Jellyfish need their beauty sleep too, researchers find

Jellyfish are like the teenagers of the sea. Blobs, aimlessly floating through life eating anything that drifts by. And if that isn't enough of a similarity, a new study concludes that they need sleep, too.

Unlike teenagers, scientists from Cal Tech couldn't ask jellyfish if they were sleeping, so to find out if they were, they performed a series of tests.

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First, researchers had to determine if jellyfish behavior changed over time, so they recorded them during the day and at night. It turned out that at night the pulse rate of the jellyfish slowed down from 58 times per minute to 39, indicating that they'd entered some sort of sleep state.

Scientists followed up those observations by stimulating the animals in different circumstances to see how they'd react.

In one test they pulled a platform out from beneath resting jellyfish. When in their sleep state, the jellyfish had a delayed reaction of about five seconds before reorienting themselves and finding a comfortable spot on the bottom once again.

In another test, researchers deprived the jellyfish of sleep by moving them every 10 seconds for 20 minutes with pulses of water. The following day, the jellyfish that were forced to stay up tried to catch up on their rest by falling asleep during times they'd usually be awake. They needed about 24 hours of rest before their sleep/wake cycles returned to normal.

All of the behaviors indicated to the scientists that jellyfish do indeed sleep.

Scientists aren't sure why animals, including humans, sleep. It could be that sleep allows our minds and bodies to recharge, recover from the trauma of the day and flush out toxins. But it's the humble jellyfish that scientists are hoping will help us better understand the reasons for sleep.

"Jellyfish are one of the first animals to develop neurons. They're really ancient and different from us, said Ravi Nath, a Cal Tech graduate student and one of the study co-authors. "And since they have this sleep state, it means that sleep has been around for a really long time."

"If we really want to understand the function of sleep, it's probably best approached in an animal that has a simple neural architecture and a simple nervous system," he said.

Rather than brains, jellyfish have diffuse nerve nets, or a collection of nerves, an early evolutionary development that is thought to have been around as long as the jellyfish – about 500 million years. Homo sapiens have only existed for some 200,000 years.

Scientists believe that the diffuse nerve net evolved over time into strong central clusters of neurons that drive different behaviors, a complex feature found in the brains of humans and animals. Experts have also found related genes and pathways in both simple organisms like nematodes and in complex ones like humans.

That's why Nath and his co-authors turned to the organism that they believed would provide insight into the simplest sleep state. They ran their tests on cassiopea jellyfish, part of the cnidarian family, which also includes sea anemone and corals. 

"I think it really shows and demonstrates that behaviors emerge somewhere in the animal lineage," said Nath. "And we can start to go in and assess behaviors by looking at how they change throughout evolutionary time." 

Nath said that the team next wants to figure out why jellyfish sleep, another opportunity that could offer insight into why we do as well.

The study was published Thursday in the journal Current Biology.