Natures wonders: even hydras sleep (Introduction)

by David Turell , Wednesday, May 19, 2021, 16:16 (1065 days ago) @ David Turell

With just a few nerve cells:

https://www.realclearscience.com/2021/05/19/sleep_evolved_before_brains_hydras_are_proo...

"The hydra is a simple creature. Less than half an inch long, its tubular body has a foot at one end and a mouth at the other. The foot clings to a surface underwater — a plant or a rock, perhaps — and the mouth, ringed with tentacles, ensnares passing water fleas. It does not have a brain, or even much of a nervous system.

"And yet, new research shows, it sleeps. Studies by a team in South Korea and Japan showed that the hydra periodically drops into a rest state that meets the essential criteria for sleep.

***

"...a counterpoint to this brain-centric view of sleep has emerged. Researchers have noticed that molecules produced by muscles and some other tissues outside the nervous system can regulate sleep. Sleep affects metabolism pervasively in the body, suggesting that its influence is not exclusively neurological. And a body of work that’s been growing quietly but consistently for decades has shown that simple organisms with less and less brain spend significant time doing something that looks a lot like sleep. Sometimes their behavior has been pigeonholed as only “sleeplike,” but as more details are uncovered, it has become less and less clear why that distinction is necessary.

"It appears that simple creatures — including, now, the brainless hydra — can sleep. And the intriguing implication of that finding is that sleep’s original role, buried billions of years back in life’s history, may have been very different from the standard human conception of it. If sleep does not require a brain, then it may be a profoundly broader phenomenon than we supposed.

***

"The evidence for sleep in creatures with minimal nervous systems seemed to reach a new high about five years ago with studies of jellyfish. The Cassiopea jellies, about four inches long, spend most of their time upside down, tentacles reaching toward the ocean surface, and pulsating to push seawater through their bodies. When Michael Abrams, now a fellow at the University of California, Berkeley, and two other graduate students at the California Institute of Technology asked if Cassiopea might sleep, they were continuing the line of inquiry that Tobler had followed when she studied cockroaches, investigating whether sleep exists in ever simpler organisms. If jellyfish sleep, that suggests sleep may have evolved more than 1 billion years ago and could be a fundamental function of almost all organisms in the animal kingdom, many of which do not have brains.

***

"The new revelations about sleep in hydras push the sleep discoveries to a new extreme. The hydra’s body and nervous system are even more rudimentary than Cassiopea’s. Yet as the researchers from Kyushu University in Japan and Ulsan National Institute of Science and Technology in South Korea demonstrated, once a hydra entered a rest state, a pulse of light would rouse it, and it too slept longer after repeated deprivation, among other findings.

"Hydra sleep has its peculiarities: Dopamine, which usually makes animals sleep less, caused the hydra to go still. The hydra does not seem to sleep on a 24-hour cycle, instead spending part of every four hours asleep. Something about the hydra’s way of life may have made these traits advantageous, Tobler suggests.

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"What, then, does sleep do in the absence of a brain? Raizen suspects that at least for some animals, sleep has a primarily metabolic function, allowing certain biochemical reactions to take place that can’t happen during waking hours. It may divert the energy that would be used by alertness and movement into other processes, ones that are too costly to take place while the animal is awake. For example, C. elegans seems to use sleep to enable the growth of its body and support the repair of its tissues. In sleep-deprived hydras, the cell divisions that are part of everyday life are paused. Something similar has been seen in the brains of sleep-deprived rats and in fruit flies. Managing the flow of energy may be a central role for sleep.

***

"Sleep might have helped to maintain the first sleeper’s rudimentary nervous system, but it could just as easily have been for the benefits of its metabolism or digestion. “Before we had a brain, we had a gut,” he said.

"Even deeper questions are now being asked. In a 2019 opinion paper, Raizen and his co-authors wondered: If sleep happens in neurons, then what is the minimum number of neurons that can sleep? Can the need for sleep be driven by other kinds of cells, as work implicating liver and muscle cells suggests?"

Comment: Fascinating. My guess is, considering the furious high speed of living biochemistry reactions, stopping to rest is necessary,