Jellyfish may be capable of sleep after all, study reports

A team of researchers have found compelling evidence that a species of brainless jellyfish can sleep in the same way other animals do.

Carmelo Sheppard | Oct 28, 2019

For the first time in history, researchers have discovered an animal without a brain that is capable of sleep.

Most brainless organisms -- such as jellyfish and corals -- do not sleep in the same way as other animals. This is because they do not have the advanced functions needed for such a process.

However, in a new study, a team of scientists from the California Institute of Technology found that a primitive, upside-down jellyfish genus known as Cassiopea may be able to rest for long periods of time. The brainless animals live in the tropical waters of the Pacific and Atlantic oceans and, unlike their relatives, spend a lot of their time on the ocean floor in a state that is similar to human sleep. This behavior has never been recorded before, and it shows sleep is a much older process than previously thought.

"It may not seem surprising that jellyfish sleepafter all, mammals sleep, and other invertebrates such as worms and fruit flies sleep," said study co-author Ravi Nath, a graduate student in Caltech's Sternberg laboratory, according to Gizmodo. "But jellyfish are the most evolutionarily ancient animals known to sleep. This finding opens up many more questions: Is sleep the property of neurons? And perhaps a more far-fetched question: Do plants sleep?"

In order for an animal to be asleep they must meet three requirements. They have to display inactivity, they need a decreased response to stimuli, and they must have negative reactions when deprived of their slumber.

To test this in the jellies, the researchers ran three separate trials to look at all of the above aspects. First, they set up cameras that tracked how many times the invertebrates pulsated over a 24 hour period. This revealed that, while the jellies pulsated 58 times per minute during the day, they only pulsated 39 times per minute at night.

Then, researchers tested the species' response to stimuli by allowing them to sit on a special platform. When the team removed the platform while the animals were awake, the jellies would quickly swim to the bottom of their tank. However, when the team removed the tank during sleep, the animals responded much more slowly.

For the final test, scientists kept the jellyfish awake by shooting jets of water at them throughout the night. Their responses showed this process made them visibly upset.

While the three tests are not definitive proof that the jellies sleep, it presents a compelling argument. However, as the research came from a lab setting, the team is not sure if the organisms would act the same way in the wild. They next plan to look at other basic organisms to see if they show similar behavior as well.

"We'd love to see whether there are other species of jellyfish that also sleep," said study co-author Claire Bedbrook, a doctoral student in bioengineering at the California Institute of Technology, according to Live Science. "We would also like to see whether or not sponges, the next level down, sleep."

The results from this study are published in Current Biology.