Jellyfish are smarter than you think

After more than 500 million years on Earth, the immense evolutionary success of jellyfish is undeniable. Still, we've always thought of them as simple creatures with very limited learning abilities.

The prevailing opinion is that more advanced nervous systems equate with more advanced learning potential in animals. Jellyfish and their relatives, collectively known as cnidarians, are considered to be the earliest living animals to develop nervous systems and to have fairly simple nervous systems and no centralized brain.

For more than a decade, neurobiologist Anders Garm has been researching box jellyfish, a group of jellyfish commonly known for being among the world's most poisonous creatures. But these lethal jellies are interesting for another reason as well: it turns out that they are not quite as simple as once believed. And this shakes our entire understanding of what simple nervous systems are capable of.

"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 behavior," says Anders Garm, an associate professor at the University of Copenhagen’s Department of Biology.

One of the most advanced attributes of a nervous system is the ability to change behavior as a result of experience – to remember and learn. The research team, headed by Jan Bielecki of Kiel University and Anders Garm, set out to test this ability in box jellyfish. The findings have just been published in the journal Current Biology.

A thousand nerve cells are more capable than once thought

The scientists studied the Caribbean box jellyfish, Tripedalia cystophora, a fingernail-sized medusa that lives in Caribbean mangrove swamps. Here, they use their impressive visual system including 24 eyes to hunt for tiny copepods among mangrove roots. While making for a good hunting grounds, the web of roots is also a dangerous place for soft-bodied jellies. 

So, as the small box jellyfish approach the mangrove roots, they turn and swim away. Should they veer too soon, they won’t have enough time to catch any copepods. But if they turn too late, they risk bumping into the root and damaging their gelatinous bodies. Thus, assessing distances is crucial for them. And here, contrast is the key, as the researchers discovered: 

"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," says Anders Garm, who continues:

"We can see that as each new day of hunting begins, box jellyfish learn from the current contrasts by combining visual impressions and sensations during evasive manoeuvres that fail. So, despite having a mere one thousand nerve cells – our brains have roughly 100 billion – they can connect temporal convergences of various impressions and learn a connection – or what we call associative learning. And they actually learn about as quickly as advanced animals like fruit flies and mice."

The new research results break with previous scientific perceptions of what animals with simple nervous systems are capable of:

"For fundamental neuroscience, this is pretty big news. It provides a new perspective on what can be done with a simple nervous system. This suggests that advanced learning may have been one of the most important evolutionary benefits of the nervous system from the very beginning," says Anders Garm.

Caribbean box jellyfish among underwater mangrove roots. Video: Anders Garm.

Seeking the brain cells where memory is housed 

The research team has also shown where the learning is happening in these box jellyfish. This has given them unique opportunities for how to now study the precise changes that occur in a nerve cell when it is involved in advanced learning.   

"We hope that this can become a supermodel system for looking at cellular processes in the advanced learning of all sorts of animals. We are now in the process of trying to pinpoint exactly which cells are involved in learning and memory formation. Upon doing so, we will be able to go in and look at what structural and physiological changes occur in the cells as learning takes place," says Anders Garm.

If the scientists are able to pinpoint the exact mechanisms in jellyfish involved in learning, the next step will be to find out whether it applies specifically to jellies or if it can be found in all animals. 

"Eventually, we will look for the same mechanisms in other animals, to see if this is how memory works in general," says the researcher.

This kind of groundbreaking knowledge could be used for a wealth of purposes, according to Anders Garm:

"Understanding something as enigmatic and immensely complex as the brain is in itself an absolutely amazing thing. But there are unimaginably many useful possibilities. One major problem in the future will undoubtedly be various forms of dementia. I don’t claim that we are finding the cure for dementia, but if we can gain a better understanding of what memory is, which is a central problem in dementia, we may be able to lay a building block to better understand the disease and perhaps counteract it," concludes the researcher.