately familiar with what a sleeping human looks like, or a sleeping dog or bird. But what does a sleeping ant look like—or a sleeping jellyfish? Despite spending a third of our lives asleep, we still haven’t cracked all its secrets, but becoming more adept at recognizing the state in other species could explain why sleep is so important to us.
Sleep—or at least some activity that looks an awful lot like it—is pretty common for all living creatures: among not just humans and our close relatives but also birds, reptiles, fish, insects and even a microscopic worm, called Caenorhabditis
elegans, found in many science labs. Still, while scientists haven’t yet confirmed that any animal can go without sleep, they also aren’t convinced that all animals need it. That’s particularly true of animals that don’t even have brains. So when a trio of Caltech grad students noticed some laboratory jellyfish were acting differently when the lights were turned off, they decided to investigate. “We just went in one night...with our iphones and some pretty simple apparatuses, and we filmed them,” Michael Abrams, one of the three lead authors on the new Current Biology paper summarizing their findings, tells
Newsweek. “You could really clearly, by eye, see a change in the behavior between the light and the dark.”
The jellyfish, called Cassiopea, are nicknamed upside-down jellyfish for their preference to hang out in shallow waters near the seafloor with their tentacles up. Cassiopea are intriguing to scientists because they have a nervous system but a very simple one—instead of a centralized brain like ours, they have a network of nerves spread through their bodies.
The jellyfish pulse their bells (the non-tentacle portions of jellyfish), creating currents around their bodies. “They’re pretty big, and it’s a big movement,” says Claire Bedbrook, another Caltech grad student on the team. And this is what the researchers noticed that first night watching the tank—the jellyfish pulsed less at night than during the day. That observation sent them on a three-year quest to determine whether what they were seeing met the definition of sleep.
While you probably don’t think about it each night as you put on pajamas, there is a biological definition of sleep, based on three criteria. The body has to move less and be less aware of stimuli—but those characteristics must be readily able to turn off (otherwise, you’re in a coma). And there’s something called homeostatic regulation, a fancy way of saying that after you go without sleep for a long time, you sleep extra to catch up.
So Abrams, Bedbrook and fellow graduate stu- dent Ravi Nath designed a sequence of experiments that could methodically determine whether those boxes could be checked for Cassiopea.
In order to do so, the team needed to figure out a way of interrupting whatever sleep the jellyfish might be getting. “We had to be pretty creative,” Abrams says, as they needed some kind of prod or nudge they could replicate precisely without the jellyfish getting used to it and ignoring it.
That meant thinking like a jellyfish, Bedbrook says. One of the runner-up techniques, putting the
Cassiopea in a flask and shaking it, visibly stressed out the jellyfish and was discarded. “This is a real issue in sleep research,” Bedbrook added.
They settled on a jellyfish elevator of sorts: a tube with a netted bottom. When they wanted to nudge a Cassiopea, they used the tube to lift the jellyfish up from its hangout at the bottom of the tank. Then they lowered the tube and let the jellyfish sink back, which Nath imagines is like a human waking up from dreaming of falling.
Once they had their perfect wake-up nudge, the researchers started running the experiments. (And yes, keeping jellyfish awake also means keeping graduate students awake.) They counted jellyfish pulses during the day and at night, after feeding and after fasting, while nudging them awake throughout the night, and during the day after a restless night. Step by step, their results checked off each component of a sleep-like state, plus the possibility that they were just seeing the effects of tired muscles. All told, they think
Cassiopea really do sleep, even if it isn’t exactly how humans do.
Bedbrook is eager to see whether other researchers find similar behaviors in their own jellyfish and other brainless species whose sleep habits haven’t yet been closely studied. That would tell scientists something important about the value of sleep, according to Isabella Capellini, a researcher at the University of Hull in the U.K. who has studied the evolution of sleep. If all species that have no brains sleep, she wrote in an email to multiple media outlets, “the presumed benefits of sleep are more universal and far more broadly shared across the tree of life than previously thought; sleep is in essence evolutionarily older than we thought.” It would also mean that sleep has a purpose beyond simply letting the brain rest.
So next time insomnia comes to visit your bedroom, consider counting jellyfish for inspiration.
One downside of sleep research: keeping jellyfish awake also means keeping graduate students awake.
WAKEUP CALL Researchers had to prod the jellyfish out of what seemed to be their sleep state without scaring them, or boring them so much that they slept through their “alarm clock.”