Short Wave - Prepare to be baffled by what we don't know about eels

Episode Date: June 5, 2026

More than a century ago, all that people knew about European eels was that they lived in the rivers and streams for decades — until they swam out to the ocean and never returned. Eventually, tiny eels would show up and the cycle would start again. Where did the adult eels go? Where did the baby eels come from? Did they even reproduce at all or just spontaneously emerge into being? Science now has some — but not all — of the answers to these questions. Today on the show, Regina G. Barber talks to fish physiologist Arjan Palstra about this mystery and how close scientists are to solving it. If you liked this episode, check out our episode on the Pacific lamprey.Interested in more science mysteries? Email us your question at [e-mail] to every episode of Short Wave sponsor-free and support our work at NPR by signing up for Short Wave+ at plus.npr.org/shortwave.See pcm.adswizz.com for information about our collection and use of personal data for sponsorship and to manage your podcast sponsorship preferences.NPR Privacy Policy

You're listening to Shortwave from NPR. Hey, Shorewaver is Regina Barber here with a modern-day eel mystery. To this day, no one knows where they come from. Well, not entirely. Centries ago, people thought that baby eels just sprang up spontaneously from morning dew. Or from mud or from slime. So they thought it was not like an animal that was reproducing with just, started to exist spontaneously from something.

Arian Paulstra is a fish physiologist at Vaganning University and Research in the Netherlands. He says eventually people started looking for eel reproductive organs like gonads to convince the world that spontaneous generation wasn't happening. Even a big name like Sigmund Freud, he started his career by looking for the gonads of eel, but never found him. A couple decades later, somebody, found an adult eel in the ocean, sex organs and all. And that part of the mystery was solved. But still, no one knew where they went to make baby eels. All they knew was that decades-old eels

living in rivers would swim out to sea and never come back. Somewhere in the 1890s, Italians discovered larvae were found in the seas around Italy. That was basically the starting point for Johannes Schmidt. Over multiple sea voyages, Danish biologists Johanna Schmidt would eventually trace smaller and smaller eel larvae to the Sargasso Sea in the middle of the Atlantic Ocean.

That's where he found larvae. And at that moment, that was the most indirect proof that eels must be spawning right there in the sagasso sea. Then a century went by with no major breakthroughs. Until the late 2010s,

when scientists attached satellite tags to a couple dozen eels by a chain of Portuguese islands in the Mid-Atlantic. When the trackers were recovered, they showed the first direct evidence that adult European eels go to the Sargasso Sea to spawn. But to this day, no one has found a mature eel in the act of spawning in the Sargasso Sea, a 2 million square mile area. We're talking about going to Mars, but in the meantime, we don't know many things of our deep seas. That discovery could help explain why eel populations have declined and guide ways to raise them in captivity, which could boost eel numbers in the wild and decrease illegal trafficking.

Today on the show, the hunt for the spawning grounds of the European eel. Plus, a look at their quirky lives and what solving this mystery would mean for the future of the species. I'm Regina Barber, and you're listening to Shortwave, the science podcast from End. PR. So, Ariane, why have scientists still not found a European eel spawning in the Sargasso Sea? That's a very good question. It may have to do with the fact that they don't have a specific site where they spawn. For instance, Japanese eel that spawns in the Marianna Ridge.

We know that they do that near sea mounts, and they do that in upwelling areas near sea mounds. and also at a specific time at new moon. So if you can pinpoint where you have to be in at what time, then you may be able to catch them in the act of spawning. And Professor Tsukamoto, he has done that already back in the 90s or the Japanese eel. But for European eels, they swim to do Segasso Sea, but not to a specific site.

it seems that they are spawning along an axis, others I guess you see, which may be as long as about 2,000 kilometers, so it's not a specific site, but it probably has to do with the Earth magnetic fields. That's awesome. Yeah. So the sea is big, but they are just like close to the surface, right? Well, no, not that close. Well, for European eel, I don't know exactly.

For Japanese eel, they're like around 100. 150 meters of depth. So that's still quite deep. And the eels are supposed to spawn even deeper, maybe about 300 meters deep. So despite these eels being so mysterious, people have learned some things about them. They are weird. Like, let's talk about some of this weirdness. Like, what do scientists know about their life cycle? Well, it's everything that happens on the continent. We know a lot about it. But what I say, the moment that they disappear into the ocean and like almost a year later we get small transparent eels back

that was always a black box. So eels are born in the Sarcastro Sea. These larvae, they turn into willow leaf-shaped larvae, they're about just a few centimeters. And by the currents they're taken to the European continent. They're metamorphose into tiny transparent glasios. and they swim up the rivers.

Actually, the moment that they swim into the freshwater, then they start to pigment. Wow. And then they spend a long life basically growing. And when they grow, that's when a lot of the differences in sex emerge, right? Yeah, there is a big difference between the sexes. I mean, when the densities are high, many fish become males, and they're quite small. They only have to produce sperm and make sure that diversity is maintained. The females, that's a different story.

When the densities are low, so mostly in upstream areas, you get more females. The females, they have to produce many eggs, so they have to be big and fat, and that takes many, many years in order to reach that stage. Okay. So imagine like a male is on average maybe 40 centimeters. while a female can be 80 centimeters, a meter. It really has to be big. The bigger and older, the better.

Finally, they return to their birthgrounds in the Sergessus Sea. You spent like 10, 20, 30 years in order to become as big and as fed in order to be able to do that. After swimming 5,000 kilometers, they spawn and they die, and the whole cycle starts again. So there is still a huge gap in their life. cycle and then particularly the oceanic part. This creates all kinds of problems. So for the European eel population has been in decline between the 1980s up to 2010. Wow.

And it's very difficult to find out which are the exact causes and what we can do about it and how we can manage the population. We do not even know how many eels leave to the Sarcastosy every year. And on the other hand, for aquaculture, it's important to artificially reproduce eels, but we still cannot. Also, because we completely lack a natural reference. We know what's happening in the lab, but how normal or natural that is, we have no idea. Sure. And for our listeners who may not know, because I didn't know before this episode, aquaculture is just breeding and raising eels in captivity.

Yeah. Okay, so scientists now know that European eels go to spawn somewhere in the Sargasso Sea, but they don't know exactly where. What are the current efforts to solve this mystery? Yeah, from every two or three years, there is a cruise organized by the group of Vaino Vhano. So basically they spent several weeks in the Chagasus Sea trying to get larvae. I've never joined one of their cruises.

Yeah, do you want to? Well, I was always a bit reluctant to be weeks and weeks at sea and spent many, many hours of looking through a microscope, whatever comes up. Okay. What are some of the hypotheses like, though? Like, what are they hoping to find? The thing which is still mysterious is catching eggs. And of course, catching adults called the act of spawning.

And perhaps they're carcasses. That must also be somewhere there. Because they still haven't found the carcasses, right? No, yeah, that's amazing. Wow. Yeah. You would expect that it would draw the attention of all kinds of predators going in that direction to eat these 100,000 of eel carcasses that are drifting there. And then finding eggs would be a major next step, I would say, which would be also interesting for us, because then we finally have a natural reference for the eggs that we find in our laboratory, because probably there are some differences, as they found.

for Japanese eel. And if you know the differences, then you know what you're doing wrong, or at least what you should improve. Yeah. So you work on eel reproduction in the lab, among other species. What have you learned by studying them in the lab? Well, first of all, the big goal is, of course, to close the cycle in the lab. If we are able to reproduce eels in the lab, We can produce glass yields. Then that's not only a source for aquaculture, but you can also release the natural population from fishing pressure. Right, because then you'd have a source of food that isn't the wild population.

So have you been able to raise glass eels from eggs in the lab? No, not yet. So nowadays, glass eels are caught and they're raised in agriculture to become big eels. and that can be used for consumption. But if you can produce these glances yourself, you don't have to fish them anymore. You can basically uncouple nature and aquaculture. Going back to the search for eel spawning grounds,

what would it mean if scientists did find them, other than solving this centuries-old mystery? Well, other than, that's the main thing, I guess. It's amazing that is now a 20, And we're still dealing with this large biological mystery, which has never been solved. But if you have discovered it, it brings along a lot of knowledge that you can use to preserve this species, to help it recover from the decline that they have suffered. You've been studying eels for decades.

Like, what draws you to these animals? Well, I think some people consider them slimy and ugly, but now for me they're beautiful and their life cycle shows how intriguing these animals are. I mean, there's still so many questions that you must be triggered by it. What's the Holy Grail for you then? Like, what would you love to learn about eels? Yeah, it's not one thing. I'm already intrigued by the fact that at which moment, do they start to swim to the Sagasy sea?

And why? Like what triggers them? Yeah, that's still a longstanding physiological question. Like, we see what happens, but we don't know why. And if we know why, we can use that information, of course. Well, some people consider it like it should always remain a biological mystery. It's so beautiful. So we should not discover the final truth.

It should be mythical forever. Yeah. On the other hand, yeah, there are so many questions. So when are they leaving? Why do some eels can get so old if they don't go to the sick as to sea? Their stories known of eels becoming 150 years. Imagine that you're 150 years and you're still like not even a puber. You're like a Peter Pan.

So if you do not reproduce your eternal life. I mean, that's great to know. I would love that. It's too late for me. I already have a kid. Yeah, for me too. Ariane, thank you so much for talking to me about Eels. I learned so much.

All right. Yeah, thank you. Short waivers, we are proud to be part of the public radio ecosystem. Help support us in making the show by following us on the NPR app or wherever you listen from. We can't do this without you. This episode was produced by Burley McCoy. It was edited by a showrunner Rebecca Ramirez, and the facts were checked by Tyler Jones. The audio engineer was Jimmy Keely.

I'm Regina Barber. Thanks for listening to Shortwave from NPR.