Jellyfish Show Evidence of Learning Despite Lacking a Brain

A recent study has revealed that Caribbean box jellyfish are capable of learning from experience, despite not having a central brain. This discovery provides new insights into the evolution and mechanisms of learning.

According to Jan Bielecki from Kiel University in Germany, “Learning is the pinnacle of nervous system performance.” Until now, it was believed that learning and behavioral adaptation based on experiences were limited to organisms with more complex biological structures and relatively larger brains, such as mice, birds, and primates. However, some studies suggested that even simpler creatures may possess this ability. This led Bielecki and his team to investigate learning in jellyfish, which represent an early stage in animal evolution.

The researchers created an experimental environment that imitated the natural mangrove-rich habitat of the Caribbean box jellyfish (Tripedalia cystophora). They placed these small jellyfish in a round tank with walls painted in white and grey stripes to resemble the vertical mangrove roots where the species usually navigates.

To create an optical illusion, the researchers used grey stripes instead of black, making the “roots” appear farther away than they actually were. As a result, the jellyfish initially bumped into the tank walls. However, after spending about seven and a half minutes in the striped tank, all 12 tested jellyfish started pivoting to avoid the walls. This suggests that they learned from the collisions and adjusted their behavior accordingly. By the end of the experiment, the jellyfish had significantly reduced their crashes and increased their successful swerves.

Bielecki highlights, “The jellies learned to stay further away from the stripes they collided with.” He was particularly amazed that it only took a few minutes for the jellyfish to learn to avoid the tank walls. “Previously, we believed that a highly developed nervous system was necessary for any form of learning, but now we have shown that even this simple and early-evolved animal is capable of learning,” Bielecki adds.

The researchers propose that, in the absence of a traditional brain, learning occurs in the jellyfish’s four visual sensory organs, called rhopalium, which are distributed throughout its body. Each rhopalium contains six lenses that sense light, providing a total of 24 eye-light lenses that aid the jellyfish in navigating and changing its course.

Simon Sprecher from the University of Fribourg in Switzerland, who was not involved in the study, comments, “This work demonstrates that nervous systems in these animals can achieve quite striking functions.”