Jellyfish have no brain, but they need their sleep, scientists discover
It was well past midnight when Michael Abrams, Claire Bedbrook and Ravi Nath crept into the Caltech lab in Pasadena where they were keeping their jellyfish.
They didn’t bother switching on the lights, opting instead to navigate the maze of desks and equipment by the pale blue glow of their cellphones. The students hadn’t told anyone that they were doing this. It wasn’t forbidden, exactly, but they wanted a chance to conduct their research without their PhD advisers breathing down their necks.
“When you start working on something totally crazy, it’s good to get data before you tell anybody,” Abrams said.
The “totally crazy” undertaking in question: an experiment to determine whether jellyfish sleep.
It had all started when Bedbrook, a graduate student in neurobiology, overheard Nath and Abrams mulling the question over coffee. The topic was weird enough to make her stop at their table and argue.
“Of course not,” she said. Scientists still don’t fully know why animals need to snooze, but research has found that sleep is a complex behaviour associated with memory consolidation and REM cycles in the brain. Jellyfish are so primitive they don’t even have a brain – how could they possibly share this mysterious trait?
Her friends weren’t so sure. “I guess we’re going to have to test it,” Nath said, half-joking.
Bedbrook was dead serious: “Yeah. Yeah, we are.”
After months of late-night research, Bedbrook has changed her mind. In a paper published Thursday in the journal Current Biology, she, Nath and Abrams report that the upside-down jellyfish Cassiopea exhibit sleeplike behaviour – the first animals without a brain known to do so. The results suggest that sleep is deeply rooted in our biology, a behaviour that evolved early in the history of animal life and has stuck with us ever since.
Further study of jellyfish slumber might bring scientists closer to resolving what Nath called “the paradox of sleep.”
Think about it, he urged. If you’re asleep in the wild when a predator comes along, you’re dead. If a food source strolls past, you go hungry. If a potential mate walks by, you miss the chance to pass on your genetic material.
“Sleep is this period where animals are not doing the things that benefit from a natural selection perspective,” Nath said.
Abrams chimed in: “Except for sleep.” Nath laughed.
“We know it must be very important. Otherwise, we would just lose it,” Bedbrook said. If animals could evolve a way to live without sleep, surely they would have. But many experiments suggest that when creatures such as mice are deprived of sleep for too long, they die. Scientists have shown that animals as simple as the roundworm C. elegans, with a brain of just 302 neurons, need sleep to survive.
Cassiopea has no brain to speak of – just a diffuse “net” of nerve cells distributed across their small, squishy bodies. These jellyfish barely even behave like animals. Instead of mouths, they suck in food through pores in their tentacles. They also get energy via a symbiotic relationship with tiny photosynthetic organisms that live inside their cells.
“They’re like weird plant animals,” Bedbrook said.
They’re also ancient: Cnidarians, the phylogenetic group that includes jellies, first arose some 700 million years ago, making them some of Earth’s first animals. These traits make Cassiopea an ideal organism to test for the evolutionary origins of sleep. Fortuitously, Abrams already had some on hand.
So the trio designed an experiment. At night, when the jellies were resting and their professors were safely out of the picture, the students would test for three behavioural criteria associated with sleep.
First: Reversible quiescence. In other words, the jellyfish become inactive but are not paralysed or in a coma. The researchers counted the jellyfish’s movements and found they were 30 per cent less active at night. But when food was dropped into the tank, the creatures perked right up. Clearly not paralysed.
Second: An increased arousal threshold. This means it’s more difficult to get the animals’ attention; they have to be “woken up.” For this, the researchers placed sleeping jellies in containers with removable bottoms, lifted the containers to the top of their tank, then pulled out the bottom. If the jellyfish were awake, they’d immediately swim to the floor of the tank. But if they were asleep, “they’d kind of strangely float around in the water,” Abrams said.
And third: The quiescent state must be homeostatically regulated. That is, the jellyfish must feel a biological drive to sleep. When they don’t, they suffer.
The scientists prevented them from sleeping by “poking” them with pulses of water every 20 minutes for an entire night. The following day, the poor creatures swam around in a daze, and the next night they slept especially deeply to make up for lost slumber.
Allan Pack, the director of the Center for Sleep and Respiratory Neurobiology at the University of Pennsylvania, was not involved in the jellyfish research, but he’s not surprised by the finding, given how prevalent sleep is in other species.
“I think it’s one of the major biological questions of our time,” Pack said. “ We spend a third of a life sleeping. Why are we doing it? What’s the point?”