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Jellyfish can learn from experience even though they lack a brain

Caribbean box jellyfish can learn from experience, even though they lack a central brain. The discovery sheds new light on the evolution and mechanisms behind learning.

“Learning is the pinnacle of nervous system performance,” says Jan Bielecki at Kiel University in Germany. Until now, researchers had generally assumed that learning from an experience and adapting a behaviour – like avoiding a hot pan after a burn – was limited to more biologically complex animals with relatively large brains, including mice, birds and primates. But some studies have hinted that simpler creatures may also have this ability, which led Bielecki and his colleagues to investigate learning in jellyfish, a group that represents a very early stage in animal evolution.

The researchers designed an experimental environment that mimicked the Caribbean box jellyfish’s (Tripedalia cystophora) native mangrove-rich habitat. They placed a fingernail-sized jellyfish in a round tank whose walls were painted with white and grey stripes to resemble the vertical mangrove roots through which the species manoeuvres in the wild.

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They used grey stripes, rather than black, so that the “roots” would look further away than they were – an optical illusion that initially caused the jellyfish to bump into the tank walls. But after a seven-and-a-half-minute session in the striped tank, each of the 12 jellyfish they tested began pivoting to avoid the walls, suggesting they learned from the collisions and changed their behaviour accordingly. By the end of the experiment, the jellies had cut their crashes in half and quadrupled their successful swerves.

“After bumping into these stripes that were closer than they thought, they learned to stay further away from them,” says Bielecki. He was especially surprised that it took the jellies just a few minutes to learn to avoid the tank walls. “Previously, we thought that it took a highly developed nervous system to be able to learn anything, but now we’ve shown that this very simple, very evolutionarily-early animal is actually able to learn.”

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The researchers think that, in lieu of a traditional brain, learning happens in the jellyfish’s four visual sensory organs, called rhopalium, which are embedded throughout its body. Each rhopalium has six lenses that sense light – a total of 24 eye-light lenses – which help guide the swimming jellyfish’s pulses and pivots.

The work demonstrates that “nervous systems in these animals can achieve quite striking functions”, says Simon Sprecher at the University of Fribourg in Switzerland who was not involved in the work.

Journal reference

Current Biology DOI: 10.1016/j.cub.2023.08.056

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