Brainless Box Jellyfish Can Learn with Bundles of Neurons, Scientists Find


The’s a little box jellyfish They live in warm coastal waters around the world, including the murky waters of the Caribbean mangrove forests. Barely the size of a fingernail, they quickly navigate dark underwater mazes of tangled roots in search of their prey, daphnia.

The bell-shaped body of this jellyfish (Tripedalia cystophora) It’s so small and simple that it doesn’t have a brain. But despite their apparent disabilities, they learn very quickly, scientists report in a recent study. current biology. They can quickly form and retain memories about the sensory information they receive from the environment and adapt their behavior accordingly. The results of this study suggest that the “brain” is not as essential to learning as we thought.

In many ways, T. cystophora They are unique creatures. They have a much more sophisticated visual system than other types of jellyfish, with four specialized visual sensory centers called rhopalia. Each roparia has 6 eyes, 24 in total, and contains about 1,000 neurons, so the jelly contains 4,000 neurons in total. (By comparison, fruit flies have 200,000 neurons.) Rhopalia generates electrical signals that control how fast the jellyfish pulses underwater. When an animal chases a flea or swerves to avoid hitting a tree root, its pulsating movements quicken.

Many other types of jellyfish can only sense light, but they sense the direction the light is coming from and follow it.T. cystophora It can also distinguish between light and dark, allowing it to form images. “Two of their eye structures are camera-shaped eyes, which are basically built the same way as your eyes and mine,” said lead author and associate professor of marine biology at the University of Copenhagen in Denmark. Anders Riddik Garm says:

Even without a brain, jellyfish can learn from experience.

Garm and study co-author Jan Bielecki, a neurobiologist at the University of Kiel in Germany, asked whether the jellyfish’s behavior (switching between dark and light) is hard-wired, or whether jellyfish We wanted to find out whether humans can learn new behaviors. In their natural habitat, jellies use their sense of visual contrast to distinguish between dark tree roots and bright water surfaces, and to avoid running into tangles of protruding roots. “We use contrast because it’s the difference between the darkness of the roots and the lightness of the water,” Garm explains. “That contrast is how they assess distance.” But can they learn to avoid obstacles that look different from their natural environment?

Garm and Bielecki began their experiment. They attached a bunch of gray and white plastic pieces to the aquarium, meant to mimic tree roots and light shining through. The trick was that to the jellyfish, the light gray color simply looked like distant roots. “Actually, it wasn’t that far away. We painted it gray so it just looked far away,” Garm explains. At first, the jellies realized that the gray “root” was far away and ran into it. However, after a few bumps, they will learn to avoid the gray bands. “They receive mechanical sensory input that tells them, OK, this root was much closer than it first appeared,” Garm says. And they start changing their behavior.

The scientists found that within 10 minutes of starting the experiment, the jelly quadrupled the number of successful pivots to avoid collisions. “In this situation, the low contrast still means the ‘roots’ are nearby, and after hitting the roots he has 3-5 mistakes to turn around early and avoid hitting them. “They learned to do it,” Garm said. “I was surprised at how quickly they learned.” This form of learning is called associative learning. The jelly learns to associate a sensory stimulus, such as an image of a gray band, with the bumps, remembers this association, and adapts its future behavior.

The findings suggest that even without a brain, jellyfish can learn from experience through visual and mechanical stimulation, the researchers said. But if jellyfish don’t have brains, where are these memories stored? Garm says this learning may occur within the cells of the distributed nervous system, particularly within the neurons of the rhopalia. There is. These neurons form memories of sensory stimuli that then become associated with specific behaviors. In other words, images of roots and ridges lead to avoidance.

“Our study suggests that learning is an essential function of neurons, or at least small neuronal circuits, due to the unique evolutionary position of jellyfish,” Bielecki says. If all we need for memory and learning are neurons or neural circuits, then perhaps we need to reconsider the definition of the term “brain,” he says. At least in the case of box jellyfish, they seem to be a bit “smart” without it.

Top photo: Jan Bielecki

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