JEL­LY­FISH SLEEP, DE­SPITE HAV­ING NO BRAINS

Focus-Science and Technology - - Discoveries -

Science still can’t fully ex­plain why we need sleep, but many cur­rent the­o­ries in­volve clear­ing the brain of waste chem­i­cals. A re­think maybe re­quired, how­ever, be­cause re­search at Cal­i­for­nia In­sti­tute of Tech­nol­ogy (Cal­tech) shows that Cas­sio­pea jel­ly­fish, which have no brain at all, ex­hibit sim­i­lar sleep be­hav­iour to hu­mans and other mam­mals.

Cas­sio­pea, the up­side-down jel­ly­fish, are known for sit­ting on the ocean floor and pul­sat­ing. So how can you tell if they’re snooz­ing? Sci­en­tists use three cri­te­ria to de­fine ‘sleep’ across the an­i­mal king­dom. Is the crea­ture less ac­tive? Is it less re­spon­sive to ex­ter­nal stim­uli? And if you de­prive it of ‘sleep’, is it more prone to such a state af­ter­wards?

The Cal­tech team mon­i­tored the jel­ly­fish 24 hours a day and found that they pul­sate about 39 times per minute at night, com­pared to 58 times per minute by day. In an ex­per­i­ment where a shelf in the wa­ter was pulled out from un­der them, jel­ly­fish in this slower-pul­sat­ing state took longer to re­ori­ent themshelves on the ocean floor than jel­ly­fish that were ‘awake’. Fi­nally, if the jel­ly­fish were ‘prod­ded’ with jets of wa­ter dur­ing the night, they tended to fall into the qui­es­cent state the next day when they would usu­ally be awake.

This sug­gests that jel­ly­fish, which are an an­cient group of an­i­mals, do in­deed sleep, sug­gest­ing sleep may be a be­hav­iour ac­quired early on in our evo­lu­tion and never aban­doned.

“Jel­ly­fish are the most evo­lu­tion­ar­ily an­cient an­i­mals known to sleep,” said re­searcher Ravi Nath. “This find­ing opens up may more ques­tions. Is sleep the prop­erty of neu­rons? And per­haps a more far-fetched ques­tion: do plants sleep?”

Cas­sio­pea jel­ly­fish have al­gae liv­ing in their tis­sues. When the jel­ly­fish lie up­side- down on the seabed, the al­gae is ex­posed to sun­light so it pho­to­syn­the­sises and pro­vides food for the jel­ly­fish

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