A mindless jellyfish… able to learn

They are barely a centimeter long and have no brain. However, a species of jellyfish is able to learn by combining vision and stimuli to avoid obstacles, a cognitive performance never seen before in these animals whose lineage goes back to the origins of the animal world.

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Tripedalia Cystophora, also called the Caribbean jellyfish or box jellyfish, has the remarkable ability to navigate through murky waters and a maze of submerged mangrove roots. There are many obstacles to avoid so as not to damage the fragile gelatinous membrane that envelops the bell-shaped body.

It works very well thanks to a device common in specimens of its species: four sensory structures, arranged as many cardinal points around its body. Each structure is called a rupalia, and contains two lens-shaped eyes and an image processing center.

All this with an economy of means that verges on asceticism, counting only about a thousand neurons per rupalia, where, for example, the little fly Drosophila, the darling of laboratories, has 200,000 neurons in its tiny brain.

Above all, unlike almost all species in the animal kingdom, cnidarians, the lineage to which jellyfish belong, do not have a brain in the strict sense of the word, but rather a dispersed nervous system. An interesting feature given their cognitive abilities.

The study, signed by Jan Bilecki, from the University of Kiel (Germany), and Anders Jarm, from Copenhagen, proves that despite this, the animal responds to “operant conditioning”. This means that training allows him to anticipate a possible outcome, in which case he hits the root.

Anders Garm points out to AFP that this ability is “a step above classical conditioning”, like that of Pavlov’s dog, where the animal cannot help but salivate at the sight of its bowl.

Through its training, the jellyfish learns to predict future trouble and try to avoid it. The study, published in the journal Current Biology on Friday, indicates that this ability has never been shown before in an animal with such a primitive nervous system.

Learn to sail

The researchers verified that the box jellyfish learns to evaluate its distance from an obstacle by associating the visual stimuli of the root and the mechanical stimuli of the shock with the latter.

They placed the animal in a small, round pen filled with water, the walls of which were colored with rather dark bands, representing the roots. She found that she quickly learned to move as wide as possible in the cage, when the straps were difficult to see and after only a few collisions with the walls.

If the tapes are highly visible, the tiny jellyfish will never hit the walls, but rather stay carefully in the center of the fence. Not ideal for walking and eating. If the strips disappear, they constantly hit the walls.

In short, Anders Jarm says: “If you deprive him of a stimulus, he will not be able to learn.” But with both, it only takes three to six attempts to learn to navigate smoothly. “It is almost the same for animals that are considered more advanced, such as a fruit fly, a crab, or even a mouse,” the scientist adds.

The researchers verified their hypothesis by repeating the experiment ex vivo, by stimulating one eye with robalia. “This supports the theory that a very small number of neurons makes learning possible,” emphasizes Jan Bilecki.

The existence of such an ability in such a simple organism “suggests that it could be a fundamental property of the nervous system,” according to Anders Jarm.

In fact, cnidarians, a group in the animal kingdom to which box jellyfish belong, are “a sister group to all other animals,” the biologist continues.

It is assumed that the common ancestor of these two groups, more than 500 million years ago, evolved a nervous system that already possessed such an ability to learn by association.