Jellyfish sleep even without brain

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In fact, jellyfish sleep, this finding is made in an article published this week in Current Biology by researchers who studied a jellyfish named Cassiopeia, which lives upside down in warm Pacific waters. This resembles small aquatic cauliflowers, with their curly tentacles. This finding may delay the onset of sleep in the history of evolution to a time prior to the onset of the central nervous system.

The starting point for Howard Hughes Medical Institute researchers at the California Institute of Technology was: Do all animals sleep? In the world of invertebrate animals, there are studies that have demonstrated that, for example, the fruit fly sleeps, and there is also an open discussion about possible “naps” of a worm (Caenorhabditis elegans, or just C. elegans) that exists in the intestines of humans.

But the subject is controversial, so notes a statement from the Howard Hughes Medical Institute about the article now published. “The discussion is further confused as we enter the topic of sleep in primitive animals such as jellyfish or sea sponges. We wanted to clarify this once and for all”, said Paul Sternberg, an HHMI researcher and one of the authors of the article.

Jellyfish have appeared in the seas for at least 600 million years (dinosaurs appeared about 230 million years ago) and belong to the group of early animals that developed neurons, nerve cells, even if they do not have a brain or central nervous system. This study of such a primitive animal raises new questions about the origin and purpose of sleep.

In an artificial environment created in the laboratory, the researchers observed the typical and rhythmic movement of contraction and relaxation of these small jellyfish, measuring between two and five centimeters in diameter. The observations led to the conclusion that the jellyfish pulsed less during the night. In the next step, 23 jellyfish were observed for six whole days and counted the “pulsations”.

It was confirmed that the pace slowed during the night and they realized that it was enough to drop a small amount of food in the tank to quickly wake the jellyfish. “It’s like the smell of coffee in the morning for us”, said Paul Sternberg.

This rapid awakening, they argue, reveals that the nocturnal behavior of the jellyfish is explained by the fact that it is sleeping and not due to a situation of paralysis or coma. The researchers performed further tests that required the jellyfish to react, thus verifying that the reaction was much slower at night. They reacted late to the stimuli, as if they were sleepy.

Finally, it was also noticed that these animals suffer the consequences of spending a night in white. To prove it, scientists spent a night waking the jellyfish with small jets of water for periods ranging from six to 12 o’clock, and then observed the rhythm of their pulsations the morning of the next day. Slower, clearer, as with us humans. But after they had recovered their sleep the next night, everything seemed to return to normal. Just like humans.

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