They are barely an inch long and have no brain. And yet one species of jellyfish is able to learn to combine vision and stimuli to avoid obstacles, a cognitive feat 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. There are so many obstacles to avoid in order not to damage the delicate gelatinous membrane that encases a bell-shaped body.
She is doing very well thanks to a device common to all specimens of her species: four sensory structures arranged around her body like many cardinal directions. Each structure, called rhopalia, includes two lens-shaped eyes and an image processing center.
And all this with an economy of resources that borders on asceticism, counting only about a thousand neurons for each rhopalia, while, for example, the tiny Drosophila fly, a favorite of the laboratories, has 200,000 neurons in its small brain.
Above all, unlike almost all species in the animal kingdom, the cnidarians, the lineage of jellyfish, do not, strictly speaking, have a brain, but rather a scattered nervous system. A fascinating feature given their cognitive abilities.
The study, signed by Jan Bielecki from the University of Kiel (Germany) and Anders Garm from Copenhagen, shows that the animal still responds to “operant conditioning”. That is, training that allows him to anticipate a possible consequence, in this case hitting a root.
This ability, Anders Garm told AFP, is “a step beyond classical conditioning” like that of Pavlov’s dog, where the animal cannot help but drool when it sees its bowl.
Through its training, the jellyfish learns to “anticipate a future problem and try to avoid it.” An ability never before demonstrated in an animal with such a primitive-looking nervous system, says the study published Friday in Current Biology.
Learn to sail
The researchers confirmed that the box jellyfish learns to estimate the distance between itself and an obstacle by associating the visual stimuli of a root and the mechanical stimuli of a shock to the latter.
They placed the animal in a small circular enclosure filled with water and whose walls were colored with more or less dark bands representing roots. And found that it quickly learned to move as far as possible in the enclosure when the bands were difficult to see and after a few collisions with the walls.
If the bands were too clearly visible, the young jellyfish never hit the walls, but instead stayed cautiously in the middle of the enclosure. Not ideal for walking and eating. When the stripes disappeared, they constantly collided with the walls.
In short: “If we deprive it of either stimulus, it cannot learn,” notes Anders Garm. But with both, it only takes three to six tries to learn how to navigate smoothly. “It is roughly the same as in animals that are considered more advanced, such as the fruit fly, the crab or the mouse,” adds the scientist.
The researchers confirmed 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 enables learning,” emphasizes Jan Bielecki.
The presence of such an ability in such a simple organism “suggests that it may be a fundamental property of the nervous system,” says Anders Garm.
In fact, the cnidarians, a group in the animal kingdom that includes the box jellyfish, were considered the “sister group of all other animals,” the biologist continues.
He assumes that the common ancestor of these two groups developed a nervous system more than 500 million years ago that already had such an ability to learn by association.