No brain, no problem. Jellyfish learn just fine with 24 eyes

Amazing stuff!

"... The Caribbean box jellyfish (Tripedalia cystophora), which doesn’t even have a brain, can alter its behavior based on past experiences, new research reveals. ...

Scientists have known for some time that animals in the phylum Cnidaria—which includes jellyfish, corals, and sea anemones—are capable of basic forms of learning when repeatedly presented with a stimulus in their environment. They will either respond to it less (what’s known as habituation) or more (known as sensitization). ...
a jellyfish’s nervous system is dispersed throughout its body, with no centralized brainlike structure, these animals have been viewed as incapable of associative learning. ...

Even among other brainless animals, T. cystophora stands out. Whereas most Cnidarians can only vaguely detect light sources, this fingernail-size, translucent creature has a whopping 24 eyes, arranged in clusters around the body to allow it to perceive the world with impressive visual detail and navigate around underwater mangrove roots in the Caribbean Sea and Central Indo-Pacific region. ...

The researchers constructed a tank lined with visual stimuli in the form of gray and white stripes to simulate the jellyfish’s mangrove habitat, with the gray stripes mimicking the shapes of distant mangrove roots.

At first, the jellyfish swam close to the stripes and frequently bumped into the tank wall. But as time went on, they seemed to associate the gray stripes with a mechanical stimulus—the pain of collision—and consistently swam away from them. T. cystophora had connected two different stimulus types to change its behavior—both hallmarks of associative learning, the team reports today in Current Biology. The jellyfish were also able to change their behavior rather quickly, after just a few run-ins with the tank wall.

To better understand the underlying neurobiology, the researchers isolated one of the jellyfish’s visual centers, known as rhopalia. Each rhopalium contains six eyes and generates electrical signals that modulate the jellyfish’s iconic pulse movement. When T. cystophora encounters an obstacle, rhopalia send out more signals, causing the animal to swerve.

The scientists showed the rhopalium a projected image of a moving gray bar to mimic a jellyfish approaching a mangrove root. At first, the visual center didn’t respond, but when the researchers added in a weak electric shock to mimic the mechanical stimulus of a collision, the rhopalium began to generate the type of high-frequency electrical signals that promote obstacle dodging. These findings suggest this structure, which functions as a type of miniature visual brain, also serves as a learning center. ...
That jellyfish are one of the world’s oldest animals—they’ve been around for more than 500 million years per some estimates ..."

From the highlights and abstract:

"Highlights

• The box jellyfish Tripedalia cystophora is capable of associative learning

• Learning is based on the combination of visual and mechanical stimuli

• The rhopalial nervous system (RNS) is the learning center of Tripedalia cystophora

Summary

Associative learning, such as classical or operant conditioning, has never been unequivocally associated with animals outside bilatarians, e.g., vertebrates, arthropods, or mollusks. Learning modulates behavior and is imperative for survival in the vast majority of animals. Obstacle avoidance is one of several visually guided behaviors in the box jellyfish, Tripedalia cystophora Conant, 1897 (Cnidaria: Cubozoa), and it is intimately associated with foraging between prop roots in their mangrove habitat. The obstacle avoidance behavior (OAB) is a species-specific defense reaction (SSDR) for T. cystophora, so identifying such SSDR is essential for testing the learning capacity of a given animal. Using the OAB, we show that box jellyfish performed associative learning (operant conditioning). We found that the rhopalial nervous system is the learning center and that T. cystophora combines visual and mechanical stimuli during operant conditioning. Since T. cystophora has a dispersed central nervous system lacking a conventional centralized brain, our work challenges the notion that associative learning requires complex neuronal circuitry. Moreover, since Cnidaria is the sister group to Bilateria, it suggests the intriguing possibility that advanced neuronal processes, like operant conditioning, are a fundamental property of all nervous systems."

No brain, no problem. Jellyfish learn just fine | Science | AAAS

Associative learning in the box jellyfish Tripedalia cystophora (no public access)

Graphical abstract