Even jellyfish sleep, proving it is a primitive ancient behavior

Cassiopea is an ancient jellyfish that has no brain or spine and basically spends its entire existence upside down on the ocean floor. But researchers from the California Institute of Technology were surprised to discover that Cassiopea actually sleeps, indicating that sleep is a primitive behavior that has survived thousands of years of evolution.

“It may not seem surprising that jellyfish sleep–after all, mammals sleep, and other invertebrates such as worms and fruit flies sleep,” said co-author Ravi Nath. “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?”

There are three conditions that constitute “sleeping.” First, sleep is marked by a state of inactivity and tranquility, also known as quiescence. While in the quiescent state, the organism must show a decreased response to stimuli. Also, the organism must show that it is more and more sleepy when deprived of sleep. In order to prove that jellyfish sleep, the research team needed to demonstrate that they meet all three sleep criteria.

To show quiescence, the researchers set up cameras to monitor the jellyfish. They found that the jellyfish go through periods of inactivity at night. During this tranquil stage, the jellyfish pulsed around 39 times per minute as opposed to 58 pulses per minute during the day.

Next, the team needed to test if the jellyfish were less responsive to stimuli during this state of decreased activity. They placed a jellyfish on a platform and pulled the platform out from underneath the animal once it had gone into a quiescent state. Instead of swimming to the bottom of the tank right away, the jellyfish floated in the water for up to five seconds before becoming alert.

In order to demonstrate the jellyfish had an increased sleep drive when deprived of sleep, researchers pulsed water at the animals for 20 minutes to keep them awake. As a result, the jellyfish were more likely to fall into the quiescent state during daytime hours when they would normally be active.

While the study proves that jellyfish exhibit sleep behavior, the underlying sleep mechanisms remain a mystery. The study is published in the online journal Current Biology.

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By Chrissy Sexton, Earth.com Staff Writer