Jellyfish surprise scientists by learning without a brain

Could jellyfish tell us why the ocean is close to our shores, or if they had brains, could help us think about things we’ve never thought about before? Probably, but jellyfish don’t have brains. Instead, they have a simple nervous system distributed throughout their transparent bodies. Because of this, it has long been thought that they are unable to learn beyond a basic level, and research seems to support this idea.

In 2021, biologist Ken Chen wrote a systematic review of learning in cnidarians (the phylum that includes jellyfish, hydra, and sea anemones).He discovered the following important evidence accustomed to This means that the animal can become accustomed to the stimulus. In other words, super basic learning.

Only a few studies have shown the potential for associative learning in sea anemones. In these, the sea anemones were simultaneously shocked and shown light. Eventually, the animals learned to withdraw when exposed to light, even if they were not shocked at the same time. This is classical conditioning, suggesting that the sea anemone formed a memory and adapted its behavior accordingly.

But there are concerns. Scientists rarely encounter shocked sea anemones in the wild, so it’s not entirely clear whether these studies demonstrated learning that helps them survive, or whether they simply induced unnatural behavior in the organisms. .

To find out if and how cnidarians learn, researchers at Kiel University and the University of Copenhagen created a more natural school for Caribbean box jellyfish. Their results cast doubt on the idea that advanced learning requires the brain.

jellyfish school

For a blueberry-sized predator with no brain or spine, the Caribbean box jellyfish is pretty good. They spend their days swimming in the sunlit tropical waters, among the roots of mangrove tree supports. These roots provide protection when hunting small crustaceans called copepods, which are the jellyfish’s favorite prey.

It all sounds idyllic, but the natural world is always full of dangers. One such danger is the roots themselves. If the box jellyfish’s fragile body collides with the roots, it can cause injury. Weather also poses risks. Silt and other particles are stirred up, making the water cloudy and inhibiting the box jelly’s ability to see the roots of the stake and move safely. (Caribbean box jellyfish is unique among jellyfish in that it has eyes on the bell part. Most other jellyfish can only sense light and darkness.)

To maximize your animal’s potential, it’s best to utilize natural behaviors that make sense for your animal.

Jan Bielecki

The researchers wanted to determine whether the box jellyfish learn to avoid the roots of the supports, or whether they have to unconsciously lick the roots. To test this, they brought the jelly to the lab and using a round tank he created three experimental conditions.

The first tanks had high-contrast black and white stripes. This condition was intended to simulate days of clear water when the propeller roots were easily visible. The second tank also included stripes, but in a low-contrast color to mimic a dark day. The final tank walls were uniformly gray.

The researchers’ goal was to create conditions similar to those that box jellyfish actually encounter in the wild, rather than situations where cnidarians would have close encounters with third species jellyfish.

“Learning is the pinnacle of nervous system performance,” said Jan Bielecki, lead author of the study and postdoctoral researcher at Kiel University. “To maximize an animal’s potential, it is best to utilize natural behaviors that make sense for the animal.”

learning is easy

As it turns out, these “brainless” creatures can be studied right away. In the low-contrast bucket, Jelly initially hit the wall, but in less than 8 minutes she began swimming an average of 50% further. She also quadrupled the number of quick turns to avoid collisions.

Inside the high-contrast bucket, the jellyfish was able to completely avoid the walls by sticking to the center. In the gray bucket, on the contrary, they were constantly ringing the bell. Taken together, these results suggest that box jellyfish began to associate turbid stripes with collisions and adjusted their behavior accordingly.

In short, they learned.

“We find that each time a new hunting day begins, box jellyfish learn from the current contrast by combining visual impressions and sensations during unsuccessful avoidance maneuvers,” said one of the study’s lead authors. said Anders Galam, associate professor of marine biology. The University of Copenhagen told Genetic Engineering & Biotechnology News.

He added: “So even though there are only 1,000 neurons, [per eye-bearing structure] — We have about 100 billion brains that can connect the temporal convergence of different impressions and learn associations — so-called associative learning. ”

Specifically, this is a type of associative learning known as operant conditioning. This advanced learning occurs when organisms learn to associate voluntary actions with stimuli and outcomes. A classic example is a lab mouse that is taught to press a blue button to get a treat and avoid a red button to give a zap.

The researchers state in their study:[This] This suggests the intriguing possibility that high-level neural processes such as operant conditioning are fundamental properties of all nervous systems. ” It’s not just centralized brain-based.

A photo of a Caribbean box jellyfish and a close-up image of its eyes.

Enlarged image of the visual system of the Caribbean box jellyfish. Each Rhopalium (B) houses 6 eyes of his 4 types. They are the lower lens eye (LLE), upper lens eye (ULE), pit eye (PE), and slit eye (SE). It also contains photosensitive neuropils (NPs). (Credit: Jan Bielecki et al., Current Biology, 2023)

In the 24 eyes of the beholder

To understand how box jellyfish learn without a brain, researchers tested the rhopalia, a sensory structure in the box jellyfish’s bell. There are four such structures in the adult box jelly, each of which contains her six eyes. These structures also generate “pacemaker signals” that control the jelly’s pulsating motion and frequency spikes when it avoids obstacles.

The researchers placed isolated rhopariums in Petri dishes facing a screen. Images showing moving bars of different contrasts were projected onto the screen, similar to the tank experiment. During the pre-trial run, rhoparium did not respond to gray or light gray bars. This is likely because Rhopalium interpreted them as being far away. However, the pacemaker signal in the dark gray bar was generated.

During the test, the researchers trained Rhopalium by delivering a small electric shock when a colored bar appeared on the screen. Within 5 minutes of the test, the rhoparium began to generate pacemaker signals in response to gray bars and even light gray bars.

These results suggest that the rhopal nervous system is the learning center of the Caribbean box jellyfish, and that this species learns from a combination of visual and mechanical stimuli.

“Our behavioral experiments show that three to five failed avoidance maneuvers are enough to change the jellyfish’s behavior and avoid hitting the roots. What’s interesting is that “That’s about the same repetition rate that fruit flies and mice need to learn,” Garm said.

The researchers published their findings in the peer-reviewed journal Current Biology.

This finding also has implications for our understanding of the evolution of learning.

Relearning our understanding of learning

In future studies, the researchers hope to determine which cells precisely control the box jellyfish’s ability to learn, and how those cells translate that information into behavior. There’s also the question of how jellyfish form memories and how long they retain them.

The study also raises the question of whether more natural studies will demonstrate similar results among other cnidarians.

“This is only the third time that associative learning has been convincingly demonstrated in cnidarians,” Chen, who was not involved in the study, told The New York Times. “And this is the coolest demonstration packed with physiological data.”

This finding also has implications for our understanding of the evolution of learning. They suggest that associative learning may be a property of all nervous systems, not just those concentrated around the brain. It has the potential to disrupt how much and how far back learning has shaped our shared evolutionary history.

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