These brainless jellyfish use their eyes and bundles of nerves to learn

For Caribbean box jellyfish, learning is literally a given.

In a new experiment, these animals learned to spot and avoid obstacles despite lacking a central brain, researchers report September 22 in Current biology. This is the first evidence that jellyfish can make mental connections between events – like seeing something and falling into it – and change their behavior accordingly.

“Perhaps learning does not need a very complex nervous system, but rather it is an integral part of nerve cells or very limited circuits,” says Jan Bielecki, a neuroethologist at the University of Kiel in Germany. If so, this new discovery could help trace the evolution of learning in animals.

The nervous system of a Caribbean box jellyfish (Tripedalia cystophora) includes four button-shaped rhopales that hang over the bell of its body. Each rhopalium houses six eyes and around 1,000 neurons. Jellyfish use their vision to move between the roots of mangroves in tropical lagoons, where they hunt for tiny crustacean prey.

Weaving between the roots is no easy task. Caribbean box jellyfish judge the distance of a root based on its darkness relative to the surrounding water, that is, its contrast. In clear water, only the distant roots fade into the background or show little contrast. But in murky waters, even the closest roots can blend into their surroundings.

Waters can quickly become cloudy due to tides, algae and other factors. Bielecki and his colleagues wondered whether Caribbean box jellyfish could learn that low-contrast objects, which might at first appear distant, were actually close.

The team placed 12 jellyfish in a round tank surrounded by alternating low-contrast gray and white stripes. A camera filmed the animals' behavior for about seven minutes. At first, the jellyfish seemed to interpret the gray stripes as distant roots and swam into the tank wall.

But these collisions seemed to lead the jellyfish to treat the gray stripes more like nearby roots in murky water, and the animals began to avoid them. The average distance of the jellies from the tank wall increased from about 2.5 centimeters in the first two minutes to about 3.6 centimeters in the last two minutes. Their average hits against the wall went from 1.8 per minute to 0.78 per minute.

“I found this really astonishing,” says Nagayasu Nakanishi, an evolutionary biologist at the University of Arkansas in Fayetteville, who has studied the nervous systems of jellyfish but was not involved in the new work. “I never thought jellyfish could really learn.”

Neurobiologist Björn Brembs views the results more cautiously, noting the small number of jellyfish tested and the variability in their performance. “I want this to be true, because that would be really cool,” says Brembs, of the University of Regensburg in Germany. Experiments with more jellyfish might convince him that animals really learn.

In other experiments, Bielecki and his colleagues cut rhopalia from jellyfish and placed these eye-bearing nerve bundles in front of a screen. A bit like this scene in A clockwork orange, says Bielecki, except that jellyfish eyes don't have eyelids to stay open. The screen displayed light gray bars of low contrast, while an electrode gave the rhopalia a weak electrical pulse, which mimicked the sensation of hitting something.

This training caused the rhopalia to begin responding to the low-contrast bars that they had initially ignored. They began sending the types of neural signals they are known to emit when a jellyfish moves away from an obstacle. This suggests that rhopalia alone can learn that seemingly distant, low-contrast obstacles are actually close enough to avoid – which, in turn, suggests that rhopalia are the learning centers of Caribbean box jellyfish. .

“This is the most interesting part of the paper,” says Ken Cheng, a behavioral biologist at Macquarie University in Sydney. “This takes us a step forward in wiring up how this works.”

For neurobiologist Gaëlle Botton-Amiot, tracing learning back to rhopalies raises new questions. “They have four of these things in their bodies, so how does it work?” she asks. “How is this coordinated?” » And if a jellyfish loses one of its rhopalia, does it forget everything its eyes have seen and everything its neurons have learned? Or do the other rhopales remember it?

Botton-Amiot's research at the University of Friborg in Switzerland suggests similar learning abilities in sea anemones. Like jellyfish, they belong to a group of animals called cnidarians. “Show that cnidarians so different [can both learn] “That means it's probably very common among them,” she says, and that maybe their common ancestor could learn it too.

“Maybe [learning] in fact, it evolved several times during the evolution of the nervous system,” explains Nakanishi. Discovering the cellular and chemical machinery behind learning in jellyfish or other animals could shed light on this point. “If there are many similarities in learning mechanisms, that suggests a common ancestry,” he says. “But if they evolved independently, then we might expect very different learning mechanisms.”