JELLYFISH SLEEP, DESPITE HAVING NO BRAINS
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