Researchers at the University of Copenhagen have found that Caribbean box jellyfish, once thought to be simple creatures, have advanced learning abilities despite having a basic nervous system. Photo credit: Jan Bielecki
Jellyfish are more developed than previously thought. A new study from the University of Copenhagen has shown that Caribbean box jellyfish can learn at a much more complex level than ever thought, despite only having about a thousand nerve cells and no central brain. This discovery changes our fundamental understanding of the brain and could shed light on our own mysterious brain.
After more than 500 million years on Earth, the immense evolutionary success of jellyfish is undeniable. Yet we have always thought of them as simple beings with very limited learning abilities.
The prevailing opinion is that a more advanced nervous system corresponds to a higher learning potential in animals. Jellyfish and their relatives, collectively known as cnidarians, are considered to be the first living animals to develop a nervous system and had a relatively simple nervous system and no central brain.
For more than a decade, neurobiologist Anders Garm has been studying box jellyfish, a group of jellyfish widely considered to be one of the most venomous creatures in the world. But these deadly frosts are interesting for another reason: It turns out they’re not as simple as we once thought. And it shakes our entire understanding of what simple nervous systems are capable of.
A Caribbean box jellyfish. The black dots embedded in the underside of the bell form the visual, sensory and learning center of the animal called Rhopalia. Photo credit: Jan Bielecki
“It was previously believed that jellyfish could only cope with the simplest forms of learning, including habituation, the ability to get used to a particular stimulus, such as a sound or a constant touch. We now see that jellyfish have a much finer learning ability and can actually learn from their mistakes. And change their behavior in the process,” says Anders Garm, associate professor in the biology department at the University of Copenhagen.
One of the most advanced properties of the nervous system is the ability to change behavior based on experience – to remember and learn. The research team led by Jan Bielecki from the University of Kiel and Anders Garm decided to test this ability on box jellyfish. The results have just been published in the journal Current Biology.
About Tripedalia cystophora
- Box jellyfish are a class of jellyfish known to be among the most venomous animals in the world. They use their poison to catch fish and large shrimp. Tripedalia cystophora has a slightly milder venom and feeds on tiny copepods.
- Box jellyfish do not have a centralized brain like most animals. Instead, they consist of four parallel, brain-like structures, each containing about a thousand nerve cells. A human brain has around 100 billion nerve cells.
- Box jellyfish have twenty-four eyes distributed among their four brain-like structures. Some of these eyes form an image and give the box jellyfish more complex vision than other jellyfish species.
- To make their way through the dark mangroves, four of the Tripedalia cystophora eyes peer through the surface of the water and use the mangrove canopy to navigate.
- Tripedalia cystophora is one of the smallest box jellyfish species with a body diameter of only about one centimeter. It lives in the Caribbean and the central Indo-Pacific.
- Unlike many species of jellyfish, Tripedalia cystophora actually mates when the male catches the female with his tentacles. The female’s eggs are then fertilized in her intestinal system, where they also develop into larvae.
A thousand nerve cells are more powerful than previously thought
Scientists studied the Caribbean box jellyfish Tripedalia cystophora, a fingernail-sized jellyfish that lives in Caribbean mangroves. Here they use their impressive 24-eye visual system to search for tiny copepods in the roots of mangroves. While the root network provides a good hunting ground, it is also a dangerous place for soft-bodied jellies.
When the small box jellyfish approach the roots of the mangrove, they turn around and move away. If they turn back too soon, they won’t have enough time to catch copepods. But if they turn around too late, they risk hitting the root and damaging their gelatinous bodies. Therefore, estimating distances is crucial for them. And this is where the contrast matters, as the researchers found out:
“Our experiments show that jellyfish use the contrast, i.e. the darkness of the root in relation to the water, to estimate the distance to the roots, which enables them to move away at the right time.” Even more interesting is that the connection “between distance and contrast changes daily due to rainwater, algae and waves,” explains Anders Garm and continues:
“We can see that box jellyfish learn from current contrasts at the beginning of each new hunting day by combining visual impressions and sensations during unsuccessful evasive maneuvers.” So even though they only have a thousand nerve cells – our brain has about 100 billion – they can do the temporal convergences link different impressions and learn a connection – or what we call associative learning. And they learn just as quickly as advanced animals like fruit flies and mice.
The new research results break with previous scientific ideas about what animals with simple nervous systems are capable of:
“This is very big news for basic neuroscience. It offers a new perspective on what can be achieved with a simple nervous system. This suggests that advanced learning may have been one of the most important evolutionary advantages of the nervous system from the beginning,” explains Anders Garm.
The Caribbean box jellyfish lives and feeds among the underwater roots of mangroves. Photo credit: Anders Gram
How they did it
The researchers recreated the mangrove conditions in the laboratory, where box jellyfish were placed in a behavioral arena. The researchers manipulated the jellyfish’s behavior by changing the contrast conditions to see what effect this had on their behavior.
They learned that jellyfish learn through failed escapes. In other words, they learn by misinterpreting contrasts and getting to the bottom of them. Here they combined the visual impression and the mechanical shock they felt every time they hit a root – and learned when to move away.
“Our behavioral experiments show that three to five failed evasive maneuvers are enough to change the behavior of the jellyfish so that they no longer touch the roots.” Interestingly, this is about the same repetition rate that a fruit fly or a mouse has to learn,” explains Different Garm.
The learning was then verified through electrophysiology and classical conditioning experiments, which also showed where learning occurs in the jellyfish’s nervous system.
In search of the brain cells that contain memory
The scientists also showed where learning takes place in these box jellyfish. This gave them unique opportunities to now study the precise changes that occur in a nerve cell when it is involved in advanced learning.
“We hope that this can become a model system for studying cellular processes in advanced learning in all animal species.” We are currently trying to find out exactly which cells are involved in learning and memory formation. This allows us to observe what structural and physiological changes occur in cells during learning,” explains Anders Garm.
If the research team succeeds in identifying the exact mechanisms of learning in jellyfish, the next step will be to find out whether this applies specifically to jellyfish or whether it is found in all animals.
“Ultimately, we will look for the same mechanisms in other animals to see if this is how memory works in general,” specifies the researcher.
According to Anders Garm, this kind of revolutionary knowledge could be used for many purposes:
“Understanding something as mysterious and extremely complex as the brain is in itself an absolutely incredible thing. But there are an incredible number of useful options. The various forms of dementia will undoubtedly pose a major problem in the future. I am not claiming that we will find the cure for dementia, but if we better understand memory, which is a central problem in dementia, we may be able to lay the foundation for a better understanding of the disease and perhaps counteract it. “, concludes the researcher.
The study is published today (September 22) in the journal Current Biology.
The study was led by Jan Bielecki from the University of Kiel and Anders Garm, Sofie Katrine Dam Nielsen and Gösta Nachman from the Department of Biology at the University of Copenhagen.