Jellyfish Can Learn Even Without Brain, Implying Adaptation in Creatures with Few Neurons and Environmental Awareness

Scientists attempted to teach brainless boxed jellyfish tricks to determine if learning requires a brain, but findings suggest creatures with a few neurons can adapt based on experience.

Neurobiologist Jan Bielecki from Kiel University in Germany and the first author of the study said that this experiment highlights the significance of learning in nervous systems.

The Incredible Nervous System of Tripedalia cystophora

In a recent paper, titled "Associative learning in the box jellyfish Tripedalia cystophora" published in Current Biology, researchers contend that Caribbean box jellyfish can acquire knowledge through experience, despite having only 1,000 active neurons at a time and lacking a central brain.

But what type of nervous system does it have? For example, octopuses excel without centralized brains, relying on a network of approximately half a billion neurons spread throughout their tentacles to solve problems. Even the unassuming sea slug Aplysia californica has demonstrated impressive learning abilities with just 20,000 nerve cells.

Regarding jellyfish, T. cystophora doesn't possess a primitive nervous system. Each of its four vision structures, called rhopalia, comprises two eyes and approximately a thousand photoreceptors. These nerves serve as both sensory systems and integration centers, enabling box jellyfish to navigate through dense mangrove root forests in pursuit of prey.

To examine if such a basic jellyfish system can learn, Bielecki and his team placed adult T. cystophora in a round tank with gray-striped walls mimicking the appearance of distant mangrove roots. Initially, the jellyfish bumped into the wall without hesitation.

However, during the 7.5-minute trial, they rapidly adjusted their behavior, pivoting four times more frequently on average and increasing their distance from the wall by half.

This swift learning ability surprised scientists, indicating that even the simplest nervous systems are capable of advanced learning. This discovery suggests a fundamental cellular mechanism that may have originated at the dawn of nervous system evolution.

Testing Their Intelligence

In an experiment, scientists exposed jellyfish to tanks with different levels of visual contrast: high-contrast black-and-white stripes resembling nearby tree roots, medium-contrast gray-and-white stripes creating the illusion of distant roots, and plain gray with no contrast.

Jellyfish in the high-contrast tanks navigated successfully without colliding with the walls, as the stark contrast helped them avoid collisions. However, they did not learn to avoid the walls entirely due to the lack of collision experience. Jellyfish in the plain gray tanks also continued to collide with the walls throughout the experiment.

Remarkably, only the jellyfish in the gray-and-white striped tanks demonstrated learning by associating the striped decor with collision risk. Initially, they bumped into the walls, but by the end of the 7.5-minute trial, they successfully avoided the walls. What surprised the researchers was the rapidity of their learning, with the jellyfish associating the stripes with the risk after just three to five collisions.

While the experiment sheds light on jellyfish learning, some scientists suggest further tests to understand the extent of their learning capabilities, such as exploring extinction and long-term memory of associations.

Nonetheless, the study highlights how simple organisms like jellyfish can reveal fundamental neural processes and challenges the notion that complex brain structures are necessary for learning, encouraging a broader examination of learning across the animal kingdom, even in simpler organisms and individual cells.