Brainless jellies need rest

Cassiopea jellyfish suffering from lack of sleep don't function well the following day. 

Cassiopea (upside-down jellyfish) is a genus of true jellyfish and the only members of the family Cassiopeidae. They are found in warmer coastal regions around the world, including shallow mangrove swamps, mudflats, canals, and turtle grass flats in Florida, and the Caribbean. The medusa usually lives upside-down on the bottom, which has earned them the common name. Where found, there may be numerous individuals with varying shades of white, blue, green and brown.

By living upside down Cassiopea jellyfish expose photosynthetic algae in their tissue to sunlight © Danita Delimont / Getty

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Animals don’t come much more distantly related than humans and jellyfish. But new research suggests we have something fundamental in common – the need for a good night’s sleep.

Sleep is not limited to mammals or even vertebrates. Worms and flies also indulge themselves in a bit of shut-eye, but this research suggests that it has ancient evolutionary roots.

Biologists at the California Institute of Technology (Caltech) have shown that Cassiopea jellyfish display a day-night cycle of activity that ticks all the sleep boxes. Cassiopea are a strange group of jellies that spend their lives upside down on the seabed, pulsating rhythmically to keep a current of plankton-rich water flowing over their tentacles.

The biologists found that not only did the pulse rate drop significantly at night, but that they became much less responsive to stimuli in this state. And crucially, just like humans, jellies deprived of sleep don’t function well the next day – they take naps when they would usually be feeding.

“Even an animal as evolutionarily distant from humans as jellyfish requires a sleep-state,” says Caltech’s Claire Bedbrook. “This study sheds light on the ancestral form of sleep that may have evolved and specialised to eventually become human sleep, though more work is needed to explicitly test why these jellyfish sleep.”

The finding is all the more remarkable for the fact that jellyfish don’t have a brain or any sort of central nervous system, just a diffuse network of nerves, or neurons, throughout their bodies. The team is now planning to test whether organisms such as sponges and slime moulds, which don’t have any neurons at all, also have wake-sleep cycles.

“My personal hypothesis is that sleep is linked to neurons, and any animal that has neurons will have a sleep state,” says Bedbrook. “Though this is just a hypothesis, so it could be possible that even plants sleep.”

Read more wildlife news stories in BBC Wildlife Magazine

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