To test sleep, researchers don’t let sleeping jellyfish lie

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The life of a jellyfish may seem like a real snooze, but until now biologists were never certain if the gelatinous blobs actually slept. Now it appears that at least one group of jellyfish needs its beauty sleep just like us.

Some species of upside-down jellyfish (Cassiopea) meet all of the criteria for entering a “sleeplike state,” a group of Caltech researchers report September 21 in Current Biology. The jellyfish seem groggy after a sleepless night and quickly waken from their slumber when fed, experiments show.

It’s a surprising find: Sleep and sleeplike states have been documented in a wide range of animals — from microscopic wormlike nematodes to, of course, humans (SN: 10/24/09, p. 16). But until now, the behavior has been observed only in animals with a centralized nervous system and brain.

Jellyfish operate on a decentralized net of nerve cells. “It’s the first animal that doesn’t have a centralized nervous system that also sleeps, that we know of,” says biology graduate student and coauthor Ravi Nath.  Adds coauthor Michael Abrams: “Sleep is not solely generated by animals with brains.”

The finding is raising new questions about when — and why — sleep evolved. Jellyfish are cnidarians, an ancient lineage of animals that evolved at least 600 million years ago. So if Cassiopea jellyfish in fact sleep, it suggests that sleep is one of the most basic requirements of life. And unlike in humans, where sleep has been linked to such brain functions as retaining memories, the same can’t be said of the role of sleep for jellyfish.

“Finding sleep in jellyfish thus raises the question of whether sleep and nervous system functions are intertwined,” says William Joiner, a neuroscientist at the University of California, San Diego, “or, alternatively, whether sleep arose before the inception of the nervous system to fulfill an as yet unidentified physiological need.”

Upside-down jellyfish spend most of their time resting their gelatinous bodies, or bells, on the seafloor with their stubby arms and tentacles sticking up. The animals regularly pulse their bells to filter feed and get rid of waste.

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To qualify as sleeping beings, the jellyfish had to pass three tests. First, did they become less active at a particular time?  By monitoring the pulsing of 23 upside-down jellyfish day and night for six days, the researchers discovered that the animals pulsed 32 percent less at night. The team could easily reverse this sleepy state by dropping food into the tank. “The jellyfish immediately responded to the stimulus and started pulsing more,” Nath says.

Second, were the jellyfish less responsive at certain times? “For people, that would be like if you’re asleep, you are less likely to respond to someone talking to you,” says Abrams. Since Cassiopea prefer to swim down to settle on a surface, the researchers hoisted up a jellyfish in a plastic pipe with mesh on the bottom, let the animal settle for five minutes and then quickly lowered the pipe. That action effectively placed the jellyfish free-floating into the water column. At night, the researchers found, it took the jellyfish longer to begin pulsing and reach the bottom of the tank than during the day.

Finally, do the jellyfish need regular periods of sleep in order to survive? After keeping jellyfish active for up to 12 hours overnight by squirting them with pulses of water, the jellyfish were significantly less active the following morning. That sluggishness shows that the jellyfish needed to make up for the loss of rest, says coauthor Claire Bedbrook.

“The authors do a good job of demonstrating that jellyfish fulfill the most fundamental criteria for sleep,” Joiner says.

A bigger question for cnidarians, and most animals, is not only if they sleep, but why. Jellyfish sleep can hardly be compared with human sleep, Abrams says, but by studying the creatures “we might be able to get at those core, fundamental components of why something sleeps.” 

Editor's note: This story was updated October 11, 2017, to correct the attribution of a quote and more closely date when cnidarians evolved.