Can a jellyfish tell you why the ocean is close to shore, or think about something that has never been there before – if it only had a brain?Perhaps, but jellyfish don't have brains. Instead, they have simple nervous systems scattered in a transparent body. For this reason, it has long been believed that they are not capable of learning beyond the basic level Xi, and research seems to support this view.
In 2021, biologist Ken Cheng wrote a systematic review of cnidarian Xi – the phylum cnidarian is made up of jellyfish, hydra and anemone. He found plenty of evidence that these animals Xi habitual, meaning they can Xi to stimulus-accustomed. In other words, super basic Xi.
Only a few studies have shown the potential of anemone associations Xi. In these **, the anemone was shocked and at the same time was shown a lamp. Over time, even if they are not subjected to electric shocks at the same time, the animal will retract its body when the light appears. This is a classic conditioned reflex that does indicate that the anemones form memories and adjust their behavior accordingly.
But there is one concern. Because anemones rarely encounter shocked scientists in the wild, it's not entirely clear whether these studies demonstrate Xi that contributes to survival, or simply induce unnatural behavior in organisms.
To find out if and how cnidarians learn Xi, researchers at Keele University and the University of Copenhagen have created a more natural school for the Caribbean box jellyfish. Their findings challenge the belief that advanced Xi need brains.
For a brainless, thornless, blueberry-sized predator, the Caribbean box jellyfish is pretty good. They spend their days swimming in the sun-drenched tropical waters, among the pillars and roots of the mangroves. These roots provide protection as jellyfish prey on their chosen prey, which are tiny crustaceans known as copepods.
It all sounds idyllic, but as always, in nature, danger abounds. One of the dangers is the root itself. If the fragile body of a box jellyfish collides with its roots, it may harm the creature. Weather also poses risks. It churns up silt and other particles, muddying the water, inhibiting the boxed jelly's ability to see the roots of the pillars and navigate safely. (Caribbean box jellyfish are unique among jellyfish because their eyes are on bells.) Most other jellyfish can only perceive light and darkness.
The researchers wanted to determine if the box jellyfish had learned to avoid the pillars or had to lick them blindly. To test this, they brought jelly into the lab and created three experimental conditions using round jars.
The first tank received a high-contrast black and white stripe. This situation is designed to simulate a day of clear water, in which the pillars are easily visible. The second tank also contains stripes, but with low color contrast to simulate a gloomy day. The last tank has uniform gray walls.
The researchers' goal was to create conditions similar to those that box jellyfish actually encountered in the wild, rather than cnidarians that come into close contact with a third species of jellyfish.
Xi is the pinnacle of neurological performance," said Jan Bielecki, first author of the study and a postdoctoral researcher at Keele University. "It's best to take advantage of its natural behavior, which makes sense for the animal, so that it can reach its full potential.
It turns out that these "brainless" creatures are studied quickly. In the low-contrast bucket, the jellies initially collide with the wall, but in less than 8 minutes, they begin to swim an average of 50% farther away. They also tripled the number of quick turn maneuvers to avoid collisions.
In a high-contrast barrel, the jellyfish manages to avoid the wall altogether by sticking to the center. Instead, in gray barrels, they constantly ring their bells. Taken together, these results suggest that box jellyfish are beginning to associate cloudy streaks with collisions and adjust their behavior accordingly.
In short, they learned.
We can see that as each day of the hunt begins, the box jellyfish learns from the current contrast by combining visual impressions and sensations, Xi in the failed evasive action," said one of the study's lead authors.
1. Anders Garam, an associate professor of marine biology at the University of Copenhagen, told Genetic Engineering & Biotech News
"So, despite having only 1,000 nerve cells (each eye structure) – and our brain has about 100 billion of them – they can connect the temporal convergence of various impressions and learn Xi connections – or what we call associative Xi," he added.
Specifically, this is an associative Xi called operant conditioning. This advanced Xi occurs when an organism learns to associate voluntary behavior with a stimulus or outcome. A typical example is a lab mouse being taught to press the blue button for hospitality and avoid pressing the red button to stimulate it.
As the researchers noted in their study: "[This] suggests an interesting possibility that higher neuronal processes, such as operant conditioning, are fundamental properties of all nervous systems." And not just those based on a centralized brain.
To tease out how the box jellyfish learned to Xi without a brain, the researchers tested the creature's rhopalia – a sensory structure located on a box jellyfish bell. An adult boxed jelly will have four such structures, each with six eyes. These structures also generate "pacemaker signals" that control the jelly's impulse motion and frequency peaks when avoiding obstacles.
The researchers place the isolated rhopalium in a Petri dish to face the screen. The image projected on the screen shows moving bars of different contrasts, similar to tank experiments. Before the trial run, rhodium did not react to gray or light gray bars, it seems that because it interprets them as distant. It does, however, generate a pacemaker signal for the dark gray bar.
During the trial, the researchers trained the iridium by giving it a small electric shock when any colored bar appeared on the screen. Within five minutes of the test, the rhodium began to produce a pacemaker signal in response to a gray bar or even a light gray bar. These results suggest that the red-bellied nervous system is the Xi center of the Caribbean box jellyfish, a species that combines visual and mechanical stimuli to learn Xi.
Our behavioral experiments have shown that three to five failed evasion maneuvers are enough to change the behavior of jellyfish so that they no longer hit the roots. Interestingly, this is about the same repetition rate that fruit flies or rats need to learn to Xi," Garm said.
The researchers published their findings in the peer-reviewed journal Current Biology.
In future research, the team hopes to determine which cells precisely control the learning and Xi abilities of box jellyfish and how these cells translate this information into behavior. There is also the question of how jellyfish form memories and how long they will remain.
The study also raises the question of whether more natural studies will demonstrate similar results in other cnidarians.
This is only the third time that association Xi has been convincingly demonstrated in cnidarians," Cheng, who was not involved in the study, told The New York Times. "It's the coolest demo, full of physiological data.
These findings also have important implications for our understanding of the evolution of learning and Xi. They argue that association Xi may be an attribute of all nervous systems, not just those centered on the brain. This has the potential to shake the extent and how long the Xi have shaped our shared evolutionary history.