Caribbean box jellyfish are barely a centimeter long and have no brain. But these minuscule gelatinous creatures are still capable of learning to avoid obstacles using visual cues – a cognitive ability never before seen in animals with such a primitive nervous system, according to new research.

Their performance of what is called "associative learning" can be compared 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 centers 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. 

Garm said that this capacity is "more complex than classical conditioning," which is best known for Russian neurologist Ivan Pavlov's experiments showing that dogs can be trained to salivate when they see their food bowl.

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 center of the tank. However this was not ideal for scrounging around for food. 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.

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