Pete Oxford/Minden Pictures/FLPA RM
Birds do it, bees do it, even enervated fleas do it. Sleep is widely believed to be common to all animals with a central nervous system, but it turns out to be even more ubiquitous than that.
Jellyfish have been found to enter a sleep-like state at night, and become dozy the next day if their rest is interrupted. This is remarkable for an animal with a simple, diffuse nervous system and no centralised brain.
Sleep has been studied in fruit flies and nematode worms, but even they are relatively complex compared with jellyfish. The findings push the origins of slumber further back in our evolutionary past.
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Cassiopea, also called upside-down jellyfish, are found in shallow seas throughout the tropics. They rarely swim, instead resting their bell on a surface and pointing their tentacles upwards. They continuously pulse by contracting and relaxing their bell about once a second, generating currents that help them to feed and get rid of waste.
Ravi Nath and his colleagues at the California Institute of Technology in Pasadena observed 23 Cassiopea continuously for six days and nights. They tested whether they met the three standard criteria for being asleep: being less active, being less responsive to stimuli, and becoming tired if they are deprived of rest.
They found that the jellyfish were less active at night, pulsing 32 per cent more slowly than during the day. However, if the jellyfish were presented with a food stimulus during the night, they quickly returned to daytime levels of activity.
In daytime, if Cassiopea are lifted to the middle of the water column, they swim downwards until they can rest on a surface again. But when the researchers did this at night, the jellyfish reacted more slowly, showing they were less responsive to stimuli.
Finally, the researchers deprived the jellyfish of rest by squirting a jet of water at them every 20 minutes. The jellyfish typically returned to their daytime pulse rate for about five minutes after each squirt. After a night of sleep deprivation, the jellyfish were 17 per cent less active and slower to respond to stimuli.
Origins of slumber
The study adds to the evidence that sleep arose just once, in a very early organism, rather than independently in different groups.
For one thing, every animal studied so far has been found to sleep. Even plants have a state akin to it – in 2016, a study found that the branches of birch trees droop towards the end of the night.
That said, there are still many more to look at. Only six out of 36 phyla – the biggest groups of animals – have been looked at so far. “The expectation of finding truly sleepless animals in the future is certainly not unreasonable,” says John Lesku at La Trobe University, Australia.
But there is more evidence for a single origin of sleep, says Nath.
Despite the vast evolutionary distance between jellyfish and backboned vertebrates like ourselves, the team found evidence for common mechanisms. The hormone melatonin, which promotes sleep in humans and our relatives, also induces quiescence in Cassiopea. So does pyrilamine, an antihistamine drug that causes drowsiness.
It is also unclear why sleep evolved. The problem is that sleep has so many benefits, it’s hard to pick out the crucial one.
One theory is that sleep arose to conserve energy needed to power neurons. Another is that animals sleep to flush away neurotransmitters that build up in the connections between neurons.
“Since sleep is found in such a basic and simple animal [as a jellyfish], it is likely that the ancestral function of sleep is basic and simple,” says Nath.
In humans, sleep is often described as helping move memories into long-term storage. Since that is an advanced function, Nath says it was probably tacked on later.
Journal reference: Current Biology, DOI: 10.1016/j.cub.2017.08.014
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