They don't have a brain or spinal cord. They float around in a way that often appears aimless. Though jellyfish lack a central nervous system, these gelatinous creatures again show that they might think more than we think they do.

Jellyfish, or medusae, belong to the group Cnidaria, members of which are already known to be capable of associative learning. This is how they can maintain awareness of their surroundings (and possible predators). Now, an international team of scientists has found that the cnidarians are capable of a slightly more advanced type of associative learning known as operant conditioning, which entails remembering the positive or negative effects of a previous action. Despite lacking a brain, Caribbean box jellies (Tripedalia cystophora) can still learn from their mistakes to avoid a potentially disastrous outcome.

Damage control

T. cystophora are about the size of a human fingernail, and while they are much less complex than vertebrates such as humans, they still have a rather sophisticated visual system for a jellyfish. The jellies have 24 eyes around their bodies—and they need them. They live in mangrove swamps where crashing into long roots is almost inevitable in murky water, and a jelly might do serious damage to its delicate body in these encounters. Its vision assists it in navigating among the roots and can be especially useful for hunting around these gnarly tangles.

This inspired Jan Bielecki of Kiel University in Germany and his research team to simulate that environment in a lab to see how the jellyfish would handle it. More specifically, they wanted to determine if the jellies could learn from making mistakes.

“Several mechanisms can shape behavioral plasticity, but the influence of previous experience—memory formation and learning—is undoubtedly among the most important,” Bielecki and his colleagues said in a study recently published in Current Biology.

To put the jellies’ obstacle avoidance behavior (OAB) to the test, the researchers covered the walls of a round tank in stripes that would appear similar to the roots in the creatures’ natural habitat, with white stripes mimicking nearby roots and gray stripes appearing as if they were further away. From the perspective of a jellyfish, the gray stripes would look like something they didn’t have to worry about immediately, even though they were located at the same distance as the white ones.

The jellies had a rough start, frequently colliding with the wall at the sites of the gray stripes. Things changed drastically in just seven and a half minutes. At that point, the jellies stayed 50 percent further from the gray stripes than they had originally. Jellies tend to pulse through the water at higher speeds when faced with visible obstacles, and they swam faster when they saw gray stripes. This was suggestive of operant conditioning.

Look, Ma, no brain

Since an actual brain was not helping the jellyfish understand their environment, something else had to be guiding them, so the research team dissected the jellyfish and studied their nervous systems to see what was.

While they lack a brain,  jellyfish do have structures called rhopalia. Six eyes are connected to each of these visual sensory centers, which give the jelly a sense of the rhythm of its movement.

Though the isolated rhopalia could not move, the scientists placed gray bars in front of each rhopalium and moved the bars as if the nerves were still attached to the moving jellyfish and heading towards an obstacle. Weak jolts of electricity were sent through the rhopalium to stimulate an actual collision in the ex vivo experiment. Rhopalia soon started firing warning signals of their own. The scientists saw this reaction as further proof that the animal had learned from its crashes when in the tank—these signals would have told the jellyfish to swerve away if the rest of its body was attached.

Operant conditioning is expected behavior in bilaterians such as arthropods, mollusks, and vertebrates. This is the first time it was seen in animals that were not bilaterians. However, even though cnidarians have a radically different nervous system, the groups Cnidaria and Bilataria are actually siblings. This might offer further insights into the evolution of more complex nervous systems like our own.

“[The relationship between these two groups] suggests the intriguing possibility that advanced neuronal processes, like operant conditioning, are a fundamental property of all nervous systems,” the researchers said in the same study.

Further research could reveal more complexities within the deceptively simple nervous systems of jellyfish and other cnidarians. Humans might be proud of our huge and complex brains, but jellies like T. cystophora could help us understand how the evolution of our brains began.

Current Biology, 2023.  DOI: 10.1016/j.cub.2023.08.056