Linda Crampton is a writer and experienced science teacher with an honors degree in biology. She enjoys writing about science and nature.
Sleep: An Interesting and Mysterious State
Sleep often seems like a mysterious state, even to some scientists. Until recently, it was thought that sleep was a function of the brain and was experienced only by the animals that possess this organ. In recent years, researchers have discovered that states resembling sleep occur in some species without a brain or without the simpler but somewhat similar cerebral ganglia. The sleepers include at least some species of jellyfish. Though some researchers prefer to call the condition in jellyfish sleep-like instead of sleep, others disagree with this terminology. Whatever term is used to describe the quiescence, it’s a fascinating phenomenon.
The so-called “true” jellyfish belong to the phylum Cnidaria and are the subject of this article. They are not the only invertebrates that exhibit sleep. I chose to highlight a cnidarian because the members of the phylum have the simplest nervous system of those animals known to enter a sleep-like state. That being said, there is some evidence that sponges (phylum Porifera), which are even simpler organisms, also sleep.
More research needs to be performed to confirm the idea that sponges sleep. Unlike jellyfish, other cnidarians, and more complex animals, sponges have no neurons, or nerve cells. They do detect certain changes in their environment and respond appropriately, however. This implies that critical information is being transmitted through the cells in at least part of their body, even though the animals don’t have specialized cells (neurons) to do this.
An Introduction to Jellyfish
Jellyfish belong to the class Scyphozoa within the phylum Cnidaria. (Box jellyfish belong to the class Cubozoa and aren’t discussed in this article.) Scyphozoans have a bell or umbrella-shaped body and tentacles around their mouth. The mouth is located under the bell at the end of a stalk called the manubrium. The contraction and relaxation of the muscles in the bell produces a pulsing motion, which enables the animal to move.
The tentacles contain cells called cnidocytes. These in turn contain nematocysts, or stinging capsules, which are used to catch prey. A nematocyst fires a structure resembling a harpoon into the prey. The harpoon injects venom into the animal. Some species of jellyfish have wider oral arms in addition to tentacles. These help to manipulate the prey.
The mouth leads to a gastrovascular cavity, where digestion occurs and the digested food is absorbed. The mouth also serves as an anus. Jellyfish have neither a respiratory nor a circulatory system. Gases diffuse into and out of their body. The pulsing of the bell helps to push fluid around the body.
Jellyfish are usually male or female. Their gonads produce sperm or eggs, which are released into the water through their mouth. When a sperm and egg meet, a larva is formed. The larva attaches itself to the ocean floor and produces a polyp. The polyp has a stalk and tentacles surrounding a mouth at the top. It is able to catch prey and eat. When a polyp is mature, it undergoes a process known as budding. Each bud is released and becomes a new jellyfish. The illustration below shows the process pictorially.
Structure and Function of a Neuron
The nervous system controls the behaviour of an animal, including its ability to sleep. Though jellyfish have a much simpler nervous system than us, their system also has some important similarities to ours, including the possession of neurons.
A neuron or nerve cell conducts an electrical impulse from a sensory structure (or from another neuron) to the target structure (or to another neuron in the chain). The impulse ends at the target structure, which produces a response as a result of the stimulation. The current in a wire consists of moving electrons. In a neuron, it consists of flowing ions, or charged atoms. The moving ions create an action potential, or a change in electric potential, as they flow along the cell membrane. A nerve consists of a bundle of neurons.
The basic structure of a neuron is shown below. Different types of neurons exist, and they have some variability in structure, but the general function of each of them is the same. In the version shown below, the current flows from the dendrites, through the cell body (which contains the nucleus of the cell), and then along the axon towards the target structure.
A neuron doesn’t stimulate the next structure in the chain directly. There is a tiny gap between one neuron and the next one or between the neuron and a muscle or gland. The region is called a synapse. Chemicals called neurotransmitters control the passage of the nerve impulse across the synapse. A variety of neurotransmitters exist in humans and in jellyfish, including some that are identical in each organism.
The illustration of a neuron shown above is not drawn to scale. The axon may be much longer than the one shown. The illustration represents a human neuron. The various types of neurons in an organism and neurons in different types of organisms may have slightly different appearances, but they follow the same general pattern.
Nature of Cerebral Ganglia
Like us, the invertebrates known to sleep have sensory structures, neurons, and target structures in their nervous system. They lack the highly specialized part of the system called the brain. Some have a cerebral ganglion (or connected cerebral ganglia) in their head, however. A ganglion is a group of cell bodies.
The cerebral ganglion structure is not as complex or as capable as a vertebrate brain, but it does have some useful and brain-like abilities, especially in some species. In these species, the structure is well developed and is sometimes referred to as a brain. Jellyfish are simpler. They have neurons and nerves in their body, but they have no cerebral ganglia and no brain. They do have some differentiation in their nervous system, however, as described below.
Diffuse Nervous System With Condensations
The body of a jellyfish has a diffuse nervous system, or one in which nerves or neurons are distributed over a wide area and form a net. Within a net, there are places where nerves or neurons are concentrated in a small area, as the quote below points out. The fact that the nervous system is not completely homogeneous could be very significant with respect to the animal’s behaviour. The condensed areas containing nerve fibres close together are not brains or cerebral ganglia, but they may allow a jellyfish to behave in a more complex manner than might be expected when examining the rest of their nervous system.
A special division of the nervous system is linked to the rhopalia. These are small sensory structures on the edge of the bell. The rhopalia contain pigmented eyespots called ocelli that detect light. They also contain statocysts (or statoliths) that respond to movement of the animal in relation to gravity. The rhopalia are connected via nerves.
Although their nervous system is relatively simple, a common misunderstanding is that all jellyfish have only a diffuse nerve net in which neurons are found homogeneously spread apart. In fact, most jellyfish species show some degree of neuronal condensation that serves as an integrative nervous system.
— Takeo Katsuki and Ralph J. Greenspan, Current Biology journal
The Exploration of Sleep in Cassiopea
Cassiopea is a genus of jellyfish. The species in the genus are sometimes known as upside-down jellyfish because they swim with the upper surface of their bell directed towards the sea bed and their tentacles towards the water surface. Like their relatives, they move by means of the pulsing motion of their bell.
In their report, researchers from the California Institute of Technology said that they were looking for the following observations in their experiment involving the possibility of sleep in Cassiopea.
“Three behavioral characteristics define a sleep state (1) behavioral quiescence, a period of decreased activity; (2) reduced responsiveness to stimuli during the quiescent state; and (3) homeostatic regulation of the quiescent state. Both behavioral quiescence and reduced responsiveness must be rapidly reversible to differentiate sleep-like states from other immobile states (e.g., paralysis or coma), and reduced responsiveness distinguishes sleep from quiet wakefulness. Homeostatic regulation results in a rebound response, i.e., a compensatory period of increased sleep after sleep deprivation.”
Researchers found that in the laboratory the test species of Cassiopea entered a quiescent period every night. During this period, jellyfish in the species either didn’t respond to a sensory stimulus that would normally cause them to react or required a stronger than normal one to change their behaviour. If the researchers repeatedly “woke the animal up,” it required more sleep than normal afterwards. The three requirements for a state of sleep mentioned above seem to have been met.
The effect of melatonin on the jellyfish was another interesting discovery. Melatonin is a chemical made by many animals. In mammals, it‘s a hormone that regulates the sleep-wake cycle and can promote sleep. When the researchers gave their jellyfish melatonin, they became quiescent, even during the middle of the day. Pyrilamine is an antihistamine used by humans. It’s known to cause drowsiness in us. It also caused the jellyfish to become quiescent. The fact that the chemicals cause similar effects in humans and jellyfish suggests that there may be similarities in the mechanism of sleep in the two species.
Many species of jellyfish exist. Only a few of them have been studied in the laboratory. Other species of jellyfish may sleep besides the ones studied in the experiment described above, but as far as I know this hasn’t been explored or at least hasn’t been publicized.
Life Without a Brain
Jellyfish are very successful creatures. They are a diverse, abundant, and widespread group of animals. Some people might wonder how an animal can get through life without a brain or at least without cerebral ganglia in control of their body. Jellyfish and simpler animals show that it’s possible for an organism to thrive under these conditions. As long as an animal’s body has the equipment to detect vital stimuli and a mechanism that responds appropriately to the stimuli, life is possible.
Though jellyfish don’t have a brain, their neurons share many features with ours. It may sound primitive to us to exist without a brain, but we don’t completely understand how a jellyfish processes information. It responds to changes in its environment, and as the quote from the scientist below says, it tries to avoid harmful stimuli. Whatever is going on in its nervous system is helpful for the animal.
I think sometimes people use its lack of a brain to treat a jellyfish in ways we wouldn’t treat another animal… they certainly have aversion to things that cause them harm.
— Rebecca Helm, Woods Hole Oceanographic Institute, via PBS
Further Exploration of Sleep in Invertebrates
One researcher mentioned in the first reference below says ”I think if it’s alive, it sleeps.” On the other hand, some researchers believe that we should continue to use the term “sleep-like” for the quiescent stage in invertebrates, at least until we learn more about the phenomenon. I hope that further research is soon performed. The experiment described in this article seems to be well respected, but it would be interesting to see what other scientists discover,
It’s hard to know exactly what another individual is experiencing, especially when that individual belongs to a different species from us. I think the information about invertebrate sleep obtained so far is intriguing, however. I’m looking forward to hearing more about the behaviour of jellyfish and in learning whether sleep-like behaviour is exhibited by simpler animals than them.
- Sleep in different species from Science Mag, AAAS, or the American Association for the Advancement of Science (This article contains an interesting table comparing sleep in different organisms.)
- Facts about jellyfish from the Smithsonian National Museum of Natural History
- Jellyfish nervous system from Current Biology journal/cell.com
- “No Brain? For Jellyfish, No Problem” from PBS (Public Broadcasting Service)
- A researcher discusses jellyfish sleep with a Science Friday interviewer (Click on “Segment Transcript” for more information.)
- The scientific report about jellyfish sleep from the Current Biology journal
This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.
© 2021 Linda Crampton