Jellyfish Shown Sleeping For The First Time As Study Offers New Insights On Sleep Evolution

Despite the fact that they have always been brainless animals, jellyfish do sleep after all. And while this behavior was only detected in one genus of jellyfish, new research suggests that sleep might have originated at a much earlier time in the history of the animal kingdom.

According to Scientific American, sleep is unavoidable for just about every kind of complex animal with a brain. Animals without a brain are the exact opposite — the brain has long been understood to be necessary for sleeping, and sleep is the perfect time for living creatures to refresh their memories, prepare for upcoming events, and get ready for a new day in general. But a new study from a trio of California Institute of Technology (Caltech) researchers suggests that brains aren’t a prerequisite for sleep, as evidenced by the Cassiopeia jellyfish.

To see whether jellyfish sleep or not, Caltech graduate students Ravi Nath, Claire Bedbrook, and Michael Abrams studied multiple species in the Cassiopeia genus. Science Magazine noted that these upside-down jellyfish pulse their bells at least once per second, using water to cleanse their bodies and provide sustenance. The animals also supply their own nutrients to their body via algae, a peculiarity among jellyfish, but far more common among corals.

A total of 23 jellyfish were monitored for close to a week, and it was discovered that jellyfish pulse their bells only 39 times per minute, down from about 60 pulses per minute during the daytime. But to further prove their theory that jellyfish do sleep, the researchers lifted slow-pulsing animals off the bottom of tanks and placed them close to the surface. At first, the jellyfish weren’t too responsive, but another attempt at lifting them off their preferred spots was far more successful, as the creatures immediately swam back to the bottom.

To determine whether jellyfish sleep like people usually do, the researchers took some of the creatures and pulsed water across them in 20-minute intervals, for a total of 6 or 12 hours per night. The jellyfish who had some of their sleep taken away from them were a bit sluggish the next morning and were even slower if they were in the 12-hour group, with activity rates going down significantly. Finally, the researchers gave melatonin to the jellyfish, a sleep-inducing substance that ultimately knocked the creatures out.

“Everyone we talk to has an opinion about whether or not jellyfish sleep. It really forces them to grapple with the question of what sleep is,” observed study first author Nath in a statement.

Kiel University (Germany) evolutionary biologist Thomas Bosch, who was not involved in the study, was quoted by Scientific American as saying that sleep could be “much older” than what was once thought. But in order to see if that theory is correct or not, more research might be needed on jellyfish and their sleeping habits. Study authors Nath, Bedbrook, and Abrams are still working on their Ph.D. theses, and as Scientific American pointed out, the three Caltech researchers are now searching for ancient genes within the animal that could possibly be involved in the process of sleep.

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