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

Summary

Podcast Summary: Short Wave (NPR)
Episode: Prepare to be baffled by what we don’t know about eels
Date: June 5, 2026
Host: Regina Barber
Guest: Dr. Ariane Palstra, Fish Physiologist, Wageningen University (Netherlands)

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Episode Overview

This episode dives into one of biology’s enduring mysteries: Where do European eels spawn? Host Regina Barber and fish physiologist Dr. Ariane Palstra discuss centuries-old misunderstandings, recent scientific breakthroughs, and the quirky, still-mysterious life cycle of the European eel. The story is part science history, part real-time research adventure — with humor and wonder woven throughout.

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Key Discussion Points & Insights

1. Early Theories and Historical Mysteries (00:21–02:36)

• Spontaneous Generation:

  • Centuries ago, people believed eels emerged “from dew, mud, or slime” ([00:40], Palstra), not from reproduction.
  • Quote:
    • “They thought it was not like an animal that was reproducing, but just started to exist spontaneously from something.” — Dr. Ariane Palstra [00:43]

• Scientific Investigation:

  • Not even Freud could find eel reproductive organs when he searched for them as a young scientist.
    • “Even a big name like Sigmund Freud...never found them.” — Dr. Ariane Palstra [01:07]

• First Clues to Spawning Grounds:

  • Early 1900s: Johannes Schmidt traces larval eels from European coasts to the Sargasso Sea (middle of the Atlantic).
  • “That was the most indirect proof that eels must be spawning right there in the Sargasso.” — Palstra [01:57]

2. Modern Research: Chasing the Spawning Secret (02:36–05:48)

• Tagging Breakthrough:

  • Late 2010s: Satellite tags on adult eels provided direct evidence that European eels migrate to the Sargasso Sea to spawn.

• Enduring Mystery:

  • No scientist has ever witnessed eels spawning, found eggs, or discovered their carcasses in the Sargasso’s vastness.

  • “We're talking about going to Mars, but in the meantime, we don't know many things of our deep seas.” — Palstra [02:36]

3. Why Is the Mystery Unsolved? (04:40–06:12)

• Spawning Behavior:

  • Unlike Japanese eels (which spawn at precise locations and times), European eels likely spawn along a vast, shifting “axis” up to 2,000 km long, possibly using Earth’s magnetic fields as cues.

    • “It's not a specific site…but it probably has to do with the earth magnetic fields.” — Palstra [05:37]

• Eels' Depth:

  • European eels are thought to spawn up to 300 meters deep, making observation even more challenging ([05:55]–[06:12]).

4. The Bizarre Life Cycle of the European Eel (06:12–08:40)

• Transformation Stages:

  • Born as tiny, transparent willow-leaf-shaped larvae (leptocephali) in the Sargasso Sea; carried by currents to Europe.
  • Metamorphose into ‘glass eels’ that become pigmented in freshwater, then grow into adults.

• Sex & Size Differences:

  • In crowded rivers, more males appear (smaller size, produce sperm); in less dense, upstream areas, more females develop (larger, to produce myriad eggs).
    • Males: ~40 cm; Females: up to 1 meter ([07:37]–[08:00])

• A Long Journey:

  • After 10–30 years, adults swim 5,000 km back to Sargasso, spawn, and die.

5. Conservation, Aquaculture, and the Lab (08:40–12:04)

• Population Decline:

  • European eel populations have dropped dramatically since the 1980s.
  • “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.” — Palstra [08:40]

• Aquaculture:

  • Essential for food supply and avoiding further stress on wild populations, but success is hampered by gaps in natural knowledge.
  • So far, scientists cannot fully breed eels in captivity (“close the cycle”); they still rely on catching wild glass eels.

• The Holy Grail:

  • If scientists could find and study naturally occurring eggs in the Sargasso, it would set a reference point for aquaculture practices and improve conservation.
    • “Finding eggs would be a major next step…then we finally have a natural reference for the eggs that we find in our lab.” — Palstra [10:21]

6. The Hunt for Spawning Grounds (09:29–12:42)

• Research cruises go out every 2–3 years to try to catch eel larvae and, one day, eggs or spawning adults.
• “There is still a huge gap in their life cycle and particularly the oceanic part, that creates all kinds of problems.” — Palstra [08:22]
• No one has found eel eggs or mass eel carcasses post-spawning—another persistent puzzle.

7. Personal Fascination and the Mythos of Eels (12:42–14:22)

• Dr. Palstra elaborates on why eels are so captivating:

  • They are misunderstood, associated with myth, persistently mysterious, and biologically fascinating.
  • “For me, they’re beautiful. And their life cycle shows how intriguing these animals are.” — Palstra [12:49]

• Lifespan Wonders:

  • Some eels live for over a century without spawning — “Imagine that you’re 150 years and you’re still…like a Peter Pan. So if you do not reproduce, you have eternal life.” — Palstra [14:08]

• Some scientists appreciate the mystery and myth as part of what makes eels so special.

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Notable Quotes & Memorable Moments

• On Earth’s Unsolved Mysteries:

  • “We’re talking about going to Mars, but in the meantime, we don’t know many things of our deep seas.” — Ariane Palstra [02:36]

• On Science’s Limitations:

  • “You would expect that it 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.” — Palstra [10:21]

• On the Beauty of Eels:

  • “For me, they’re beautiful. And their life cycle shows how intriguing these animals are.” — Ariane Palstra [12:49]

• On Eel Immortality:

  • “Imagine that you’re 150 years and you’re still…like a Peter Pan. So if you do not reproduce, you have eternal life.” — Ariane Palstra [14:08]

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Important Timestamps

| Timestamp | Segment | |-----------|---------| | 00:21 | Introduction of eel mystery & historical perspective | | 01:07 | Freud’s early eel investigation | | 01:47 | Johannes Schmidt and the Sargasso Sea | | 02:36 | The challenge of locating spawning eels today | | 04:40 | Why haven’t scientists observed spawning? | | 06:24 | Eel life cycle explained | | 07:17 | Sex differences in eels discussed | | 08:40 | Conservation challenges and population decline | | 10:06 | The search for eggs and carcasses | | 11:34 | Aquaculture and breeding efforts in the lab | | 12:14 | What finding the spawning grounds would mean | | 12:49 | Dr. Palstra’s personal fascination with eels | | 14:08 | Eel longevity and the “Peter Pan” phenomenon |

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Episode Tone & Language

• Tone:

  • Warm, curious, sometimes playful — never taking itself too seriously, but always respectful of the scientific mystery.

• Language:

  • Approachable and engaging, with clear explanations and an undercurrent of awe at nature’s complexity.

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Summary:
This Short Wave episode invites listeners into a centuries-old mystery, with European eels as the enigmatic protagonists. From mistaken beliefs about origin, through ocean-crossing journeys, to desperate hunts for proof in the Sargasso Sea, the episode mixes science history, modern research, and philosophical wonder. Dr. Palstra and Regina Barber highlight both the frustrating limits of current knowledge and the endless fascination of one of nature’s most elusive creatures. Even in 2026, the European eel’s spawning story remains one of the greatest unsolved puzzles in biology.

Transcript

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[NPR Host]

You're listening to Short Wave from NPR.

[Regina Barber]

Hey, short waivers. Regina Barber here with a modern day eel mystery. To this day, no one knows where they come from. Well, not entirely. Centuries ago, people thought that baby eels just sprang up spontaneously from morning dew

[Ariane Palstra]

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.

[Regina Barber]

Ariane Palstra is a fish physiologist at Wageningen University and research in the Netherland. He says eventually people started looking for eel reproductive organs like gonads to convince the world that spontaneous generation wasn't happening.

[Ariane Palstra]

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

[Regina 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.

[Ariane Palstra]

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

[Regina 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.

[Ariane Palstra]

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

[Regina 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.

[Ariane Palstra]

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

[Regina 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 year. 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 npr.

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from Integrative Therapeutics, makers of Cortisol Manager named Best Overall Ashwagandha supplement by Vogue. Use code STRESS2026 for 20% off of cortisol manager on Amazon.com so Ariane, why

[Regina Barber]

have scientists still not found a European eel spawning in the Sargasso Sea?

[Ariane Palstra]

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 in the Mariana Ridge, we know that they do that near sea mounts and they do that in in upwelling areas near sea mounds and also at a specific time at New Moon. So. Oh wow. If you can pinpoint where you have to be and at what time, then you may be able to to catch them in the act of spawning. And Professor Tsukamoto, he has done back in the in the 90s for the Japanese eel, but for European eels, they swim to the Sagasso Sea, but not to a specific site. It seems that they are spawning along an axis sea which may be as long as about 2,000km. So it's not a specific site, but it probably has to do with the earth magnetic fields.

[Regina Barber]

That's awesome.

[Ariane Palstra]

Yeah.

[Regina Barber]

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

[Ariane Palstra]

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.

[Regina 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?

[Ariane Palstra]

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 Sargasso Sea. These larvae, they turn into willow leaf shaped larvae, leptocephaly 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 glacials and these swim up the rivers. Actually, the moment that they swim into the fresh water, then they start to pigment.

[Regina Barber]

Wow.

[Ariane Palstra]

And then they spend a long life basically growing.

[Regina Barber]

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

[Ariane Palstra]

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

[Regina Barber]

when

[Ariane Palstra]

the densities are low. So mostly in upstream areas, you get more females and 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. Okay, so imagine like a male was 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 Sargasso Sea. You spend like 10, 20, 30 years in order to become 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. 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.

[Regina Barber]

Wow.

[Ariane Palstra]

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, 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. We have no idea.

[Regina 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.

[Ariane Palstra]

Yeah.

[Regina 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?

[Ariane Palstra]

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

[Regina Barber]

Yeah. Do you want to?

[Ariane Palstra]

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

[Regina Barber]

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

[Ariane Palstra]

The thing which is still mysterious is catching eggs and of course catching adults called the act of spawning. And perhaps their carcasses must also be somewhere there, but because they still haven't

[Regina Barber]

found the carcasses, right?

[Ariane Palstra]

Like, 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 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 lab. 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.

[Regina Barber]

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

[Ariane Palstra]

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.

[Regina 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?

[Ariane Palstra]

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

[Regina 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?

[Ariane Palstra]

Well, other than that's the main thing, I guess it's. Yeah, it's amazing. That is now 2026 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, that you can use to preserve this species and to help it recover from the decline that they have suffered.

[Regina Barber]

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

[Ariane Palstra]

Well, I think some people consider them slimy and ugly, but 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.

[Regina Barber]

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

[Ariane Palstra]

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?

[Regina Barber]

Like what triggers them?

[Ariane Palstra]

Yeah, that's still a long standing 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.

[Regina Barber]

It should be mythical forever.

[Ariane Palstra]

Yeah. On the other hand. Yeah, there are so many questions. So when are they leaving? Why do some eels get so old if they don't go to the Sikh soc. Their stories known of eels becoming a hundred, 150 years. And imagine that you're 150 years and you're still like not even a puber. You're like, you're like a Peter Pan. So if you do not reproduce, you have eternal life.

[Regina Barber]

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

[Ariane Palstra]

Yeah, for me too.

[Regina Barber]

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

[Ariane Palstra]

All right. Yeah. Thank you.

[Regina Barber]

Short wavers. 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 Taylor Jones. The audio engineer was Jimmy Keeley. I'm Regina Barber. Thanks for listening to shortwave from npr.

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