Are Jellyfish Smarter Than We Think?

10:03 minutes

Caribbean box jellyfish, Tripedalia cystophora, with a dead fish in its transparent stomach, Bocas del Toro, Panama
A Caribbean box jellyfish with a fish inside its transparent stomach. Credit: Shutterstock

Jellyfish are known for their graceful, hypnotic movement through the water—and for occasionally stinging swimmers. One thing they’re not known for, however, is intelligence. A study published in the journal Current Biology, however, challenges the idea of the ‘brainless’ jellyfish by showing that at least one species of jelly may be capable of associative learning. 

The scientists were studying the Caribbean box jellyfish, which normally lives amongst a forest of tangled mangrove tree roots. In the lab, they painted false roots on the walls of the jellyfish’s tank, and watched to see what happened. At first, the jellies judged the low-contrast gray roots to be far away, and tried to swim through them. After a few collisions with the tank, however, the jellies learned that the false roots were closer than they appeared, and learned to keep their distance. 

Dr. Anders Garm, an associate professor of marine biology at the University of Copenhagen in Denmark, joins guest host Dessa to explain the experiment, and what it tells researchers about the connection between the behavior of small groups of neurons and the process of learning.

Segment Guests

Anders Garm

Dr. Anders Garm is an associate professor of marine biology at the University of Copenhagen in Copenhagen, Denmark.

Segment Transcript

DESSA: This is Science Friday. I’m Dessa.

For the rest of the hour, two stories about the wonderful world of jellyfish. Jellyfish are easy to admire for their translucent, otherworldly beauty, their hypnotic locomotion, but they haven’t been celebrated for their intelligence. Scrabble partners they are not. But researchers have found that even though these fragile organisms don’t really have a brain, they’re able to learn and change their behaviors in response to past mistakes.

Joining me now to talk about this discovery is Dr. Anders Garm. He’s an associate professor of marine biology at the University of Copenhagen, and one of the authors of a paper on the findings in the journal Current Biology. Welcome to Science Friday.

ANDERS GARM: Thank you very much.

DESSA: So jellyfish don’t have brains. What do they have?

ANDERS GARM: Oh, they have lots of things. These jellyfish we’re talking about here are pretty special jellyfish. Those are the one we call the box jellyfish, or cubomedusae. They have a structure hanging beneath the bell– actually, they have four of them– we call them rhopalia. And in there, we have sort of a central nervous system, even though pretty simple. So each of these four structures have about 1,000 neurons each. So not much, but still some nervous system.

DESSA: And can you just provide a little bit of context here? How much is 1,000 neurons? How many would you find in a mouse or something simple like a fruit fly?

ANDERS GARM: Yeah, many more. Take a fruit fly, which is one of the model systems we use to study the brain, and they have between 200,000 and 300,000 neurons. If you take a mouse, it’s way many more. It’s hundreds of millions. So 1,000 is very few.

DESSA: So can you walk me through the experiment as you conducted it and tell me a little bit about your findings?

ANDERS GARM: Yes. The ones we did first were a set of behavioral experiments. And what we really wanted to make sure here was that we’re testing natural behavior. So these jellyfish are found in the mangroves in the Caribbean, and they are living in between the prop roots of the mangrove trees. So we copied their natural habitat, the mangrove root areas, in the lab in our setup. And in there, we could change the appearance of these root mimics we were using.

We could give them a low contrast, which would signal them being far away, even though they were close by. We could give them high contrast signaling that they were close by, as they were. Or we could remove contrast completely. And by combining these three different types of root mimics, we could show that only when they were fooled by a root that was nearby, but appearing far away and thereby bumping into them, and then combining this impression visually of the low contrast, but still getting the mechanical stress of bumping into it, they would learn that, now, actually, the low contrast would mean close by and then they would learn to avoid it.

DESSA: Got it. So they learned that these gray bars in the tank didn’t mean faraway roots; they were an obstacle that the jellies had to avoid?

ANDERS GARM: Exactly. So we were tricking them into learning by giving them a mimic that appeared further away than it was.

And then the two other setups were control experiments. The high contrast roots, where they would never bump into them, showed that, when taking away the mechanical stimuli– the mechanical stress of bumping into things– they didn’t learn, they didn’t change behavior. And opposite, when we used the no contrast uniform gray wall and saw that the animals were bumping into the wall constantly, meaning that they got lots of stress signal there, but no visual input, they wouldn’t change their behavior either and didn’t learn not to avoid the wall.

So again, showing that it is the combination of the visual input, low contrast, and the mechanical stress that enables them to learn. And this is what we call associative learning or operant conditioning.

DESSA: And how long did it take them to learn to avoid these painted stripes?

ANDERS GARM: That was where we were surprised. I mean, I have to admit that we were expecting that they would learn because it would make so much sense for them to be able to learn this in their habitat. But the speed of learning was quite a surprise. Because it turns out that only repeating these faulty avoidances, getting the mechanical stimuli, three, four, or five– sometimes they needed six times– but between three and six times was enough.

Then they had learned the distance contrast relationship and started avoiding. And this low number of repetitions is basically comparable to a lot of other animals that are classical in learning experiments, like flies, crabs, and mice.

DESSA: Once they had made the association, right– I see these stripes, I’ve learned to avoid them, because otherwise I’d bash myself into the wall– how long did they retain that information?

ANDERS GARM: Yeah, that we are not completely sure. We haven’t looked at whether it’s only short-term memory or some kind of long-term memory. But what we can assume is they would not remember it for too long because that wouldn’t make sense to them. Because they need to have it updated with how the water changes. So we expect that this would maybe last half an hour, an hour, and then they would need to relearn.

DESSA: Wow. And when you see the water change, it’s like, hey, if the water is really murky, I need to adjust the level of contrast that I would expect from nearby stuff to far away stuff. So you’ve got to update that constantly, is that right, if you’re a jellyfish?

ANDERS GARM: Yeah, exactly. Yes.

DESSA: OK. Now, can you differentiate this kind of learning from a reflexive action? Are the jellyfish planning to evade these obstacles?

ANDERS GARM: These are different behaviors. So the thing about learning is that it’s one of the major mechanisms behind plasticity and behavior. So it’s basically not just a response to a stimulus. It’s, you can say, a clever or intelligent way to respond to a stimulus that is dependent on previous experiences. And that’s what takes learning apart from other things. It’s not a reflex because it requires prior experience.

DESSA: We mentioned at the very beginning that there’s no centralized system like a brain. There are these approximately 1,000 neurons throughout the jellyfish’s body. Does that mean that, without a centralized processing center, certain parts of the jellyfish can learn what other parts of it doesn’t know?

ANDERS GARM: Yeah. A small correction here. I mean, they have more than 1,000 neurons, but in the center there that is learning there is 1,000 neurons– this part of the rhopalium nervous system, we call it. And we could show in really nice experiments also because this rhopalium can be detached from the animal. And the interesting part about this is that, with a simple animal like this, you can take a part of the body out and that would not really realize it’s not part of the body anymore. So it will behave like it’s still on the jellyfish.

And we have this neuronal fingerprint– the nerve activity fingerprint– of this avoidance behavior. So we can actually measure directly from what this part of the nervous system is trying to tell the animal whether it wants it to do the behavior or not. And that was how we actually could show that it happens in these 1,000 neurons.

But you’re completely right. What is an almost philosophical question here is that since they have this repeated four times along the bell, four of these rhopalia, if one of the rhopalia is learning, is it able to transmit what it has learned to the other four rhopalia? Which is an extremely interesting question, and we would love to look into this. And these are actually future plans we want to do.

DESSA: Speaking of future plans, what is the next step? For your own research, does this lead to the next big question about how jellyfish are functioning in the wild?

ANDERS GARM: Yeah. I mean, what we would actually like to take is take it out of the jellyfish. Because we think that with this jellyfish, we have a really cool model system for understanding some of the fundamental processes, the cellular processes, that happens when a cell or a nerve circuit is learning.

So with these 1,000 neurons– and this is what we’re doing now– we hope to be able to make a complete circuitry– what we call a connectome– of these 1,000 neurons, map exactly how these neurons look like, where they’re arranged in the body, and how they communicate with synapses.

And once we have this diagram, we can pinpoint what parts of the circuitry is most likely involved in these learning processes, depending on how they connect. And then we can go in afterwards, both with molecular methods and with physiological methods, and examine these neurons and compare naive neurons to neurons that has been part of this learning process, and then detect what has actually changed both on the cellular level, but also on the circuitry level. And in this way, we hope to be able to get a much more in-depth understanding of advanced learning, like associative learning and operant conditioning.

DESSA: OK. Speaking of associative learning and revising one’s behavior from past mistakes, the Caribbean box jellyfish, that’s venomous, right? Have you personally been stung by a jellyfish in your trials?

ANDERS GARM: I have been stung by a high number of different jellyfish by now. One of the reasons why we have chosen this exact jellyfish, the Tripedalia cystophora, as it’s called, is it’s a copepod eater. And there is this very close connection in jellyfish between the size of their prey and the strength of their venom.

And since copepods are really small animals, this one here is about as venomous as the common moon jelly. And this actually means you would need to kiss it to actually feel the venom because that’s one of the places you have live skin that is sensitive enough.

DESSA: Oh, I hope you haven’t felt as much. Thank you so much for joining me today. That was Dr. Anders Garm, an associate professor of marine biology at the University of Copenhagen, in Denmark. Thanks for taking the time.

ANDERS GARM: Thank you very much.

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