Jellyfish are More Advanced Than Previously Thought

The enormous evolutionary success of jellyfish is clear after more than 500 million years on Earth. Nonetheless, they were always seen as primitive creatures with limited learning ability.

According to popular belief, more complex nervous systems in animals correspond to more advanced learning abilities. Jellyfish and their relatives, also known as cnidarians, are thought to be the first living organisms to acquire nervous systems, with very primitive nervous systems and no centralized brain.

For more than a decade, neurobiologist Anders Garm has been studying box jellyfish, a type of jellyfish notorious for being among the deadliest in the world. But these poisonous jellies are also intriguing for another reason: it turns out that they are not as simple as previously thought. And this fundamentally alters the idea of what rudimentary nervous systems are capable of.

It was once presumed that jellyfish can only manage the simplest forms of learning, including habituation—i.e., the ability to get used to a certain stimulation, such as a constant sound or constant touch. Now, we see that jellyfish have a much more refined ability to learn, and that they can actually learn from their mistakes. And in doing so, modify their behavior.”

Anders Garm, Associate Professor, Department of Biology, University of Copenhagen

The ability of a brain’s nervous system to alter behavior as a result of experience—to remember and learn—is one of its most advanced features. Box jellyfish were chosen as the subject of this experiment by the study team, which was led by Jan Bielecki of Kiel University and Anders Garm. The research results were just released in the journal Current Biology.

A Thousand Nerve Cells are More Capable Than Once Thought

The medusa, Tripedalia cystophora, which is about the size of a fingernail and inhabits the mangrove swamps of the Caribbean, was the subject of the researchers’ study. Here, they search amid the mangrove roots for microscopic copepods using their remarkable visual system, which includes 24 eyes. The network of roots is an excellent hunting area, but it is also deadly for soft-bodied jellies.

The little box jellyfish turn and swim away when they get close to the mangrove roots. They won't have enough time to grab any copepods if they turn too quickly. But if they turn too late, they run the danger of injuring their gelatinous bodies by running into the root. As a result, determining distances is essential for them. The researchers found that contrast is crucial here:

Garm added, “Our experiments show that contrast, i.e., how dark the root is in relation to the water, is used by the jellyfish to assess distances to roots, which allows them to swim away at just the right moment. Even more interesting is that the relationship between distance and contrast changes on a daily basis due to rainwater, algae and wave action.”

“We can see that as each new day of hunting begins, box jellyfish learn from the current contrasts by combining visual impressions and sensations during evasive maneuvers that fail. So, despite having a mere one thousand nerve cells – our brains have roughly 100 billion—they can connect temporal convergences of various impressions and learn a connection—or what we call associative learning. And they actually learn about as quickly as advanced animals like fruit flies and mice.”

How They Did it?

Box jellyfish were placed in a behavioral arena, and the circumstances of a mangrove swamp were mimicked in the lab. In this instance, the researchers changed the contrast settings to alter the behavior of jellyfish and observe the results.

They discovered that unsuccessful evasion attempts are how jellyfish learn. In other words, they pick up knowledge via misreading contrast and running into roots. Here, they coupled the mechanical stress and visual shock they experienced every time they ran into a root, learning when to divert off course.

Garm stated, “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 the roots. It is interesting that this is roughly the same repetition rate that a fruit fly or mouse needs to learn.”

Electrophysiology and classical conditioning tests were used to further confirm the learning and to pinpoint the location of the learning process in the jellyfish nervous system.

The findings of this new study contradict earlier scientific theories about the capabilities of animals with basic nervous systems.

“For fundamental neuroscience, this is pretty big news. It provides 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 benefits of the nervous system from the very beginning,” Garm added.

Seeking the Brain Cells Where Memory is Housed

The study team has also identified the areas in these box jellyfish where learning takes place. They now have the opportunity to analyze the specific changes that take place in a nerve cell as it engages in advanced learning due to these rare possibilities.

“We hope that this can become a supermodel system for looking at cellular processes in the advanced learning of all sorts of animals. We are now in the process of trying to pinpoint exactly which cells are involved in learning and memory formation. Upon doing so, we will be able to go in and look at what structural and physiological changes occur in the cells as learning takes place,” Garm stated.

About Tripedalia Cystophora

• Box jellyfish are a species of jellyfish that is notorious for being one of the deadliest organisms on the planet. They grab fish and huge shrimp with their poison. Tripedalia cystophora’s venom is milder and it feeds on small copepods
• Box jellyfish, unlike other animals, do not have a centralized brain. Instead, they have four parallel brain-like structures, each of which contains around a thousand nerve cells. There are around 100 billion nerve cells in the human brain
• The box jellyfish has twenty-four eyes spread over four brain-like structures. Box jellyfish have more complicated eyesight than other forms of jellyfish because some of their eyes are image generating
• Four of Tripedalia cystophora’s eyes stare up through the surface of the water and navigate utilizing the mangrove canopies to make their way through murky mangroves
• Tripedalia cystophora is one of the tiniest box jellyfish species, having a body diameter of less than one centimeter. It can be found in the Caribbean Sea and the Central Indo-Pacific
• Tripedalia cystophora, unlike many jellyfish species, mates by the male capturing the female with its tentacles. The eggs of a female are then fertilized in their gastrointestinal system, where they grow into larvae

If the scientists can isolate the precise processes in jellyfish involved in learning, the next step will be to determine whether it is unique to jellies or if it is prevalent in all animals.

Garm noted, “Eventually, we will look for the same mechanisms in other animals, to see if this is how memory works in general.”

According to Anders Garm, this new information could be utilized for a variety of purposes:

Garm concluded, “Understanding something as enigmatic and immensely complex as the brain is in itself an absolutely amazing thing. But there are unimaginably many useful possibilities. One major problem in the future will undoubtedly be various forms of dementia. I don’t claim that we are finding the cure for dementia, but if we can gain a better understanding of what memory is, which is a central problem in dementia, we may be able to lay a building block to better understand the disease and perhaps counteract it.”