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Jellyfish Have No Brain But Are Able To Learn From Experience

A Caribbean box jellyfish
Credit: Jan Bielecki under CC BY-SA
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Jellyfish are ancient and deeply weird animals. They have been bobbing around our oceans and waterways for roughly 500 million years. Having evolved so early, jellyfish have structures that look very little like those of other animals. That includes their nervous system. New research shows for the first time that despite lacking a central brain, jellyfish can still learn from their environment and change their behavior accordingly. The study is published in Current Biology.

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An existence spent drifting around the emptiness of the ocean has imposed little evolutionary pressure on jellyfish to evolve a body structure that we would recognize, says Anders Garm, an associate professor of marine biology at the University of Copenhagen and senior author of the new study. “They have this habit of distributing everything all along their edges,” says Garm. The “bell” – the main, umbrella-like section of a jelly’s body – has no less than 24 “eyes”, distributed among four sensory centers around its rim. These sensory centers are called rhopalia. 

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Garm’s study examined the box jellyfish Tripedalia cystophora, part of the same evolutionary class of jellies as the outrageously toxic sea wasp jellyfish, which can kill humans with its venom. T. cystophora is less deadly, and about the size of a fingernail. At home in the Caribbean, T. cystophora drifts around the roots of mangrove trees, hunting plankton-like prey in the murky water. Working alongside Jan Bielecki, a group leader at Kiel University, Garm wanted to test whether T. cystophora could learn in response to signals from its environment. To assess their jelly’s smarts, the team designed an experiment faithfully inspired by the animals’ natural habitat.

The jellies were placed in a round tank, which was lined with white and gray stripes, the gray stripes colored to look like far-away mangrove roots. Initially, when the jellies were placed in the tank, they swam right into the tank wall where the gray “roots” were. But by the experiment’s end, just seven minutes later, the jellies had learned to avoid the gray roots and had cut the number of collisions with the wall in half, increased their distance from the wall by 50% and were four times more likely to pivot away from the potential collisions.

Getting jellyfish ready to dodge

The team then placed one of the jellies’ rhopalia in a petri dish, where its response to moving bars, like the ones shown to the intact jellyfish, was recorded. These structures didn’t respond to the gray bars, seeing them as distant structures. But after the rhopalia were stimulated with electrical signals – which mimicked those transmitted upon collision – while viewing the bars, the rhopalium started generating “dodge” signals, suggesting that the combination of visual and electrical stimuli allowed the jellyfish to learn to avoid objects.

Garm and Bielicki note that the jellyfish seemed to forget what they had learned roughly 20 minutes after the experiment finished, suggesting that the type of learning involved is not the long-lasting and durable type that humans are capable of, but rather a more ephemeral function. The exact mechanism by which T. cystophora learns remains unclear.

A more natural experiment

T. cystophora have roughly 1,000 neurons in their nervous system, and it is perhaps only their alien body structures that have prevented them from being a more widely used model organism. Garm and Bielicki agree that their experimental approach was key to maximizing the benefit of T. cystophora as a test animal: “The strength in our story is to wait during your experiment until you understand your animals so well that you can pinpoint the exact behavior where it makes sense for them to learn and then replicate that as naturally as possible in your lab.” The pair point to experiments on mice – nocturnal animals – that take place during the day as an example of research that is less naturalistic. Perhaps by taking a deeper look at what drives animals – even weird and ancient ones – in their natural habitat, we can make more of their valuable contribution to science. “You have to ask your animal,” concludes Garm. “You can’t judge a fish by its ability to climb a tree.”

Reference: Bielecki J, Nielsen SKD, Nachman G, Garm A. Associative learning in the box jellyfish Tripedalia cystophora. Current Biology. 2023. doi:10.1016/j.cub.2023.08.056