No brain, no problem: Tiny jellyfish can learn from experience

Cnidarians -- the animal group which includes jellyfish, sea anemones and coral -- are brainless, instead getting by with a "dispersed" central nervous system

September 23, 2023 03:20 pm | Updated 03:20 pm IST

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

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.

Also Read | India adds 664 animal species to its faunal database in 2022, 339 taxa to its flora

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.

Also Read | Walk with the wild side: Celebrating PFA’s 36000th rescue in Bengaluru 

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.

Also Read | Massive extinct whale ‘may be heaviest animal that ever lived’

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.

Top News Today

Sign in to unlock member-only benefits!
  • Access 10 free stories every month
  • Save stories to read later
  • Access to comment on every story
  • Sign-up/manage your newsletter subscriptions with a single click
  • Get notified by email for early access to discounts & offers on our products
Sign in

Comments

Comments have to be in English, and in full sentences. They cannot be abusive or personal. Please abide by our community guidelines for posting your comments.

We have migrated to a new commenting platform. If you are already a registered user of The Hindu and logged in, you may continue to engage with our articles. If you do not have an account please register and login to post comments. Users can access their older comments by logging into their accounts on Vuukle.