‘Brainless’ jellyfish can learn at much more complex level than imagines, find scientists

“It was once presumed that jellyfish can only manage the simplest forms of learning, including habituation – i.e., the ability to get used to a certain stimulation, such as a constant sound or constant touch. Now, we see that jellyfish have a much more refined ability to learn, and that they can actually learn from their mistakes. And in doing so, modify their behaviour,” said Anders Garm, corresponding author of the paper, in a press statement. Garm is an associate professor at the University of Copenhagen’s Department of Biology.

One of the most advanced attributes of complex nervous systems is their ability to allow organisms to change their behaviour as a result of experience. In essence, to remember and learn. The research team found evidence of this ability in box jellyfish.

No brain, no problem

For the study, the scientists looked at Tripedalia cystophora, a fingernail-sized box jellyfish that lives in Caribbean mangrove swamps. These creatures use their visual systems, which include 24 eyes, to hunt for tiny “copepods” in the roots of mangroves.

While the mangrove roots make great hunting grounds, they are also a dangerous place for soft-bodied jellyfish.
So, when they approach the mangrove roots, they turn and swim away quite fast. If they turn away too soon, they won’t have enough time to catch any copepods. But if they turn too late, they might bump into the root and damage their soft bodies. This means assessing distances is crucial to their survival.

“Our experiments show that contrast, i.e., how dark the root is in relation to the water, is used by the jellyfish to assess distances to roots, which allows them to swim away at just the right moment. Even more interesting is that the relationship between distance and contrast changes on a daily basis due to rainwater, algae and wave action,” explained Garm.

They saw that as each new day of hunting began, the box jellyfish learned from current contrasts by combining visual inputs and sensations during evasive manoeuvres that failed. Despite just having one thousand nerve cells, they can connect the experiences from previous manoeuvres to understand what to do in the future—what is called associative learning. “And they actually learn about as quickly as advanced animals like fruit flies and mice,” adds Garm.

This discovery could be big news for fundamental neuroscience, widening the perspective on what can be achieved by rudimentary nervous systems. This also suggests that advanced learning may have been one of the most important evolutionary benefits of the nervous system from a long, long time ago.