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

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 https://www.npr.org/people/1082526815/regina-g-barber talks to fish physiologist Arjan Palstra https://www.wur.nl/en/persons/drir-ap-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 [email protected] mailto:[email protected]. Listen 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 http://plus.npr.org/shortwave. See pcm.adswizz.com https://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 https://www.npr.org/about-npr/179878450/privacy-policy

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• 0:18Speaker 00

  You're listening to Short Wave from NPR.

• 0:23Regina Barber?

  Hey, Short Wavers. Regina Barber here with a modern day eel mystery. To this day, no one knows where they come from. Well, not entirely.

• 0:36Regina Barber?

  Centuries ago, people thought that baby eels just sprang up spontaneously from morning dew.

• 0:42Speaker 02

  Or from mud or from slime. So they thought it was not like an animal that was reproducing, but just started to exist spontaneously from something.

• 0:53Regina Barber?

  Ariane Polstra is a fish physiologist at Wageningen 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.

• 1:09Speaker 02

  Even a big name like Sigmund Freud, he started his career by looking for the gonads of eel, but never found them.

• 1:17Regina Barber?

  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.

• 1:36Speaker 02

  Somewhere in the 1890s, Italians discovered larvae were found in the seas around Italy. That was basically the starting point for Johannes Schmidt.

• 1:49Regina Barber?

  Over multiple sea voyages, Danish biologist Johannes Schmidt would eventually trace smaller and smaller eel larvae to the Sargasso Sea in the middle of the Atlantic Ocean.

• 1:59Speaker 02

  That's where he found larvae. And that, at that moment, that was the, like, the most indirect proof that eels must be spawning right there in the Sargasso Sea.

• 2:09Regina Barber?

  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.

• 2:38Speaker 02

  We're talking about going to Mars, but in the meantime, we don't know many things of our deep seas.

• 2:46Regina Barber?

  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, 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 Short Wave, the science podcast from NPR.

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• 4:48Regina Barber?

  So, Ariane, why have scientists still not found a European eel spawning in the Sargasso Sea?

• 4:55Speaker 02

  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 Mariana Ridge, we know that they do that near sea mounds. And they do that in upwelling areas near sea mounds and also at a specific time, at new moon. So...

• 5:19Regina Barber?

  Oh, wow.

• 5:20Speaker 02

  And if you can pinpoint where you have to be and 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 for the Japanese eel. But for European eels, they swim to the Sargasso Sea, but not to a specific site. It seems that they are spawning along an axis over the Sargasso Sea, 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 field.

• 5:57Regina Barber?

  That's awesome.

• 5:59Speaker 02

  Yeah.

• 5:59Regina Barber?

  So, the sea is big, but they are just like close to the surface, right?

• 6:04Speaker 02

  Well, no, not that close. Well, for European eel, I don't know exactly. For Japanese eel, they're like around 150 meters of depth. So that's still quite deep. And the eels are supposed to spawn even deeper, maybe about 300 meters deep.

• 6:22Regina Barber?

  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?

• 6:34Speaker 02

  Well, it's everything that happens on the continent, we know a lot about it. But what I say at 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 Sarkasso Sea. These larvae, they turn into willow leaf-shaped larvae, like the safety larvae. They're about just a few centimeters. And by the currents, they're taken to the European continent. There they metamorphose into tiny transparent glass eels. And they swim up the rivers. Actually, the moment that they swim into the freshwater, then they start to pigment.

• 7:15Regina Barber?

  Wow.

• 7:16Speaker 02

  Then they spend a long life, basically growing.

• 7:21Regina Barber?

  And and when they grow, that's when a lot of the differences in sex emerge, right?

• 7:26Speaker 02

  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.

• 7:47Speaker 02

  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 to reach that stage.

• 8:01Regina Barber?

  Okay.

• 8:01Speaker 02

  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 birth grounds in the Sarkasso Sea. You spend like 10, 20, 30 years in order to become

• 8:22Regina Barber?

  Oh, wow.

• 8:22Speaker 02

  as big and as fat in order to to be able to do that. After swimming 5,000 kilometers, they spawn and they die, and the whole cycle starts again.

• 8:33Speaker 02

  So there is still a huge gap in their life cycle and then particularly the oceanic part. That creates all kinds of problems. So for the European eel population has been in decline between the 1980s up to 2010.

• 8:49Regina Barber?

  Wow.

• 8:50Speaker 02

  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 Sargasso Sea 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.

• 9:21Regina Barber?

  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.

• 9:29Speaker 02

  Yeah.

• 9:30Regina Barber?

  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?

• 9:39Speaker 02

  Yeah, from every two or three years, there is a cruise organized by the group of Hainavano. So basically, they spend several weeks in the Sargasso Sea trying to catch larvae. I've never joined one of their cruises.

• 9:56Regina Barber?

  Yeah, do you want to?

• 9:57Speaker 02

  Well, I was always a bit reluctant, like, to be weeks and weeks at sea and spend many, many hours looking through microscopes, whatever comes up.

• 10:11Regina Barber?

  Okay. What are some of the hypotheses like, though? Like, what are they hoping to find?

• 10:16Speaker 02

  The thing which is still mysterious is catching eggs and of course catching adults called the active spawning and perhaps their carcasses that must also be somewhere there.

• 10:28Regina Barber?

  Because they still haven't found the carcasses, right?

• 10:31Speaker 02

  No, yeah, that's amazing. Yeah. You would expect that that would draw the attention of all kinds of predators going in that direction to eat these hundred thousands 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.

• 11:11Regina Barber?

  Yeah, so you work on eel reproduction in the lab, among other species. What have you learned by studying them in the lab?

• 11:21Speaker 02

  Oh, 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, and we can produce glass eels, then that's not only a source for aquaculture, but you can also release the natural population from fishing pressure.

• 11:44Regina Barber?

  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?

• 11:52Speaker 02

  No, not yet. So nowadays, glass eels are caught and they're raised in aquaculture to become big eels, and that can be used for consumption. But if you can produce these glass eels yourself, you don't have to fish them anymore. You can basically uncouple nature and aquaculture.

• 12:13Regina Barber?

  Going back to the search for eel spawning grounds, like, what would it mean if scientists did find them, like, other than solving this like centuries-old mystery?

• 12:24Speaker 02

  Well, other than, that's the main thing, I guess. It's amazing that it's now 2026, and we're still dealing with this large biological mystery, which has never been solved. But if you have discovered that, 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.

• 12:52Regina Barber?

  You've been studying eels for decades. Like, what draws you to these animals?

• 12:59Speaker 02

  Well, I think some people consider them slimy and ugly. But no, 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.

• 13:15Regina Barber?

  What's the holy grail for you, then? Like, what would you love to learn about eels?

• 13:22Speaker 02

  Yeah, it's not one thing. I'm already intrigued by the fact that at which moment do they start to swim to the Sargasso Sea? And why?

• 13:31Regina Barber?

  Yeah, like, what triggers them?

• 13:33Speaker 02

  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?

• 13:54Regina Barber?

  It should be mythical forever.

• 13:56Speaker 02

  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 Sargasso Sea? There's stories known of eels becoming 150 years. And imagine that you're 150 years and you're still like, not even a puer. You're like a Peter Pan. So if you do not read the Dusy's Eternal Life.

• 14:24Regina Barber?

  I mean, that's great to know. I would love. It's too late for me, I already have a kid.

• 14:29Speaker 02

  Yeah, for me, too.

• 14:32Regina Barber?

  Ariane, thank you so much for talking to me about eels. I learned so much.

• 14:38Speaker 02

  All right, yeah, thank you.

• 14:44Regina Barber?

  Shortwavers, 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 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 Short Wave from NPR.

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