2023-10-01 15:08:59
Researchers at the University of Copenhagen have discovered that Caribbean box jellyfish, previously thought to be simple creatures, possess advanced learning abilities despite having a basic nervous system. Credit: Jan Bielecki
Jellyfish are more advanced than previously thought. A new study from the University of Copenhagen has demonstrated that Caribbean box jellyfish can learn at a much more complex level than ever imagined, despite only having around a thousand nerve cells. and the absence of a centralized brain. This discovery changes our fundamental understanding of the brain and might shed light on our own mysterious brains.
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 creatures with very limited learning capabilities.
The prevailing view is that a more advanced nervous system equates to more advanced learning potential in animals. Jellyfish and their relatives, collectively known as cnidarians, are considered the first living animals to develop a nervous system and to have a fairly simple nervous system and no centralized brain.
For more than a decade, neurobiologist Anders Garm has studied box jellyfish, a group of jellyfish commonly known to be one of the most venomous creatures in the world. But these killing frosts are interesting for another reason: It turns out they’re not as simple as we once thought. And it shakes up our entire understanding of what simple nervous systems are capable of.
A Caribbean box jellyfish. The black dots embedded in the bottom of the bell constitute the visual, sensory and learning center of the animal called rhopalia. Credit: Jan Bielecki
“It was once thought that jellyfish might only handle the simplest forms of learning, including habituation, that is, the ability to become accustomed to a certain stimulation, such as a sound or touch constant. We now see that jellyfish have a much more refined capacity for learning and can actually learn from their mistakes. And in doing so, change their behavior,” says Anders Garm, associate professor at the Department of Biology at the University of Copenhagen.
One of the most advanced attributes of the nervous system is the ability to change behavior as a result of experience – to remember and learn. The research team, led by Jan Bielecki of the University of Kiel and Anders Garm, decided to test this ability in 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 venom to catch fish and large shrimp. Tripedalia cystophora has a somewhat milder venom and feeds on tiny copepods. Box jellyfish do not have a centralized brain like most animals. Instead, they have four parallel brain-like structures, each containing regarding a thousand nerve cells. A human brain has approximately 100 billion nerve cells. Box jellyfish have twenty-four eyes distributed among their four brain-like structures. Some of these eyes form an image, giving box jellyfish more complex vision than other types of jellyfish. To make their way through the dark mangroves, four of them Tripedalia cystophora eyes look up through the water surface and navigate using the mangrove canopies.
Tripedalia cystophora is one of the smallest box jellyfish species, with a body only regarding a centimeter in diameter. It lives in the Caribbean Sea and the central Indo-Pacific. Unlike many species of jellyfish, Tripedalia cystophora in fact, it mates when the male captures 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 capable than previously thought
Scientists have studied the Caribbean box jellyfish, Tripedalia cystophora, a fingernail-sized jellyfish that lives in the mangroves of the Caribbean. Here, they use their impressive visual system including 24 eyes to hunt for tiny copepods among the roots of mangroves. While providing a good hunting ground, the network of roots is also a dangerous place for soft-bodied jellies.
So when the little box jellyfish approach the roots of the mangrove, they turn around and move away. If they turn too early, they won’t have enough time to catch copepods. But if they turn around too late, they risk bumping into the root and damaging their gelatinous bodies. So, assessing distances is crucial for them. And here, contrast is key, as the researchers discovered:
“Our experiments show that contrast, that is, the darkness of the root relative to the water, is used by jellyfish to judge distances to the roots, which allows them to move away at the right time. What is even more interesting is that the relationship between distance and contrast changes daily due to rainwater, algae and wave action,” explains Anders Garm, who continues:
“We can see that as each new hunting day begins, box jellyfish learn from current contrasts by combining visual impressions and sensations during unsuccessful avoidance maneuvers. So although they only have a thousand nerve cells – our brain has regarding 100 billion – they can link the temporal convergences of various impressions and learn a connection – or what we call associative learning. And they learn as quickly as advanced animals like fruit flies and mice.
The new research findings break with previous scientific perceptions of what animals with simple nervous systems are capable of:
“For fundamental neuroscience, this is very big news. It offers a new perspective on what can be done 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. Credit: Anders Gram
How they did it
The researchers replicated mangrove conditions in the laboratory, where box jellyfish were placed in a behavioral arena. Here, 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 hitting the roots. Here, they combined the visual impression and the mechanical shock they felt every time they hit a root – and, in doing so, they learned when to move away.
“Our behavioral experiments demonstrate that three to five failed avoidance maneuvers are enough to modify the behavior of jellyfish so that they no longer touch the roots. Interestingly, this is regarding the same repetition rate that a fruit fly or a mouse needs to learn,” explains Anders Garm.
The learning was then verified through electrophysiology and classical conditioning experiments, which also showed where learning takes place in the jellyfish’s nervous system.
In search of the brain cells where memory is located
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 this can become a model system for studying cellular processes in advanced learning in all kinds of animals.” We are currently trying to identify exactly which cells are involved in learning and memory formation. By doing so, we will be able to observe what structural and physiological changes occur in cells as learning occurs,” explains Anders Garm.
If the research team can identify the exact mechanisms involved in learning in jellyfish, the next step will be to find out whether this applies specifically to jellyfish or whether it can be 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.
This type of revolutionary knowledge might be used for many purposes, according to Anders Garm:
“Understanding something as enigmatic and extremely complex as the brain is in itself an absolutely incredible thing. But there are an unimaginable number of useful possibilities. The various forms of dementia will undoubtedly be a major problem in the future. I am not claiming that we find the cure for dementia, but if we can better understand what memory is, which is a central problem in dementia, we may be able to lay the foundations for a better understanding of the disease and perhaps counteract it,” concludes the researcher.
The study will be published today (September 22) in the scientific 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.
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