How to train your jellyfish: brainless box jellies learn from experience

Mangrove box jellyfish
Mangrove box jellyfish

In an experimental study, scientists have been able to demonstrate that just like humans, jellyfish too can learn from their past experiences. The team from Denmark and Germany trained a certain kind of jellyfish native to the Caribbean Sea, thus called the Caribbean box jellyfish – Tripedalia cystophora – to learn to spot and dodge obstacles.

Box Jellyfish
Box Jellyfish

And it’s a no-brainer that it must use its brain, right? Well, no, because just like most jellyfish, T. cystophora doesn’t possess a single centralized brain. Box jellyfish may lack a brain, but they use two dozen eyes. It has a concentration of neurons in its four eye-bearing sensory structures, which it uses for visual processing and to navigate using landmarks above the water.

The study challenges previous notions that advanced learning requires a centralized brain and sheds light on the evolutionary roots of learning and memory, according to a press release.

“Learning is the pinnacle performance for nervous systems,” said Jan Bielecki, first author of the study. He said that in order to teach jellyfish new tricks, “it’s best to leverage its natural behaviors, something that makes sense to the animal, so it reaches its full potential.”

The box jellyfish demonstrated a type of associative learning that has previously only been demonstrated in bilaterian animals like vertebrates, arthropods, and mollusks. Associative learning happens through conditioning. For example, if you touch a hot iron and it burns you, then you may associate the hot iron with pain and get conditioned not to repeat touching the iron again.

“If you want to understand complex structures, it’s always good to start as simple as you can,” said Anders Garm, senior author of the paper. “Looking at these relatively simple nervous systems in jellyfish, we have a much higher chance of understanding all the details and how it comes together to perform behaviors.”

Associative learning and how it works

Two unrelated elements become connected in our brains. In an experiment, the team put box jellyfish inside a round tank with grey and white stripes mimicking mangrove roots from a distance. When presented with gray obstacles, the jellyfish initially swam along the arena wall, which allowed it to see the obstacles (grey stripes) and touch them via wall contacts, which were the stimuli. 

The matured version of Tripedalia maipoensis, the newly discovered box jellyfish species.
The matured version of Tripedalia maipoensis, the newly discovered box jellyfish species.

The team found that within 7 minutes and 30 seconds of the trial period, the jellyfish learned to significantly increase the distance to the arena wall and the frequency of avoidances. Importantly, they also more than halved their wall contacts. The team concluded that these results suggest associative learning in T. cystophora by combining visual and mechanical stimuli.

“It’s surprising how fast these animals learn; it’s about the same pace as advanced animals are doing,” added Garm. “Even the simplest nervous system seems to be able to do advanced learning, and this might turn out to be an extremely fundamental cellular mechanism invented at the dawn of the evolution nervous system.”

Jellyfish are notorious for one other thing. Their sting may cause prolonged pain or even result in death. T. cystophora has a weaker toxin, so its sting may cause days of pain but not death.

The study was published in the journal Current Biology.

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