Sleep is very common in higher animals, such as us humans, who spend nearly a third of our lives sleeping. But before closing your eyes, did you ever consider whether the simple animals needed to sleep?
Guess which creatures below sleep?
The evolution of the nervous system and its representative animals. Image: Silverthorn, D. U. / Humanphysiology (2007)
Over the past dozen years, scientists have just confirmed that "brained" organisms, including zebrafish, fruit flies, and even nematodes, have sleep-like behaviors. That is, in the image above, from flatworms (B) to mammals (F) will sleep. So the new question is, will "brainless" jellyfish sleep?
"Brainless" jellyfish. Image: Egor Kamelev/Pexels
Jellyfish have the most primitive and simplest reticular nervous system in multicellular animals. Its neurons are simply connected to each other to form a loose network, and there is no central and obvious functional partition, which belongs to the typical "brainless" animal. Therefore, as long as you find out whether the jellyfish will sleep, you can know whether the brain is necessary for sleep.
How to prove that jellyfish sleep?
Jellyfish have no eyes, can't speak, and can't make lying motions, so how can we tell if they're sleeping? Fortunately, there are three criteria for sleep behavior, which are currently generally accepted in many species and can be used as a reference:
(1) Quiescence, that is, a decrease in activity.
(2) Reduced responsiveness to external stimuli.
(3) Homeostatic rebound, that is, there will be a rebound after sleep deprivation.
The three standard translations are:
(1) Less movement when asleep.
(2) Slower response when asleep.
(3) After staying up late and not sleeping, you will be sleepy and have to make up for sleep.
After staying up late, I couldn't open my eyes at all. Image: Tom and Jerry
If you use common pattern organisms such as mice and fruit flies, it is easy to find materials, but few people invite sea spirits such as jellyfish into the laboratory. Therefore, if you want to carry out this research, it seems that you must first go to the sea to catch jellyfish, otherwise you can only sigh .
Three PhD students from the California Institute of Technology talked about it in an after-dinner chat. It just so happened that one of them had just kept a couple of tanks of jellyfish doing other research. So the three of them immediately rushed to the laboratory to find out—when they turned off the lights in the water tank, something magical happened, and the regular contractions of these jellyfish seemed to slow down, as if they were sleeping. So they decided to test it out using the strict criteria described above, whether this was real sleep behavior.
Test 1: Differences in diurnal activity
This is a jellyfish that is cultivated in the laboratory belonging to the genus Cassiopea. Unlike the flying jellyfish that are common in aquariums, fairy jellyfish sit "upside down" on the ocean floor most of the time, making them very easy to keep and observe. Usually, they shrink regularly.
Cute "little fairy". Image: Nath, R. D. et al. / Current Biology（2017）
The researchers used a camera to record in detail the shrinkage behavior of jellyfish during the day and night. It was found that during the day these jellyfish contracted regularly every one or two seconds. At night, not only do they stop from time to time and stop shrinking, but the frequency of contraction is also reduced a lot.
Above and below the image is the frequency of contraction during the day and night, respectively.
图片：Nath, R. D. et al. / Current Biology（2017）
Importantly, such results do not occur only in individual jellyfish. Of all 23 jellyfish they observed, they were all significantly more active during the day than at night. They contract an average of 58 times per minute during the day and 39 times per minute at night.
Test 2: Response to external stimuli
Another feature of sleep is a decreased response to stimuli to the outside world. To test whether jellyfish also have this feature at night, the three doctoral students also opened their minds and "tailored" an exclusive detection method according to the habits of fairy jellyfish. Fairy jellyfish naturally like to "stay" - whether day or night, whenever there is a sudden external force pulling them up from the comfortable "bed" (the bottom of the sea), they will swim back quickly. Therefore, the way to test the speed of the reaction is to check how quickly the fairy jellyfish reacts when it swims back after suddenly floating up.
They put a fairy jellyfish in a small tube with a bottom and placed it in a water tank. Each time they measure, they will slowly lift the tubule and then drop it in an instant, at which point the jellyfish floats in the water due to inertia. They then record when the jellyfish begins to swim and swims back to the bottom of the tubule.
Can you still let the jellyfish sleep well? Image: Nath, R. D. et al. / Current Biology（2017）
The results of the experiment showed that the fairy jellyfish who "slept in a daze" at night really needed more time to realize that they were not in place than during the day, and it also took longer to swim back to the bottom of the tube. That is, the response of fairy jellyfish to external stimuli at night does slow down.
Test 3: Effects of Staying Up Late
To verify the impact of "staying up late" on jellyfish, the first difficulty encountered is how to make jellyfish "insomnia"? After all, fairy jellyfish neither stay up late to brush their mobile phones nor catch deadlines, so what reason is there for "insomnia"? However, in the eyes of three doctoral students such as (xin) spring (bing) (kuang), this is not a difficult task. They quickly found a way to "toss them around" — just blowing the poor little jellyfish up every 20 minutes with a stream of water and continuing for 10 seconds would make them sleep restlessly throughout the night.
Participating in the "Stay Up Late" experiment with (poor) fairy jellyfish (5 times the true speed).
So they used this method to "toss" the fairy jellyfish all night and then recorded how active they were next. Unsurprisingly, on the second day of "staying up late", the activity of the jellyfish decreased significantly compared with usual; even on the second night that followed, they slept more "deeply" than usual.
As a control experiment, if the jellyfish are "tossed" during the day, their activity will not change much compared with usual in the following night and day. It seems that the "sleep" of fairy jellyfish does have a steady-state regulation mechanism.
More evidence
Through these three sets of experiments, the three have actually demonstrated that there is a behavior in jellyfish that is very close to what we often call "sleep." However, they wanted to see further whether this behavior was similar to the sleep behavior of other animals in terms of chemical regulation mechanisms.
So they tried to add to the tank molecules commonly used in vertebrates to regulate sleep and photoperiods: melatonin. This molecule also has a more widely known common name – "brain platinum". For most daytime active mammals, serum melatonin concentrations decrease during the day and rise at night, regulating systemic sleep activity.
Miraculously, melatonin does reduce the activity of fairy jellyfish during the day, and the degree of reduction is also positively correlated with the concentration of melatonin. That said, melatonin, a molecule involved in the regulation of mammalian sleep, may be surprisingly evolutionarily conserved.
After the addition of melatonin, the activity of jellyfish quickly decreases.
It turns out that "brain platinum" is not a patent of brain animals, as early as the "brain" did not appear, melatonin has been regulating the sleep behavior of the nervous system.
The results of the above series of studies may mean that sleep is a non-brain-dependent property of the animal nervous system that can directly rely on the entire neural network itself. This has led to new ideas for scientists to understand "why do we sleep?"
So what practical implications does this discovery have for jellyfish rearing?
The answer is: no
But that's how science was, when magnetism was discovered and shown to aristocrats and officials
Ridiculed: What's the use of this, can it be exchanged for money?
Retort: In the future you may be able to tax it.
Later the facts have proved everything.
Everybody is watching
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