Jellyfish are more advanced than people think. A new study from the University of Copenhagen shows that despite having only a thousand nerve cells and no central brain, the learning ability of Caribbean box jellyfish is much more complex than imagined. This discovery changes our fundamental understanding of the brain and may enlighten us about our own mysterious brains.
After more than 500 million years on Earth, there is no doubt about the great evolutionary success of jellyfish. Despite this, we have always thought of them as simple creatures with very limited learning abilities.
It is generally believed that the more developed the nervous system of an animal, the greater its learning ability. Jellyfish and their relatives (collectively known as cnidarians) are thought to be the first living animals to have nervous systems, which are rather simple and lack a central brain.
For more than a decade, neurobiologist Anders Garm has been studying box jellyfish, a group of jellyfish known for being some of the most venomous creatures in the world. But these deadly jellyfish are interesting for another reason: It turns out they're not as simple as once thought. This shakes up our understanding of simple nervous systems.
"It was once thought that jellyfish could only carry out the simplest learning, including habituation learning - the ability to get used to a certain stimulus, such as a continuous sound or a continuous touch. Now, we have found that jellyfish's learning capabilities are much more refined and they can actually learn from their mistakes." Anders Garm, associate professor at the Department of Biology at the University of Copenhagen, said.
One of the most advanced properties of the nervous system is its ability to change behavior based on experience - memory and learning. A research team led by Jan Bielecki and Anders Garm of Kiel University set out to test this ability in box jellyfish. The findings have just been published in the journal Current Biology.
The box jellyfish is one of the most venomous jellyfish in the world. They use their venom to catch fish and large shrimps. The box jellyfish (Tripedaliacystophora) has mild venom and feeds on small copepods.
Box jellyfish don't have a centralized brain like most animals. Instead, they have four parallel brain-like structures with about a thousand nerve cells in each. The human brain has approximately 100 billion nerve cells.
The box jellyfish has twenty-four eyes spread across four brain-like structures. Some of these eyes can form images, giving box jellyfish more sophisticated vision than other types of jellyfish.
To find its way through the gloomy mangroves, Tripedaliacystophora uses its four eyes to look through the water and uses the mangrove canopy to navigate.
Tripedaliacystophora is one of the smallest box jellyfish species, with a body diameter of only about one centimeter. It lives in the Caribbean and central Indo-Pacific.
Unlike many jellyfish species, the male jellyfish of Tripedaliacystophora uses his tentacles to capture the female during mating. The female's eggs are then fertilized in their intestinal system, where they develop into larvae.
The scientists studied the Caribbean box jellyfish (Tripedaliacystophora), a fingernail-sized jellyfish that lives in Caribbean mangrove swamps. Here, they use their powerful visual system, which includes 24 eyes, to hunt tiny copepods in the roots of mangroves. While tree root nets are a great place to hunt, they can also be a dangerous place for mollusk jellyfish.
So when box jellyfish get close to the roots of a mangrove forest, they turn around and swim away. If they turn too fast, they don't have enough time to catch the copepod. But if they turn too late, they risk being hit and damaging their gelatin. Therefore, assessing distance is crucial for them. Researchers found that contrast is key:
"Our experiments show that jellyfish use contrast, the depth of their roots relative to the surface of the water, to assess the distance to the roots, allowing them to swim away at the right time. What's even more interesting is that the relationship between distance and contrast changes every day due to the action of rain, algae and waves," Anders-Gam continues: "We can see that as each day a new hunt begins, the box jellyfish They learn the current contrast by combining visual impressions with the sensations of failed avoidance movements. So even though they have just over a thousand nerve cells (our brain has about a hundred billion nerve cells), they can connect the temporal convergence of various impressions and learn the connections - what we call associative learning, in fact, at about the same rate as advanced animals like fruit flies and mice."
New research results break the previous scientific understanding of animals with simple nervous systems:
"This is huge news for basic neuroscience. It provides a new perspective on what simple nervous systems can do," Anders-Gam said. "This suggests that advanced learning may have been one of the most important evolutionary advantages of nervous systems from the beginning."
Caribbean box jellyfish live and feed underwater in mangrove roots. Image source: Anders Gramm
The researchers replicated mangrove swamp conditions in the laboratory, placing box jellyfish in a behavioral arena. Here, researchers manipulated the behavior of jellyfish by varying contrasting conditions to see what effect this had on their behavior.
They learned that jellyfish learn through failed escapes. That is, they learn by misinterpreting contrast and bumping into tree roots. Here, they learn when to turn by combining the visual impression of hitting a tree root with the mechanical impact.
"Our behavioral experiments demonstrate that three to five failed evasive maneuvers are enough to change the jellyfish's behavior so that they no longer hit tree roots. Interestingly, this is about the same repetition rate required for learning in fruit flies or mice," says Anders-Gam.
Electrophysiology and classical conditioning experiments further validated this learning method and also showed where learning occurs in the jellyfish nervous system.
The scientists also showed where learning occurs in box jellyfish. This now provides them with a unique opportunity to study the precise changes that occur in nerve cells as they participate in advanced learning.
"We hope that this will become a super model system for studying the cellular processes of advanced learning in various animals. We are now trying to determine exactly which cells are involved in learning and memory formation," Anders-Gam said. "This way, we can delve into what structural and physiological changes occur in cells during the learning process."
If the research team can identify the exact mechanism by which jellyfish are involved in learning, the next step will be to find out whether this mechanism is specific to jellyfish or if it is found in all animals.
"Ultimately, we will look for the same mechanism in other animals to see if this is how memory in general works," the researchers said.
Anders-Gam believes that this groundbreaking knowledge could be used for a variety of purposes: "Understanding something as mysterious and extremely complex as the brain is in itself a very remarkable thing. But it also has many unimaginable useful possibilities. A major problem for the future will undoubtedly be the various forms of dementia. I am not claiming that we have found a cure for dementia, but if we can better understand what memory is, which is a core problem in dementia, we may be able to lay the foundation for a better understanding of the disease and perhaps fight it."
The research will be published today (September 22) in the scientific journal Current Biology.