Even without a central brain, jellyfish can learn from past experiences like humans, mice, and flies. The researchers trained Caribbean box jellyfish (Tripedalia cystophora) to learn to spot and dodge obstacles. The study published in Current Biology has challenged previous notions that advanced learning requires a centralised brain and sheds light on the evolutionary roots of learning and memory. No bigger than a fingernail, these seemingly simple jellies have a complex visual system with 24 eyes embedded in their bell-like body. Living in mangrove swamps, the animal uses its vision to steer through murky waters and swerve around underwater tree roots to snare prey. Scientists demonstrated that the jellies could acquire the ability to avoid obstacles through associative learning, a process through which organisms form mental connections between sensory stimulations and behaviours. The researchers dressed a round tank with grey and white stripes to simulate the jellyfish’s natural habitat, with grey stripes mimicking mangrove roots that would appear distant. They observed the jellyfish in the tank for 7.5 minutes. Initially, the jelly swam close to these seemingly far stripes and bumped into them frequently. But by the end of the experiment, the jelly increased its average distance to the wall by about 50%, quadrupled the number of successful pivots to avoid collision and cut its contact with the wall by half. The findings suggest that jellyfish can learn from experience through visual and mechanical stimuli.

Caribbean box jellyfish are barely a centimetre long and have no brain. 
| Photo Credit: AFP

Caribbean box jellyfish are barely a centimetre long and have no brain.

But these gelatinous, fingernail-sized creatures are capable of learning from visual cues to avoid swimming into obstacles — a cognitive ability never before seen in animals with such a primitive nervous system, researchers said on Friday.

Their performance of what is called “associative learning” is comparable to far more advanced animals such as fruit flies or mice, which have the notable benefit of having a brain, the researchers said.

The Caribbean box jellyfish, or Tripedalia cystophora, is known to be able to navigate through murky water and a maze of submerged mangrove roots.

These scenarios throw up plenty of dangers that could damage the jellyfish’s fragile gelatinous membrane which envelops its bell-shaped body.

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But they avoid harm thanks to four visual sensory centres called rhopalia, each of which has lens-shaped eyes and around a thousand neurons.

For comparison, fruit flies are packing 200,000 neurons in their tiny brains.

Cnidarians — the animal group which includes jellyfish, sea anemones and coral — are brainless, instead getting by with a “dispersed” central nervous system.

Despite this considerable disadvantage, the Caribbean box jellyfish responds to what is called “operant conditioning,” according to the study in the journal Current Biology.

This means they can be trained to “predict a future problem and try to avoid it,” said Anders Garm, a marine biologist at the University of Copenhagen and the study’s lead author.

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Garm told AFP that this capacity is “more complex than classical conditioning,” which is best known for Russian neurologist Ivan Pavlov’s experiments showing that dogs cannot help but salivate when they see their food bowl.

‘Very intriguing’

To test the jellyfish, the researchers put them in a small, water-filled tank with stripes of varying darkness on the glass walls to represent mangrove roots.

After a few bumps into the walls, the jellyfish quickly learned to move through the parts of the enclosures where the bars were least visible.

If the bars were made more prominent, the jellyfish never hit the walls, remaining safely in the centre of the tank. However this was not ideal for scrounging around for food.

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If the stripes were removed entirely, the jellyfish constantly ran into the walls of the tank.

“If you separate the two stimuli, there is no learning,” Garm concluded.

The jellyfish learned their lesson in between three to six tries, “which is basically the same amount of trials for what we would normally consider an advanced animal, like a fruit fly, a crab or even a mouse,” he said.

They said their research supports the theory that even animals with a very small number of neurons are capable of learning.

That such a simple organism is able to achieve this feat “points to the very intriguing fact that this may be a fundamental property of nerve systems,” Garm said.

Cnidarians are a “sister group” to the animal group that includes most other animals — including humans.

Garm suggested that some 500 million years ago, a common ancestor of the two groups could have developed a nervous system that was already able to learn by association.