2023-09-22 15:57:51
They are barely a centimeter long and have no brain. And yet, a type of jellyfish is capable of learning combining vision and stimuli to avoid obstacles, a cognitive performance never before observed 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 a remarkable ability to navigate through murky water and a maze of submerged mangrove roots. So many obstacles to avoid in order not to damage the fragile gelatinous membrane which envelops a bell-shaped body.
She does very well thanks to a device common to specimens of her species: four sensory structures, arranged like so many cardinal points around her body. Each structure, called rhopalia, includes two lens-shaped eyes and an image processing center.
All with an economy of means that borders on asceticism, counting only around a thousand neurons for each rhopalia, where for example the tiny Drosophila fly, a darling of laboratories, has 200,000 neurons in its little brain.
Above all, unlike almost all species in the animal kingdom, the cnidarians, the lineage to which the jellyfish belongs, do not have a brain strictly speaking, but rather have a dispersed nervous system. An intriguing feature given their cognitive abilities.
The study signed by Jan Bielecki, of the University of Kiel (Germany), and Anders Garm, of that of Copenhagen, establishes that despite this, the animal responds to “operant conditioning”. That is to say training allowing him to anticipate a possible consequence, in this case bumping into a root.
This capacity, notes Anders Garm to AFP, is “a step above classical conditioning”, like that of Pavlov’s dog, where the animal cannot help but salivate when seeing its bowl.
With its training, the jellyfish “learns to foresee a future problem, and to try to avoid it”. An ability never before demonstrated for an animal with such a primitive-looking nervous system, notes the study published in Current biology on Friday.
Learn to sail
The researchers verified that the box jellyfish learns to evaluate the distance separating it from an obstacle by associating the visual stimuli of a root and the mechanical stimuli of a shock with the latter.
They placed the animal in a small round enclosure filled with water and whose walls were tinted with more or less dark bands, representing roots. And found that it quickly learned to move as widely as possible in the enclosure, when the bands were difficult to see and following a few collisions with the walls.
If the bands were too visible, the young jellyfish never hit the walls but remained carefully in the center of the enclosure. Not ideal for walking and eating. If the strips disappeared, then they constantly collided with the walls.
In short, “if we deprive it of one of the two stimuli, it cannot learn,” remarks Anders Garm. But with both, it only takes three to six tries to learn to navigate smoothly. “This is roughly the same as for animals considered more advanced, such as the fruit fly, the crab or even the mouse,” adds the scientist.
The researchers validated their hypothesis by repeating the ex-vivo experiment, by stimulating a single eye with rhopalia. “This supports the theory that a very small number of neuronal cells makes it possible to learn,” emphasizes Jan Bielecki.
The presence of such a capacity in such a simple organism “indicates that it might be a fundamental property of nervous systems”, according to Anders Garm.
Indeed, the cnidarians, a group in the animal kingdom to which box jellyfish belong, considered a “sister group to that of all other animals,” continues the biologist.
He hypothesizes that the common ancestor of these two groups, more than 500 million years ago, developed a nervous system already possessing such a capacity for learning by association.
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