Brainless Jellyfish Are Capable of Learning, Study Suggests

Scientists provide evidence that tiny Caribbean box jellyfish—which lack a central nervous system—can learn to navigate through mangrove roots

Translucent jellyfish against black backdrop
Box jellyfish are about the size of a grape. Jan Bielecki

In the warm waters of the Caribbean, box jellyfish carefully dodge the roots of mangroves while hunting for small crustaceans to eat. Whether the water is murky or clear, the tiny creatures—which are about the size of a grape—avoid bumping into the roots, which could easily injure their soft bodies.

But here’s the catch: Jellyfish have no central brain. And while Caribbean box jellies (Tripedalia cystophora) are among the few species that have eyes, their eyes are simple. Yet, they have figured out how to swim around the roots, even when low water quality makes them difficult to see. So, how do they do it?

“The hypothesis was, they need to learn this,” says Anders Garm, a biologist at the University of Copenhagen, to the New York Times’ Veronique Greenwood. “When they come back to these habitats, they have to learn: How is today’s water quality? How is the contrast changing today?”

Now, Garm and other researchers say they’ve confirmed this hunch: that the brainless creatures are capable of learning, despite their lack of a central nervous system. They shared their findings in a new paper published Friday in the journal Current Biology.

Scientists weren’t surprised the Caribbean box jellyfish—a smaller, non-lethal counterpart to the deadly Australian box jellyfish—could learn. But during a series of experiments, they were shocked by just how quickly the jellies could adjust their behavior—the animals learned in mere minutes.

To better understand the box jellies’ cognitive abilities, scientists replicated their natural environment in the lab. First, the researchers lined the inside of a small bucket with images of alternating vertical stripes. In one scenario, the stripes were high-contrast black and white, which the scientists intended to mimic mangrove roots in clear water. In another, the stripes were gray and white, which was meant to resemble the roots in murky water.

They filled the tanks, dropped in the jellyfish and recorded their reactions over seven and a half minutes. As expected, in the bucket with the starkest contrast, the jellies easily avoided colliding with the wall.

In the murky water condition, however, the creatures initially bumped into the wall, presumably because they couldn’t see the gray stripes very well. But it didn’t take long for them to adjust their behavior.

In less than eight minutes, they learned to avoid running into the wall, the researchers observed. Their average collisions declined from 1.8 to 0.78 per minute, and their average distance from the tank wall increased from about 1 inch (2.5 centimeters) to 1.4 inches (3.6 centimeters), reports Science News’ Maria Temming.

In a final scenario, they placed the jellyfish into a bucket with no stripes—just a solid gray wall. In this case, the creatures repeatedly collided with the wall and never changed their behavior.

“There was no visual cue, so they didn’t learn anything,” says study co-author Jan Bielecki, an electrophysiologist at Germany’s Kiel University, to CNN’s Jenna Schnuer. “They just kept bumping into stuff and not responding.”

“It was only when they had a combination of visual stimulation and mechanical stimulation that they would actually learn something,” Bielecki tells Nature News’ Dyani Lewis.

Next, the researchers tested how vision might play into jellyfish navigation. Box jellyfish have four centers of vision—known as rhopalia—and each contains visual neurons and six eyes, for a total of 24 eyes per animal. In an experiment, the team removed the rhopalia from the jellyfish and put them in a dish. They gave the cells a small electrical pulse—meant to simulate the jellyfish bumping into a mangrove root—while exposing them to striped images.

In just about five minutes of this training, the cells appeared to make the connection between the two occurrences and began sending signals that would cause a jellyfish to swim in the opposite direction.

Based on the results of the experiments, scientists believe the rhopalia are “where learning happens,” as Bielecki tells Nature News.

If that’s true, scientists are still left puzzling over how the four rhopalia work together to make up a functioning, brain-like system.

“How is this coordinated?” says Gaëlle Botton-Amiot, a neurobiologist at Switzerland’s University of Fribourg who was not involved in the paper, to Science News. If one of the rhopalia were removed, writes the publication, would the other three remember what had been learned, or would the jellyfish forget?

These experiments provide evidence that box jellyfish are capable of associative learning, or the process of linking two unrelated stimuli together. This concept was made famous by Russian neurologist Ivan Pavlov, who trained dogs to associate the sound of a bell with food.

In addition, the findings may also be evidence of “some amount of short-term memory,” says Michael Abrams, a molecular and cell biologist at the University of California who was not involved in the project, to CNN. That brings up a question for possible future experiments, he adds. How long can box jellyfish remember what they’ve learned?

More broadly, the discovery could help scientists understand the evolution of learning. Jellyfish appear in the fossil record about 500 million years ago, and they may have been the first creatures to split off from early multicellular life and begin evolving separately. On a more practical note, the findings could also prove useful for developing smarter robots that recognize patterns, per Nature News.

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