Even Jellyfish Sleep (2022)

When Ravi Nath asks people if jellyfish sleep, he finds that everyone thinks they know the answer. Roughly half say yes, and half say no. Some scientists assert that only mammals and birds could be said to truly sleep. Other people think that even plants have something akin to sleep. “Every person we’ve asked has an opinion,” Nath says. “Even a 10-year-old kid has a response.”

Nath has an answer, too. Along with his friends and fellow California Institute of Technology students Claire Bedbrook and Michael Abrams, he put a jellyfish called Cassiopea through a gauntlet of clever experiments, which confirmed that they do indeed enter a sleeplike state. Every night, they become less active and less responsive. They can be easily roused from this state, but if they’re deprived of their slow periods for too long, they become even more inactive and unresponsive the next day—as if they were reeling from an all-nighter. And if the trio are right, their discovery has big implications for understanding both how sleep evolved—and why.

Sleep is widespread across the animal kingdom. Fish sleep. Flies sleep. Even nematode worms, which Nath studies, sleep. But jellyfish belong to one of the most ancient animal groups, which split off from those other creatures at least 600 million years ago. If they also have a version of sleep, it suggests that the roots of this behavior are more ancient than anyone suspected.

It’s still unclear why exactly animals sleep at all, but there’s no shortage of explanations. Scientists have variously argued that sleep helps individuals to flush toxins from their brains, to consolidate new memories, or to reset their brains for a fresh day of learning. But none of these hypotheses make much sense for a jellyfish because they don’t have brains at all. They just have a nerve net—a loose ring of neurons that runs around the rim of their pulsating bells. “Maybe the drive for this sleep state was something more basic, like conserving energy,” says Bedbrook. “It might be something that’s required if you have a nervous system, regardless of how simple or complex it is.”

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She and her friends started studying Cassiopea in their own apartment, eyeballing the movements of their bells by the light of their iPhones. Cassiopea is an upside-down jellyfish. It rarely swims, and instead sits inverted on a surface, using its pulsating bell to waft water over its upward-pointing tentacles. And these pulsing movements became slower at night. “You can really tell when you look at them at night that they’re less active than they are during the day,” says Bedbrook.

Rather than relying on their eyes, the trio designed an imaging system that would automatically count the jellyfish’s pulses over several days and nights. “The second we had the setup working, we could see the pattern right away,” Bedbrook says. The jellyfish are 30 percent less active at night. They pulse less frequently, and they go through several pauses, 10 to 20 seconds long, where they stop pulsing altogether.

But sleep isn’t just about inactivity. It’s defined by several other criteria—and the trio started checking off every one.

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First, the inactivity must be reversible—if you can’t wake up, you’re more comatose than asleep. And indeed, when the trio roused their jellyfish by offering them a night-time snack, they saw that the animals became just as active as they’d normally be by day.

Second, a sleeping animal should be unresponsive—it should take more effort to rouse them when they’re asleep than when they’re awake. To test for that, the trio would place their jellyfish in a PVC pipe with a screen bottom. By lifting the pipe and then dropping it, they could briefly suspend the animal in mid-water. Cassiopea doesn’t like floating freely, and will typically swim down to a surface. But at night, they were much slower to do so; it’s as if they were groggy after having just woken up.

“This was when things became very convincing to me,” says Bedbrook. “Sometimes when we dropped the jellyfish in the water at night, they wouldn’t pulse. They’d just float to the bottom of the tank. It’s so different to how they respond to the stimulus during the day.” But if they repeated the experiment just 30 seconds later, the jellyfish responded as they would do during the day. “Now they’re awake!” says Abrams.

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Finally, a sleeping animal should suffer if it’s deprived of sleep, and rebound the night after. “If we pull an all-nighter, we’re really tired afterwards,” explains Rath. He and his friends duplicated this effect in their jellyfish by blasting them with jets of water every 20 minutes. After each blast, the animals started pulsing more quickly before dropping back to their inactive state. And after a full night of this horrendous snooze alarm, they were less active the following day. By contrast, the same water jets, delivered during the day when the animals are typically awake, had no such effect.

That’s an impressive set of results. But for Nadine Gravett from the University of Witwatersrand, whose work on sleeping elephants I’ve covered before, it’s not enough. She says that scientists typically study sleep by strapping electrodes to the heads of animals and measuring the electrical activity of their brains. Without that, “they can’t claim that jellyfish have a sleep-like state,” Gravett says. “The most that can be said is that under controlled laboratory conditions, they show distinct periods of immobility—and it’s possible that sleep could happen during these periods.”

But Isabella Capellini from the University of Hull notes that electrical recordings are only used to quantify sleep in mammals and birds, and not in, say, fish or insects. “When studying sleep in such animals, we can only rely on the behavioral definition,” she says.

And based on that definition, the Caltech trio’s experiments “strongly argue for the existence of sleep in jellyfish,” adds Cheryl Van Buskirk from California State University. “It suggests that sleep may not be a product of nervous-system complexity, but rather may be a basic property of neurons. Why excitable cells would require periods of dampened activity is still a mystery, but it wouldn’t surprise me if the answer were to come from studies, like this, of simple nervous systems.”

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Nath gets the skepticism. It’s the same reaction that researchers got when they first started claiming the flies or worms can sleep—something that’s now more accepted. But he’s not saying that jellyfish sleep is exactly the same as human sleep. It’s something more ancient and ancestral. It has many of the same core features but, as an example, there’s no obvious analog of REM sleep—the period where our closed eyes flicker rapidly, our muscles relax, and our brains conjure vivid dreams. Do jellyfish dream of gelatinous sheep? No one can say.

The team’s next move is to look at the jellyfish’s genes. The genes that influence sleep in flies and worms are the same as those that affect the slumbers of mice and humans—and based on a preliminary glance at Cassiopea’s genome, they seem to exist there too. “We’d have to block them in some way to see how that affects their behavior,” says Abrams.

Certainly, the jellyfish seems to respond to the same sleep-inducing chemicals that work on human brains. When the trio dosed their animals with melatonin, a hormone that makes people sleepy, they started pulsing more slowly. And when they treated the jellyfish with an antihistamine—a class of drugs that often makes people drowsy—they also started pulsing more slowly.

“We did try caffeine and we didn’t see anything though,” says Bedbrook.

(Video) Even jellyfish need to sleep

“So not everything works,” adds Abrams. “We wanted to say something about giving coffee to jellies, but it didn’t work out.”

FAQs

How long does a jellyfish sleep? ›

They pulse less frequently, and they go through several pauses, 10 to 20 seconds long, where they stop pulsing altogether. But sleep isn't just about inactivity.

Do jellyfish dream when they sleep? ›

A team of researchers have found that even, the jellyfish, a brainless organism, exhibit sleep-like behaviour.

Are there any animals that don't sleep? ›

Regardless of their preferred mode, bats, elephants, frogs, honeybees, humans and more have something in common: They all sleep. In fact, scientists have yet to find a truly sleepless creature.

Can jellyfish feel pain? ›

Can jellyfish feel pain? Jellyfish don't feel pain in the same way that humans would. They do not possess a brain, heart, bones or a respiratory system. They are 95% water and contain only a basic network of neurons that allow them to sense their environment.

Do jellyfish get tired? ›

The following day and night, the jellyfish exhibited much lower levels of activity than normal, suggesting sleep deprivation. As a bonus, the researchers also showed that jellyfish get sleepy when exposed to melatonin, just as humans do — a hint that their underlying sleep mechanism may be similar to ours.

Does a jellyfish have brain? ›

Jellyfish have no brain!

They have a basic set of nerves at the base of their tentacles which can detect touch, temperature, salinity etc. Since they don't have a brain, they depend on automatic reflexes in response to these stimuli! Catching prey is also a matter of chance.

Can jellyfish see? ›

Their eyes can also sense changes in light. All jellyfish species' eye spots give them these abilities – and in some species, like the box jellyfish, their eye spots are very well-developed so they can see and hunt their prey.

Do animals without brains sleep? ›

We think of sleep as restoring our brains: a time to process memories, cleanse our cells of toxins, and prepare for a new day. But even animals that lack brains need to snooze. Biologists have discovered that, like people, jellyfish hit the hay and have the same trouble we do waking up.

Which animal has pink milk? ›

The milk produced by hippo females is pink in color. This is because two distinct kinds of acids are produced by them. These acids are norhipposudoric acid and hipposudoric acid. The former is orange in color, while the latter is red and also called blood sweat.

Do spiders sleep? ›

The research showed the spiders' overnight movements looked a lot like REM in other species, she said — like dogs or cats twitching in their sleep. And they happened in regular cycles, similar to sleep patterns in humans.

Do trees sleep? ›

According to research, while trees may not sleep in the same way animals do, they do relax their branches during nighttime, which suggests that yes, trees have activity-rest cycles. These cycles can also vary depending on the tree species.

Are jellyfish smart? ›

Jellyfish are not very smart. “They have very simple sensory organs, and no brain to process any information,” says marine biologist Stein Kaartvedt. Which is why it's so interesting that, off the coast of Norway, scientists have observed what they're calling “apparent social behavior” in the helmet jellyfish.

Are jellyfish asexual? ›

Throughout their lifecycle, jellyfish take on two different body forms: medusa and polyps. Polyps can reproduce asexually by budding, while medusae spawn eggs and sperm to reproduce sexually.

Does jellyfish have gender? ›

Jellyfish are usually either male or female (with occasional hermaphrodites). In most cases, adults release sperm and eggs into the surrounding water, where the unprotected eggs are fertilized and develop into larvae.

How many hearts does a jellyfish have? ›

Lacking brains, blood, or even hearts, jellyfish are pretty simple critters. They are composed of three layers: an outer layer, called the epidermis; a middle layer made of a thick, elastic, jelly-like substance called mesoglea; and an inner layer, called the gastrodermis.

Does jellyfish have a heart? ›

Jellyfish don't have brains

They don't have a heart, lungs or a brain either! So how does a jellyfish live without these vital organs? Their skin is so thin that they can absorb oxygen right through it, so they don't need lungs. They don't have any blood so they don't need a heart to pump it.

How do jellyfish live without a heart? ›

They don't have hearts either, but jellyfish do have a very basic set of nerves at the base of their tentacles. These nerves detect touch, temperature, salinity, etc. Sentences in statocysts are weird.

What do jellyfish do during the day? ›

Most jellyfish live near the surface of the ocean. Some jellyfish drop to deeper waters during the day and return near the surface a night to find food. Other jellyfish do the opposite. They spend their days finding food near the surface and drop to deeper water during the night.

Do fishes sleep? ›

While fish do not sleep in the same way that land mammals sleep, most fish do rest. Research shows that fish may reduce their activity and metabolism while remaining alert to danger. Some fish float in place, some wedge themselves into a secure spot in the mud or coral, and some even locate a suitable nest.

How do jellyfish mate? ›

In this species, a male will use his tentacles to grab a female's tentacles; he will then drag her around for a while before pulling her in close to him. Next, he will produce and release a spermatophore (sperm packet), and use his tentacles to pass it to one of the female's tentacles.

Do jellyfish know they are alive? ›

Jellyfish sting for the same reason many sharks bite, they bump into something they think might be food and try and eat it. Are jellyfish conscious? Jellyfish have no brains and therefore are not aware of their own existence. So no, while alive they are not “conscious”.

Do jellyfish hunt humans? ›

Jellyfish sting their prey with their tentacles, releasing a venom that paralyzes their targets. Jellyfish don't go after humans, but someone who swims up against or touches one — or even steps on a dead one — can be stung all the same. While jellyfish stings are painful, most are not emergencies.

How long does a jellyfish live? ›

Jellyfish Lifespan

Most jellyfish live one to three years, but certain species can live from days to decades. Jellyfish have complex life cycles, beginning with fertilized larvae floating in the ocean.

What eats a jellyfish? ›

Among the predators of the jellyfish, the following have been identified: ocean sunfish, grey triggerfish, turtles (especially the leatherback sea turtle), some seabirds (such as the fulmars), the whale shark, some crabs (such as the arrow and hermit crabs), some whales (such as the humpbacks).

Do jellyfish glow? ›

Most jellyfish bioluminescence is used for defense against predators. Jellyfish such as comb jellies produce bright flashes to startle a predator, others such as siphonophores can produce a chain of light or release thousands of glowing particles into the water as a mimic of small plankton to confuse the predator.

What is the deadliest jellyfish? ›

The box jellyfish is the deadliest jellyfish in the world, and quite possibly the deadliest marine creature as well. While they are difficult to avoid, it is best to know the symptoms of a box jellyfish sting in case you or someone around you ever has an unfortunate encounter with the creature.

Do plants sleep? ›

Although plants do not sleep in the same way that humans do, they do have more and less active times and they have circadian rhythms—internal clocks that tell them when it is night and when it is day. And like many people, plants are less active at night.

What animal sleeps the longest? ›

Koalas are the longest sleeping-mammals, about 20–22 hours a day.

What is the simplest animal with a brain? ›

Hydra have the simplest 'brain' in the history of the earth, so we might have a shot at understanding those first and then applying those lessons to more complicated brains,” he says.

Which animal milk is black? ›

Answer: The female black rhinoceros or Diceros bicornis give black milk. Explanation: Black milk is the slimmest milk containing very little amount of fat (0.2%) is produced by Black rhinoceros.

What animals have no blood? ›

Flatworms, nematodes, and cnidarians (jellyfish, sea anemones, and corals) do not have a circulatory system and thus do not have blood. Their body cavity has no lining or fluid within it.

Which animal milk is blue? ›

Hippopotamus gives the milk of blue color. Blue milk, also known as Bantha milk, was a rich blue-colored milk produced by female banthas. Sentients drank it, and also used it in bantha butter, blue bantha buttermilk biscuits, blue-milk cheese, blue milk custard, ice cream, and yogurt.

Do spider like humans? ›

For one thing, spiders tend to avoid people, and have no reason to bite humans because they aren't bloodsuckers and don't feed on humans, Buddle said. "They are far more afraid of us than we are of them," he said. "They're not offensive."

Do spiders drink? ›

Yes, they do drink. And unlike what most people think (including me at one time) they don't drink through their fangs. Their fangs are used to inject venom to subdue prey."

Why do spiders hate light? ›

Spiders are not naturally attracted to light. What gets their attention is the many bugs that tend to surround lighted places. Remember, spiders will seek out any valuable food source. Therefore, they will be in the light as long as there are also insects there.

Do plants feel pain? ›

Elizabeth Van Volkenburgh, all living organisms perceive and respond to painful touch, but plants do not perceive or “feel” pain the same way that animals do because they lack a nervous system and brain.

Do flowers sleep? ›

The truth is flowers never tire like we humans do, they are exhibiting a natural behavior known as nyctinasty. Nyctinasty is a mechanism that causes plant movements in response to the day-night cycle or temperature changes. Plants perform this movement for a variety of reasons.

Why do trees not touch? ›

The most likely theory is that the trees simply do not want to hurt themselves. The phenomenon is often observed in windy areas, leading scientists to assume that the gaps between the crowns make it less likely that branches will collide or get wedged together. This reduces the "risk of injury."

What is the IQ of a jellyfish? ›

Jellyfish have no brains at all, so they do not even have an IQ.

How do jellyfish talk? ›

How does a jellyfish sting? - Neosha S Kashef - YouTube

How long can a jellyfish live out of water? ›

After a while and looking at my watch's timer, I blurted out to the group: “48 minutes.” Now we learned that jellyfishes could survive that long out of sea water.

Can jellyfish change gender? ›

No switching is necessary. Other species of jellyfish – sequential hermaphrodites – are either male and then female, or vice-versa, but not both simultaneously. These are natural transgender jellyfish. No special treatment is necessary.

How many babies can a jellyfish have? ›

Some jellyfish can lay as many as 45,000 eggs in a single night.

How is a jellyfish born? ›

Just like butterflies, which that are born from the transformation of caterpillars, jellyfish are born by asexual reproduction from polyps that – unlike caterpillars – remain alive for many years.

How does a jellyfish get pregnant? ›

There are a few jellyfish species that receive sperm through their mouths to fertilise eggs inside the body cavity, but most jellyfish just release sperm or eggs directly into the water. Under favourable conditions they will do this once a day, usually synchronised to dawn or dusk.

Is jelly fish edible? ›

Most species of jellyfish are edible and are a rich source of nutrients. They have low calories with high antioxidant properties, which prevent cell injury.

What are female jellyfish called? ›

Each individual saucer develops into a tiny jellyfish which separates itself from the stack and becomes free swimming. In a few weeks, they grow into an adult jellyfish, called a medusa. The dominant and conspicuous medusa are either male or female. The reproductive organs (gonads) develop in the lining of the gut.

Do jellyfish have brains? ›

Most animals have some sort of centralized nerve center, aka a brain, although many have only simple versions called ganglia – concentrations of nerves that control other nerves around them. Jellyfish have no such central place; in fact, they have two nervous systems.

Can jellyfish think? ›

Jellyfish have no brain!

Since they don't have a brain, they depend on automatic reflexes in response to these stimuli! Catching prey is also a matter of chance. They don't actively hunt down their food; they just wait for prey to come in contact with their tentacles.

What do jellyfish do during the day? ›

Most jellyfish live near the surface of the ocean. Some jellyfish drop to deeper waters during the day and return near the surface a night to find food. Other jellyfish do the opposite. They spend their days finding food near the surface and drop to deeper water during the night.

Can jellyfish see? ›

Their eyes can also sense changes in light. All jellyfish species' eye spots give them these abilities – and in some species, like the box jellyfish, their eye spots are very well-developed so they can see and hunt their prey.

Does jellyfish have gender? ›

Jellyfish are usually either male or female (with occasional hermaphrodites). In most cases, adults release sperm and eggs into the surrounding water, where the unprotected eggs are fertilized and develop into larvae.

Are jellyfish smart? ›

Jellyfish are not very smart. “They have very simple sensory organs, and no brain to process any information,” says marine biologist Stein Kaartvedt. Which is why it's so interesting that, off the coast of Norway, scientists have observed what they're calling “apparent social behavior” in the helmet jellyfish.

What kills jellyfish? ›

Jellyfish are eaten by seabirds, turtles, and crabs. Grey triggerfish, ocean sunfish, seabirds, turtles, whale sharks, crabs, and whales eat jellyfish naturally. However, the main predators of jellyfish are usually other different types of jellyfish.

What is the IQ of a jellyfish? ›

Jellyfish have no brains at all, so they do not even have an IQ.

What is the smartest jellyfish? ›

The skyward gaze of one set of eyes belonging to box jellyfish provides evidence that these creatures -- which lack a conventional brain -- are capable of sophisticated behavior.

Does jellyfish have a heart? ›

Lacking brains, blood, or even hearts, jellyfish are pretty simple critters. They are composed of three layers: an outer layer, called the epidermis; a middle layer made of a thick, elastic, jelly-like substance called mesoglea; and an inner layer, called the gastrodermis.

How long can a jellyfish live out of water? ›

After a while and looking at my watch's timer, I blurted out to the group: “48 minutes.” Now we learned that jellyfishes could survive that long out of sea water.

Do jellyfish like to be alone? ›

Some species tend to stick together as a group and others live alone. The smaller species of Jellyfish tend to stay around each other for protection from predators. The larger species of Jellyfish though tend to be more isolated in their habitat.

Are jellyfish asexual? ›

Throughout their lifecycle, jellyfish take on two different body forms: medusa and polyps. Polyps can reproduce asexually by budding, while medusae spawn eggs and sperm to reproduce sexually.

What eats a jellyfish? ›

Among the predators of the jellyfish, the following have been identified: ocean sunfish, grey triggerfish, turtles (especially the leatherback sea turtle), some seabirds (such as the fulmars), the whale shark, some crabs (such as the arrow and hermit crabs), some whales (such as the humpbacks).

Do jellyfish glow? ›

Most jellyfish bioluminescence is used for defense against predators. Jellyfish such as comb jellies produce bright flashes to startle a predator, others such as siphonophores can produce a chain of light or release thousands of glowing particles into the water as a mimic of small plankton to confuse the predator.

Videos

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