Box jellyfish show surprising ability to learn without a brain

Creating and storing memories of experiences and changing behavior based on those memories are fundamental functions of the nervous system. This type of associative learning is called operant conditioning and has been established in flies, mice, and, of course, humans. The ability to learn from previous experiences and act in different ways is especially important for survival.

Now, researchers have shown for the first time that a simple box jellyfish has a similar ability, capable of learning from past negative experiences and changing its behavior accordingly.

“This study is the first to convincingly demonstrate operant conditioning in the form of avoidance learning in any animal in the phylum Cnidaria, including jellyfish, box jellyfish, sea anemones, corals and hydras,” Macquarie University researchers Ken Chen said. Australia, which was not involved in the study, also wrote in an email.

Learning without a centralized brain

Cnidarians, the group of animals to which jellyfish belong, are thought to be some of the first living things to have primitive nervous systems. “Because this entire phylum is a sister group to the giant bilateria complex that includes insects, molluscs, and us vertebrates, we know that associative learning is widespread in animals with all kinds of nervous systems. It could mean something,” Chen added.

Tripedalia cystophora, a type of box jellyfish found in the Caribbean, lives in sunny surface waters. When box jellyfish hunt small crustaceans that live among the underwater roots of mangroves, they risk injury to their delicate bodies if they hit the roots.

Although jellyfish do not have a centralized brain, they do have a visual system located in four neuron clusters. Each cluster, called a roparia, has several thousand neurons and her six eyes. In addition to assisting with visual cues, this rhopal nervous system also has a variety of other functions, including acting as a pacemaker for swimming.

Previous research has shown that when box jellyfish visually sense underwater mangrove roots, their swimming pacemaker induces them to exhibit avoidance behavior and quickly move away from the barrier. Jellyfish use the visual contrast between the support roots and the water to determine their distance from obstacles.

In response to visual stimuli, Rhoparia causes contractions that cause it to swim away from the jellyfish’s roots. However, constantly changing water conditions and the presence of algae can affect the jellyfish’s ability to correctly assess contrast and distance, making it difficult to avoid collisions without limiting foraging.

overcome obstacles

Jan Bielecki, a researcher at the University of Kiel in Germany, and his colleagues conducted a study to determine whether jellyfish learn from past encounters with obstacles.

“We designed the experiment using a sort of bottom-up approach; we let the animals decide how to conduct the test, in the sense that you can’t judge a fish by its ability to climb a tree. We took great care to use stimuli that were familiar to the animal in its natural habitat,” Bielecki said in an email. “We took advantage of animal obstacle avoidance behavior and the observation that animals must have appropriate regulatory mechanisms to protect their fragile bodies.”

A fingernail-sized jellyfish was placed in an opaque circular aquarium 16 cm in diameter in three visual scenes: a plastic cylinder with alternating black and white vertical stripes, a gray and white stripe, or a completely gray space. ). The black and gray stripes were intended to imitate the roots of the prop.

When faced with gray roots, the jellyfish swam along the walls of the cylinder. But over the course of seven and a half minutes, they learned to stay away from the wall. Jellyfish’s distance from walls has been increased by 50% of her, and the number of times she hits barriers has been halved. Gray stripes and wall bumps served as visual and mechanical stimuli, respectively. Researchers have found that both types of stimuli are necessary for associative learning.

On the other hand, the jellyfish were almost completely separated from the black-striped wall, so there were few collisions and no mechanical stimulation. In this scenario, the jellyfish did not increase the number of avoidance behaviors. But they slightly increased the distance from the wall.

Even when faced with a uniform gray wall, the jellyfish continued to make contact. However, because there was no visual stimulus, learning did not occur and conflicts continued throughout the experiment.

Identifying learning centers

To find the learning centers of these jellyfish, Bielecki’s team isolated the loparigues. The researchers placed an isolated rhopalium in front of a projection screen, providing visual stimulation to the lens, and electrical pulses acted as mechanical stimulation.

As the party responsible for the contraction of swimming faster away from the obstacle, the signal frequency of the swimming pacemaker was used as a proxy for avoidance behavior. “Short bursts of high frequency in response to visual stimuli indicate an obstacle avoidance response,” Bielecki added.

Swimming pacemaker responses were faster when exposed to both visual and mechanical stimuli. But when the two stimuli were separated, the pacemaker no longer responded as it normally would. Researchers were surprised by how quickly the jellyfish learned. Five training sessions were sufficient to achieve learning. This confirms that Roparia is a learning center for jellyfish.

“Box jellyfish exhibit associative learning with a very sparse nervous system,” Bielecki says. Even primitive, dispersed groups of neurons can facilitate learning, suggesting that “learning has been an integrated part of neurons from the beginning of nervous system evolution.” In the future, the research team plans to understand jellyfish learning at the cellular level.

References: Jan Bielecki et al. Associative learning in the box jellyfish Tripedalia cystophora, Current Biology (2023). DOI: 10.1016/j.cub.2023.08.056

Feature image credit: Isaiah B on Unsplash

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