Thinking out of the box

Wise men say only fools rush in— researchers show that jellyfish can make (and learn from) that exact mistake.

Reference: Bielecki, J., Dam Nielsen, S. K., Nachman, G., & Garm, A. (2023). Associative learning in the box jellyfish Tripedalia cystophora. Current Biology: CB, 33(19), 4150–4159.e5.

Cover image credit: Adapted from Bielecki et al., 2014. Licensed under Creative Commons Attribution-Share Alike 4.0

Animals display an impressive ability to learn—rats can learn how to navigate mazes, parrots can learn how to mimic human speech, and dogs can learn how to sit, stay, or roll over. Researchers at Kiel University and the University of Copenhagen add another, more unusual example to that list: box jellyfish. Specifically, researchers showed that box jellyfish display associative learning—the process of linking one stimulus to another and modifying their behavior accordingly— and were unimpeded by the fact that they don’t have a brain.

Embarking on a crash course

Unlike humans, jellyfish do not have a centralized brain. Instead, they have a network of neurons —specialized cells that transmit electrical signals—that are dispersed throughout its body. This network consists of an extensive visual processing system, with 24 eyes distributed across the box jellyfish’s grape sized body. With the ability to see its surroundings, researchers wanted to know if the jellyfish could learn from them as well.

Box jellyfish live in mangrove swamps, foraging amongst the roots of mangrove trees for small prey. The researchers recreated their habitat by lining a cylindrical tank with black and white stripes, mimicking mangrove roots as they would appear on a clear, high-visibility day. The box jellyfish never collided with the tank walls, showing that they could discern where the “roots” were and avoid them.

Roots of a mangrove tree partially submerged underwater.
Roots of a mangrove tree partially underwater. Image credit: US National Oceanic and Atmospheric Administration on Wikimedia Commons. Under public domain.

But visibility is highly variable in mangrove forests, as their waters often become thick with mud and silt. To recreate this murky water, the researchers replaced the black and white stripes with lower contrast grey and white stripes. The box jellyfish began colliding into the walls of the tank far more frequently, as they did not perceive the grey stripes as an imminent obstacle.

A moment of clarity

After only a few minutes, however, the researchers observed a distinct change in behavior. The box jellyfish began increasing their distance from the tank walls, and pulsed rapidly to change direction when they got too close. By the end of the experiment, the box jellyfish had increased their average distance from the wall by half and quadrupled the number of rapid pivots made to avoid bumping into the wall. The box jellyfish started associating the grey stripes with the sensation of collision and responded by swimming away from the tank walls. They connected the visual stimuli of the grey stripes with the mechanical stimuli of collision and demonstrated associative learning by avoiding the walls more often.

The power of the mind’s eye

Researchers additionally studied this behavior in an isolated physiological unit: the rhopalium. Rhopalium are combination “eye-brain” structures, with each consisting of 6 eyes and a nerve center. The rapid pivoting behavior used to avoid collisions is initiated by the rhopalium—when it detects obstacles, it sends high frequencies of electrical signals to the jellyfish’s body, prompting the rapid pulsing needed to pivot. Researchers thus wanted to know if learning could be observed even at the level of a single processing center.

Diagram of the rhopalium, an eye-brain complex used by the box jellyfish to respond to visual stimuli
Diagram of the rhopalium of a box jellyfish. Image credit: F.S Conant, The Cubomedusae, in Selected Morphological Monographs, Johns Hopkins University, 1900. Under public domain.

Researchers isolated a single rhopalium by dissecting it from the body of a jellyfish, where it remained temporarily active in seawater. Though the isolated rhopalium could not move, researchers simulated the box jellyfish moving toward a mangrove root by projecting an image of a grey stripe moving towards the rhopalium. Initially, the rhopalium did not respond. Then, the researchers added a weak electric shock to mimic the mechanical stimulus of colliding into a root. After a few minutes, the rhopalium began to generate high frequencies of signals, similar to what it would generate to prompt the rapid pulsing necessary to pivot the jellyfish away from the obstacle. This showed that learning was taking place at the level of a single rhopalium, not just in a whole organism.

It’s a hard knock life

These findings may have important implications for how learning evolved in the animal kingdom. Jellyfish evolved over 500 million years ago, suggesting that learning emerged early in animal evolution. The researchers even postulate that learning—once thought to be a hallmark of more complex adaptations—might be a fundamental cellular property of the nervous system. But perhaps the jellyfish’s capacity for learning isn’t so surprising, as learning is crucial to survival. Having survived five mass extinctions throughout its time on Earth, the jellyfish has clearly learned some savvy lessons from the proverbial school of hard knocks.

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