Evie Summers
Do jellyfish need sleep? It's a question that has puzzled many, with some scientists asserting that only mammals and birds can be said to truly sleep. However, recent research has found that jellyfish do exhibit a sleep-like state.
The jellyfish Cassiopea, also known as the upside-down jellyfish, has been observed to enter a state of reduced activity and responsiveness at night, fulfilling the criteria for sleep. This discovery has significant implications for our understanding of sleep, as jellyfish belong to one of the most ancient animal groups, suggesting that sleep is a behaviour that evolved long ago and has been conserved throughout the animal kingdom.
So, do jellyfish need sleep? While they may not have brains, the sleep-like state they enter seems to be essential for their survival and function, indicating that sleep is not just a result of complex nervous system organisation but may be an intrinsic property of neurons themselves.
| Characteristics | Values |
|---|---|
| Activity level | 30% less active at night |
| Response to stimuli | Less responsive at night |
| Homeostatic regulation | If deprived of sleep, they become less active and responsive the next day |
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What You'll Learn
Jellyfish exhibit a sleep-like state
Firstly, Cassiopea exhibit decreased activity that is rapidly reversible. They become less active at night, with a 32% reduction in the rate of their bell's pulsation. When researchers offered them a midnight snack, they perked up and started pulsing at daytime rates, indicating that their quiescent state was easily reversible.
Secondly, Cassiopea show reduced responsiveness to stimuli during their quiescent state. When placed in a small container made of PVC pipe with a mesh bottom and suspended in water, they took longer to start pulsing at night than during the day. This behaviour is similar to the grogginess experienced by humans when waking up.
Thirdly, the sleep-like state in Cassiopea is under homeostatic control. When researchers disturbed the jellyfish with gentle pulses of water every 20 minutes during the night, they showed a decline in pulsing in the following day, as if they were tired. After a full night without disturbances, the jellyfish returned to normal activity levels.
The discovery that jellyfish exhibit a sleep-like state has important implications for our understanding of sleep. It suggests that sleep is an ancient behaviour that evolved early in the history of animal life and may be a fundamental property of neurons. The finding also raises questions about the role of sleep in animal life and its evolutionary purpose.
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They slow down and stop moving at regular intervals
The upside-down jellyfish Cassiopea regularly pulses. Using these pulses as a measure of nervous system activity, researchers observed sleep-like behaviour in Cassiopea. It slows its pulsing at night, even stopping briefly. If the researchers dunked it in cold water while the pulsing was slowed, it took a few seconds to "rouse" and react to the cold water. This is different behaviour to when they were pulsing at normal speeds.
To measure activity, the researchers counted the rate of the bell's pulsation in 23 jellyfish for six straight days and nights. They found that the rate dropped by 32% at night, going from about 1,155 pulses per 20 minutes during the day to 781 pulses per 20 minutes at night.
The jellyfish Cassiopea exhibits a sleep-like state with three characteristics. Firstly, a period of decreased activity or behavioural quiescence. Secondly, reduced responsiveness to stimuli during the quiescent state. Thirdly, homeostatic regulation of the quiescent state.
Cassiopea rarely swims and instead remains stationary with its bell on a surface, using its pulsating bell to waft water over its upward-pointing tentacles. These pulsing movements become slower at night. Cassiopea is an ideal jellyfish for behavioural tracking because it rarely swims and remains stationary.
To test whether Cassiopea exhibits reduced responsiveness to stimuli during their nighttime quiescent state, the researchers designed an experiment to deliver a consistent arousing stimulus to the jellyfish. They observed that Cassiopea prefer staying on solid surfaces. If Cassiopea are released into the water column, they quickly reorient and move to the bottom of the tank. The researchers used placement into the water column as a stimulus to compare responsiveness during the night versus the day.
Cassiopea were put inside a short PVC pipe with a screen bottom. This was lifted to a fixed height, held for 5 minutes to allow the jellyfish to acclimate, and then rapidly lowered, which placed the jellyfish free-floating into the water column. At night, the jellyfish showed an increase in the time to first pulse and the time to reach the bottom, compared to the day. This increased latency in response to the stimulus indicates that Cassiopea has reduced responsiveness to the stimulus during the night.
To test if the increased latency at night is rapidly reversible, a second drop was initiated within 30 seconds of the first drop. During the night, there was a large decrease in the time to first pulse and time to reach the bottom, after the second drop when compared to the first drop. During the day and night, the time to first pulse and time to the bottom after the second drop were indistinguishable, demonstrating that after perturbation, animals have similar arousal levels during the day and night. These results indicate that Cassiopea has rapidly reversible reduced responsiveness to a stimulus during the night.
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They are less responsive to stimuli during these periods
Jellyfish exhibit a sleep-like state, during which they are less responsive to stimuli. This is evidenced by the fact that when jellyfish are in this state, they take longer to react to stimuli such as being dropped into water or being fed. In one study, jellyfish took 2 seconds to start pulsing during the day, but 6 seconds at night.
The sleep-like state of jellyfish is also homeostatically regulated, meaning that if they are deprived of their quiescent state, they will show decreased activity and responsiveness the next day. This is similar to the grogginess experienced by humans when they are sleep-deprived.
The discovery that jellyfish exhibit a sleep-like state has significant implications for understanding the evolutionary roots of sleep. Jellyfish belong to one of the most ancient animal groups and do not have a brain or a centralised nervous system. Therefore, the finding suggests that sleep may be a basic property of neurons rather than a product of nervous system complexity.
Further research is needed to confirm the existence of sleep in jellyfish and to understand the underlying mechanisms.
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They suffer ill effects if they are kept awake
The Cassiopea jellyfish, also known as the upside-down jellyfish, exhibits a sleep-like state, which is distinguishable by specific behavioural characteristics. Firstly, the jellyfish shows decreased activity and is less responsive to stimuli during the night. Secondly, this state is rapidly reversible, as the jellyfish can be easily roused from it. Finally, the jellyfish suffers ill effects if it is kept awake. When deprived of their slow periods for too long, they become even more inactive and unresponsive the next day, as if they were reeling from a sleepless night.
In one experiment, researchers blasted the jellyfish with jets of water every 20 minutes to keep them awake. After each blast, the animals started pulsing more quickly before dropping back into their inactive state. Following a full night of this treatment, they were less active the next day.
In another experiment, the jellyfish were suspended in water, which they dislike, and their responsiveness was tested. When suspended at night, the jellyfish took longer to start pulsing and reach the bottom of the tank, indicating that they were groggy and had to shake off their sleep before they could react.
These findings suggest that sleep is an essential function for the jellyfish and that sleep is deeply rooted in the biology of all animals, even those without a brain or central nervous system.
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They need extra catch-up sleep if they are sleep-deprived
Jellyfish do need extra catch-up sleep if they are sleep-deprived. Researchers found that if jellyfish are regularly disturbed during their quiescent period, they become more likely to enter a similar state during times when they are normally active.
In one study, jellyfish were kept awake all night by shooting them with cold water every time they tried to slow down. The jellyfish showed slower reaction times the next day, as if they were trying to catch up on lost sleep. This is similar to how humans feel groggy and less alert when we don't get enough sleep.
The researchers also tested the effects of partial sleep deprivation by disturbing the jellyfish for only 6 hours during the night. They found that the jellyfish showed a decrease in activity during the first 4 hours of the following day, indicating that they needed extra sleep to recover from the partial sleep loss.
These findings suggest that jellyfish, like many other animals, require a certain amount of sleep to function optimally and will try to make up for lost sleep if they are deprived. This highlights the importance of sleep for all creatures, even those without a central nervous system.
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Frequently asked questions
Yes, jellyfish do sleep.
Researchers observed that jellyfish exhibit a sleep-like state, displaying decreased activity, reduced responsiveness to stimuli, and homeostatic regulation.
Homeostatic regulation refers to the innate "drive" toward sleep, where the animal needs sleep to function optimally.
Jellyfish have a nerve cell net, which allows them to exhibit sleep-like behaviour.
The reason why jellyfish need to sleep is still unknown. However, it is speculated that sleep is an emergent property of neurons, meaning that nerve cells connected in a network may exhibit sleep-like behaviour.
