[RADIOLAB INTRO]

JAD ABUMRAD: All right, Bobby. Would you like to—would you like to do it?

ROBERT KRULWICH: Yeah. Go ahead and start me and I'll just go—I'll just take it through.

JAD: Okay.

JAD: Three, two, one. Hey, I'm Jad Abumrad

ROBERT: I'm Robert Krulwich.

JAD: This is Radiolab.

ROBERT: And today I've brought us a little—a little story.

LUCY COOKE: Hello!

ROBERT: Lucy?

ROBERT: That is ...

LUCY COOKE: Robert!

ROBERT: Oh, great! Okay ...

ROBERT: A sort of slippery mystery.

JAD: Slippery mystery. Okay.

ROBERT: And the mystery comes from this woman, Lucy Cooke.

LUCY COOKE: I'm the author of The Truth About Animals.

ROBERT: And it's a rip-roaring ...

LUCY COOKE: Which is filled with strange stories.

ROBERT: It's really—like, we're, like, feasting on this book. Okay.

ROBERT: So Lucy—so Lucy wrote a book, and the book is actually a collection of animal profiles which come from her journalism. And I just—I love it. I love it a lot. And so I called her up, and we just started talking about ...

LUCY COOKE: Frogs.

ROBERT: All sorts of animals.

LUCY COOKE: Bats.

ROBERT: Sloths.

LUCY COOKE: The island of dwarf stone sloths in Panama.

ROBERT: I have no idea what that means.

ROBERT: We talked about birds.

ROBERT: Birds are like turtles?

LUCY COOKE: No, no, no, no, no!

ROBERT: We talked about ...

LUCY COOKE: If you're a female panda ...

ROBERT: ... bears.

LUCY COOKE: What you're really looking for in a male panda is one that can squirt his pee quite high up a tree.

ROBERT: But what I really wanted to talk to Lucy about ...

ROBERT: Well, I'm gonna stop right there and switch quickly to eels.

ROBERT: ... was eels.

ROBERT: Because we only have 33 minutes left, and I don't want to ...

LUCY COOKE: Oh my God, and the eel's so good.

ROBERT: And the eels are so good.

ROBERT: Because as you will soon learn, the story of the eel is really—and this is strange to think about—the limits of human knowledge.

LUCY COOKE: Yeah, are you ready to rock and roll?

ROBERT: You could just start with, like, tell me when you first—you know, first encountered this creature.

LUCY COOKE: Oh, I was a very geeky only child, and what I loved to do more than anything else in the world was, my father sunk an old Victorian bath into the garden and that became my Narnia. And I'd sort of disappear into this watery kingdom and, you know, was obsessed with creating the perfect pond ecosystem out of this rather sterile tub for human ablution. It became my—it was—it was—it was everything to me. So every Sunday, I'd nag my dad. "Can we go to the ditches of Romney Marsh? Please, Dad!" And he'd sort of take me off and we'd go. He made me a net to catch things with—out of a pair of old net curtains that I'd sort of trawl through the ditches of Romney Marsh and catch all these wonderful creatures. You know, pond skaters and newts and frogs, and—and I'd bring them back to my—my tub. But eels became my holy grail, because the thing about eels—I don't know Robert, have you ever tried to catch one?

ROBERT: No, I would never even think to try to catch them because they're ucky and slimy and they remind me of snakes.

LUCY COOKE: Yeah. No, I wanted to, because I wanted to have all animals, much like Noah. I wanted all animals to be represented in my pond.

ROBERT: You had a list!

LUCY COOKE: Yeah, I had a list. And the eel, it was impossible. I caught them, but then trying to grab them with my own bare hands was always a complete disaster because they are, as you say, extremely slippery. They would just slither out of my hands and then shoot off in the grass, more like a snake than a fish out of water. But had I managed to get eels to join my—my happy pond party, I would have been a little bit horrified because I now know they would have eaten all the other guests, because they are voracious predators and they will eat any other creature that they can get their mouths around, including each other.

ROBERT: Which, Lucy explained, was proven rather graphically in a famous 1930s experiment in France.

LUCY COOKE: Yeah. So basically in the 1930s, there are a couple of researchers in Paris who placed a thousand elvers which are young eels, they're about three inches long in a tank of water. And they fed them every day. But even so, after a year of the one thousand elvers there were only 71 left.

ROBERT: Because they all got sick and died, or what?

LUCY COOKE: No. They ate each other.

ROBERT: Oh!

LUCY COOKE: And so the 71 survivors a year later were three times as long as they were before. And then three months later, after what a local journalist reported as, "daily scenes of cannibalism," there was one champion that was left and it was a female—whoo-hoo!—measuring a foot in length. And she lived four more years on her own, and could have lived a lot longer if the Nazis hadn't invaded Paris and inadvertently cut off her supply of worms and she died.

ROBERT: So ...

LUCY COOKE: Those Nazis have got a lot to answer for. The eel story has got it all.

ROBERT: It does. It has everything.

LUCY COOKE: It's got it all. It's even got Naz—it's got Freud, it's got Nazis. It's got an international gonads championship. It's got everything.

JAD: Wait. International gonads championship?

ROBERT: Yes, yes. Because there has been and there still is a very simple question we've been asking about eels, and that is: where do they come from?

JAD: That's a question?

ROBERT: Yes. People have been fishing them and eating them and hunting them and studying them, as you're about to find out, forever and ever and ever. For such an important animal, it's remarkable it has kept the secret of its origins from every human being.

JAD: Wait, how can it be—how can it—how can that be that—how can that be true?

ROBERT: It's not for lack of trying.

LUCY COOKE: It's taken a very, very, very long time to try and figure out the mysteries of the eel.

ROBERT: And Lucy says this question: where do eels come from?

LUCY COOKE: That is something that has tormented men of science since Aristotle. So he thought that eels were spontaneously generated by the action of water on mud, and that the worm casts that we see in sand were actually embryonic eels boiling out of the earth.

ROBERT: Wait a second. So water would slap, slap, slap, slap, slap against seashore rocks or sand or something, and then—squirt!—up would pop an eel?

LUCY COOKE: Yeah. Exactly. That was—that was Aristotle's solution. That wasn't his finest hour. And then the great Roman naturalist Pliny the Elder, he thought that eels rubbed themselves against rocks and the scrapings came to life.

ROBERT: You mean, like it's dandruff? Like, eel dandruff? Like, rub, rub, rub, rub—boing!—I've got a baby?

LUCY COOKE: [laughs] Exactly, yeah. The ideas of where they came from were utterly fanciful. You know, there was one Reverend Bishop who claimed in the Middle Ages that he'd seen eels being born out of the thatching of his roof. There were others that thought that they came from dewdrops, but only at certain times of year.

ROBERT: Well, what about when you get a microscope? Like, these are the superstars of science. So van Leeuwenhoek, he looked at them, right? So he could have seen it.

LUCY COOKE: Yeah. So he thought that they gave birth to live young like mammals do, because he sort of got his microscope out and he looked inside an eel and he thought he saw little baby eels swimming around inside a bigger eel. But actually, what he'd seen were parasitic worms inside a swim bladder. So he was wrong.

ROBERT: What about Carl Linnaeus? As long as we're dealing with the great—Linnaeus who categorized all life, did he have an eel idea?

LUCY COOKE: Yeah, he did. He thought that they were—gave birth to live young. But the unfortunate thing was that he was actually looking at the wrong creature. It wasn't an eel. It was a very similar-looking fish called an eelpout, which is a completely unrelated type of fish. So he was very wide of the mark.

ROBERT: So let me just—let me get rid of the clutter of all of these non-Italians. Because in your story, it turns out that Italians are the ones for—I guess because they are—they have eels in their rivers, they just thought, "We've got to figure this out." I noticed that Leonardo in The Last Supper, are the disciples eating eel for their Passover seder? Is that what's going on there?

LUCY COOKE: I believe so. I think there are—there are eels on the table in the painting of The Last Supper.

ROBERT: Wow. Okay.

LUCY COOKE: So I mean, the Italians who are very good with food, took a great interest in the riddle of the origins of the eel. And at a time in the 18th century, while Italy at that time was a load of warring states, there was no sort of sense of national identity. And the—there were a sort of small band of Italian scientists who decided that somehow that they would—they would forge an identity for their nation, not through revolution but by finding the gonads of the eel. It's a novel approach to politics.

ROBERT: [laughs] Yes! Did they succeed in Italianizing the gonad? Like, did they find them in Italian eels?

LUCY COOKE: Oh, they made a lot of attempts, basically. So it started in 1707 when a local surgeon found an unusually plump eel amongst the many thousands that were caught every day on the Po River delta. And he sliced it open and he saw what he thought was an ovary and eggs, and he sent this pregnant fish to his friend the esteemed naturalist Vallisneri, who hastily proclaimed the centuries-long search for the evils—the evils?—the eels' private parts was finally over. And unfortunately, he was wrong. But this then sort of ignited this interest amongst the Italian scientific establishment who decided it was a matter of extreme importance to find the true ovaries of the eel, and they came up with this plan. They thought, "I know what we're going to do. All we have to do is put out there a reward."

ROBERT: Like, if you find the gonads of an eel, you'll get a thousand ducats.

LUCY COOKE: Exactly. So what happened was they got an eel stuffed with eggs, but unfortunately the wily fisherman had filled his eel with the eggs from another fish.

ROBERT: Oh! He cheated! He put in another—oh!

LUCY COOKE: Yeah, he cheated. So there was a bit of a blow for the Italians' gonad hunt. And that went on pause for about 50 years. And then in 1777, a fresh, fat, slimy suspect flopped up on the shores of Comacchio, and it was examined by the great anatomist Carlo Mondini, who was a professor at the nearby University of Bologna. And he realized that the frilled ribbons inside the eel's abdomen weren't fringes of fatty tissue, which is what they'd previously thought, but they were the female eel's evasive ovaries. So bingo! Ovaries found.

JAD: Wait. Frilled fringes? What exactly did the ovaries look like?

ROBERT: So if you think of a—if you think of seaweed.

JAD: Mm-hmm.

ROBERT: When you open up a female eel, there is a little place in the middle of her tummy that looks like little wispy bits of flesh with little dots on the end.

JAD: Oh, that look like seaweed.

ROBERT: Yeah.

JAD: That's the ovaries?

ROBERT: Yeah. Turns out that those are their ovaries.

JAD: How'd they—how'd they miss that?

ROBERT: I don't know. You know, I think you—they don't look like anything like mammalian ovaries, they don't look like other fish ovaries, so they just—they were hiding in plain sight.

JAD: And so then the male ovaries, do you have any sense of what those ...?

ROBERT: The male testes they're called.

JAD: The male testes, I'm sorry. Yeah, male testes.

LUCY COOKE: Testicles still missing, though. And what's amazing about the eel story which is just absolutely extraordinary, is we now have another character who turns up. The mission to complete the eels' genital jigsaw you might say, fell to an unlikely character, Sigmund Freud.

ROBERT: He's an eel hunter?

LUCY COOKE: Well, he was a student at the time at the University of Vienna, and it was his first ever academic job was he spent a summer trying to track down the testicles of the eel. He was—he was investigating the claim of a Polish professor who had claimed that he had discovered the testicles of the eel, but he hadn't saved them or he hadn't used a microscope, or for some reason there was no proof. So Freud was given the task of proving this claim. So for weeks, every day from 8:00 in the morning until 5:00 in the afternoon in a hot smelly laboratory, he sliced open long, phallocentric fish in search of their testicles. Freud was completely consumed by it and—and failed. He just—he couldn't substantiate this Polish professor's claims, and he moved on to look for the seat of desire in—in another animal, namely the human.

ROBERT: So Lucy says now jump ahead about 30 years to around the 1890s.

LUCY COOKE: A male eel finally exposed itself. And it was to this chap Giovanni Grassi.

ROBERT: An Italian biologist who one day ...

LUCY COOKE: Found an eel swimming off the coast of Italy.

ROBERT: Scooped it up, brought it back to his lab, cut it open. And saw finally and with certainty, this is a little cloudy—cloudy how he knew this, but he said, "Yup. These are the right things. This is the testes and those are the sperm."

LUCY COOKE: Nothing short of miraculous.

ROBERT: So did he get a big, like—a prize? A reward?

LUCY COOKE: Well, I like to think that there was an enormous great big cup shaped like a pair of testicles.

ROBERT: [laughs] Okay, let's just plop—let's just keep that thought in our heads and move on.

JAD: But okay. So testes located, ovaries located.

ROBERT: Right.

JAD: Two great mysteries of nature solved.

ROBERT: But there's another one. There's another even bigger one.

JAD: Mmm!

ROBERT: Where do they go to do this thing? Where do the testes and the eggs go—squick, squick—and make a baby?

JAD: What do you mean? Where, like, in the water?

ROBERT: In the water, yes.

JAD: But ...

ROBERT: But where on Earth?

JAD: Huh.

ROBERT: On Earth, Jad. That turns out to be an even deeper mystery. That's after the break.

[MEGAN: Hi, this is Megan and I'm calling from cloudy Ithaca, New York. Radiolab is supported in part by the Alfred P. Sloan Foundation, enhancing public understanding of science and technology in the modern world. More information about Sloan at www.sloan.org]

JAD: Radiolab. Okay, so Robert you left us with the eel mysteries one and two solved.

ROBERT: We know where their sex parts are, yeah.

JAD: Testes located, ovaries located.

ROBERT: Right. Now you want to know how they have sex and where they have sex.

JAD: I don't—it's funny, I didn't—I mean, now that you ask the question I do want to know the answer, but I wasn't ...

ROBERT: Well, this has proven to be, Jad, a very hard question. And it was only a while ago that we found a clue.

LUCY COOKE: Grassi was on a bit of a roll. So he'd found the testicles of the eel.

ROBERT: And the clue was found by our Italian scholar, Grassi.

LUCY COOKE: He does something incredibly ingenious, because as early as the 1850s there were these tiny, weeny transparent fish the shape and thickness of a willow leaf with bulbous black eyes and these—grrr!—snaggly, gruesome buck teeth, had been documented washing up in huge numbers on the shores of Italy. And these sort of minuscule monsters were just sort of dismissed as just another one of the many, many millions of nondescript marine creatures that inhabit the sea. But Grassi, he thought to himself, "I think that those are actually—they're not an adult fish. They're a larvae of a fish."

ROBERT: Oh, so they're a baby. I just don't know what they're about to grow up and to become.

LUCY COOKE: Yeah. And what he did that was so incredibly clever was that he counted their vertebrae, and it averaged at about 115. And then he looked for a match in an adult species of fish, and he found it in the European freshwater eel. It's just an—an amazing piece of biological detective work, I think.

ROBERT: And it revealed that baby eels were living along the coastlines and then washing into the mouths of rivers all over the world. Italy and Spain and Japan and ...

BECCA BRESSLER: And where are we right now?

ROBERT: ... the great state of New York.

CHRIS BOUSER: We are on the Fall Kill Creek in Poughkeepsie, New York, where this creek enters the Hudson River.

ROBERT: We have eels right around here in New York City.

JAD: We do.

ROBERT: We do. In Poughkeepsie.

CHRIS BOUSER: About 75 miles north of New York City.

ROBERT: So our producer Becca Bressler and I went up to Poughkeepsie to meet this man Chris Bouser.

CHRIS BOUSER: I am the Hudson River estuary educator for the New York State Department of Environmental Conservation.

ROBERT: And Chris explained to us when these tiny little baby eels ...

CHRIS BOUSER: Like little pieces of transparent linguine.

ROBERT: ... arrive in the Hudson River ...

CHRIS BOUSER: They're only about two inches long. And they arrive here by the hundreds of thousands. And then they quickly when they get here ...

ROBERT: They start transforming.

CHRIS BOUSER: ... going through these physiological changes that makes puberty look like a kid's birthday party what these eels go through.

ROBERT: Basically, these little eels settle down at the bottom of the river.

CHRIS BOUSER: And blend in with the river bottoms.

ROBERT: And grow ...

CHRIS BOUSER: And grow ...

ROBERT: And grow and grow and grow and grow and grow.

CHRIS BOUSER: There's only one way to see how big the eels in there are, and that's to go in there and see them. We want to catch you some adult eels. Or ...

ROBERT: I'm happy to do it. Not exactly happy to do it. I'm just kind of ...

CHRIS BOUSER: Or if you'd rather just get the eels, our team can go ahead and get them right now.

ROBERT: No, no, no, no. I'm not gonna wuss out on this. I'm gonna do this.

CHRIS BOUSER: Keep going then.

ROBERT: Okay.

CHRIS BOUSER: Just keep going.

ROBERT: But let me ask you. Like, the first time you actually touched one with your own hands, what—tell me what it felt like?

CHRIS BOUSER: Silky love. People think of eels as being slimy. No, they're satiny, silky. They just feel like these beautiful, muscular, well-adapted miracles of evolution. You cannot touch an eel or hold an eel in your hand without feeling some weird love and wisdom coming to pass.

ROBERT: Do you have any brothers and sisters?

CHRIS BOUSER: I do. They think I'm strange.

ROBERT: Okay. So ...

ROBERT: How am I gonna catch anything with these rubber gloves?

CHRIS BOUSER: Ah! Here comes Aiden to show you how.

ROBERT: Chris and a few of his colleagues and Becca and I, we put on these canvas overalls, rubber boots, rubber gloves.

FEMALE RESEARCHER: This is for you.

ROBERT: And we were given nets.

ROBERT: Oh!

FEMALE RESEARCHER: This is what we're gonna be using to catch the eels.

ROBERT: Okay.

CHRIS BOUSER: Scoop them with that net!

ROBERT: Okay.

ROBERT: And one of the researchers, she was netless. Instead, she had this long metal rod that connected to a backpack.

FEMALE RESEARCHER: When—I'm going to turn the backpack on. Whenever the backpack is on it's putting electricity in the water. So ...

ROBERT: Basically, electrical current comes out of the backpack into the metal rod into the water ...

CHRIS BOUSER: To gently stun and coax these eels out of their hiding places.

ROBERT: And once that happens, that's my cue to grab one.

FEMALE RESEARCHER: All right. So is everybody ready?

BECCA: Ready. Yes.

CHRIS BOUSER: Sarah Mount, you're in charge. Sarah Mount's giving directions now.

ROBERT: And so we walked out deeper into the creek.

ROBERT: Okay. We're now mid-creek.

ROBERT: And eventually we stopped. And Sarah, the researcher with the backpack ...

SARAH MOUNT: All right. Backpack on.

ROBERT: ... sent some electricity into the water. And ...

BECCA: There's an eel!

ROBERT: Up they came.

SARAH MOUNT: Oh, there's one right there. There you go.

ROBERT: One after another, these long shiny creatures slithering through the water.

BECCA: There's two eels in here.

ROBERT: Some of them are over three feet long. So I tried to net one.

ROBERT: Oh! That wasn't a small one! Whoa! Almost had it.

CHRIS BOUSER: Wait, before you do that. Everybody look up here and smile, just because it's a great picture. Go get them!

SARAH MOUNT: Everybody ready?

ROBERT: But I just kept missing and missing ...

ROBERT: But I didn't catch any.

ROBERT: ... and missing. Until finally in my net, a two-foot long shiny eel.

SARAH MOUNT: If you want to take your glove off and touch it with your bare hand, you're welcome to.

ROBERT: Yes, I would love to.

SARAH MOUNT: You can feel that silky, smooth ...

ROBERT: Silky love coming my way. If I can get off my rubbery glove for the silky love. Okay, mister eel. Ooh, yeah it is kind of—it is kind of silky. Yeah. Oh, he doesn't feel bad at all. What do you think little fella?

SARAH MOUNT: Do you feel any danger?

ROBERT: No. I'm going to call her Florence. Florence the eel. Hey, here's your tail.

ROBERT: I should say that eels can survive out of water, even crawl through the grass for hours. But eventually, I did let Florence go back into the water, only to learn from Chris and his team that Florence wasn't actually wasn't a Florence. That Florence in fact wasn't either a male or a female. Because this particular eel didn't have ovaries or testes. And in fact, most of the eels that we saw on that day in the river didn't have ovaries or testes and were all sexless.

JAD: Hmm.

ROBERT: Because this is the thing that happens the world over is that when eels are in rivers they hang out, they grow, they get bigger and they're waiting essentially for this moment that comes almost at the midnight hour of their lives. It's—it's like the ringing of a bell. When just as they're about to get their long-awaited ovaries and testes, they come up from the bottom of the riverbed and then they make their way down the river and back out into the sea. And then they just disappear. You can't find them.

JAD: What do you mean they just disappear? They—like, they can't follow them?

ROBERT: They cannot be followed.

JAD: Oh, is that why it was so hard for them to find the sex parts, because maybe the ones they were finding ...

ROBERT: Well, they had to find an older one, yeah.

JAD: That didn't have them yet.

ROBERT: Yeah.

JAD: Oh, that explains a lot of it. Yeah.

ROBERT: So they're like the opposite of salmon. Salmon start in the—in the rivers and go down to the sea. Spend a long time in the ocean and then come back up the rivers. This is the—eels are the un-salmon.

JAD: Oh, interesting.

ROBERT: They—they start in the ocean somewhere. They go up to the rivers, they get big and mature, then they head back to the ocean somewhere. The question is where?

JAD: Oh, interesting.

ROBERT: Nobody knows! I mean, this is weird. Like ...

JAD: Do they all go to the same place?

ROBERT: Well, you'll find out.

LUCY COOKE: Okay, all right. Well, shall I—shall I try and tell the story of Johannes Schmidt very quickly then?

ROBERT: So back to those early days of scientific eel-hunting in the late-1800s, nobody had seen an adult eel out in the sea, strange as that may be. But then there's this guy ...

LUCY COOKE: Oceanographer, Johannes Schmidt.

ROBERT: Who thought, "Well, wait a second. The eels we know go out somewhere into the ocean, die, and have babies. We see those babies showing up at the tops of rivers all over the world, so let's track the babies backwards," he thought. Starting with the biggest ones, then look for smaller babies then smaller babies still. And the smallest, smallest babies will probably be right at the nest where the parents die and those babies are born.

LUCY COOKE: It's worth noting that Johannes Schmidt was described as being, and I quote, "Pathologically ambitious."

ROBERT: Because remember what Schmidt's looking for are these tiny, tiny little eels.

LUCY COOKE: The shape and thickness of a willow leaf.

ROBERT: Like a three-inch long translucent piece of wiggly flesh. So small. And we're talking about the Atlantic Ocean. So, like, how are you gonna pull this off?

LUCY COOKE: Well, he had rather fortuitously—he'd just married the heiress to the Carlsberg Brewery, and they were probably the best lager company in the world for an aspiring oceanographer to hitch his wagon to, because they were known to fund ocean exploration. So he'd married the heiress and then he was then, because he had all these funds, he was able to spend, after she'd married him he then disappeared to sea for 20 years.

ROBERT: 20 years!

LUCY COOKE: Combing the world's oceans.

ROBERT: From Cairo and Alexandria all the way to Virginia.

LUCY COOKE: With progressively finer nets.

ROBERT: Looking for smaller and smaller and then smaller baby eels.

LUCY COOKE: His breakthrough came in 1921 when he found one that was a quarter of an inch long, which he presumed could be no more than a day or two old.

ROBERT: And Schmidt found this eel ...

LUCY COOKE: Slap-bang in the middle of the Bermuda Triangle.

ROBERT: In the Sargasso Sea. First of all, it's so far away from Italy and Spain and Europe, it's kind of near the Carolinas off North America. And it's a very strange place.

LUCY COOKE: It's unusually salty. It's filled with an underwater forest of sargassum seaweed and very, very, very deep. I think at some places it's four miles deep.

ROBERT: Whoa!

ROBERT: It is the only sea, the only thing we call a sea that doesn't have land around it. It's this zone of very quiet water surrounded by roaring currents going in a big circle around it.

JAD: And the idea is this is the place where all eels come to have their babies?

ROBERT: No, not all eels. Because there's eels all over the world. But for the Atlantic eel, the Sargasso is the place. At least that's—that's the theory. Because since Schmidt's big discovery in 1921, we have been trying to confirm it. So for instance, a few years ago, scientists in Europe tagged 400 eels to see where they would go. They went into the Mediterranean, but most of them died and very few got out into the Atlantic. And, you know, a few of them went kind of wandering toward Africa. Then a year ago roughly, there was another story about a single tagged eel.

ROBERT: Can you tell me the story of that eel?

CHRIS BOUSER: I sure can.

ROBERT: It was an eel that started up in the St. Lawrence River in Canada, who would eventually become a female eel.

CHRIS BOUSER: She heads south, sniffs around. According to the tag, she then heads due east.

ROBERT: Heads first towards Scotland, then reverses her way back to Maine.

CHRIS BOUSER: And starts a quick beeline south. I don't mean south-ish. I mean, south. South like she is following one of the magnetic longitudes of the planet. She heads down, and somewhere at the edge of the Sargasso Sea the tag comes off. We don't know if it falls off, we don't know if it was eaten. We don't know what happens. All we know is that's the end of the road of the tag, but the fate of that female eel we don't know.

LUCY COOKE: Exactly. And we don't know because even today—and in fact, I got an email yesterday from perhaps the world's leading eel scientist, and he explains to me that sadly after many, many, many years, much effort, and millions of dollars spent, still no one had managed to actually track an eel all the way from the rivers of Europe to the Sargasso.

ROBERT: But they tried, right?

LUCY COOKE: Exactly. They—they sunk swollen eel temptresses into the Sargasso, hoping that they would attract males and they could catch them in flagrante and then finally prove that—that definitely it was observed eels mating. But still, it's never been seen, it's never been witnessed.

ROBERT: Maybe the guys aren't there. Maybe they got the wrong place. Or maybe the guys are shy or maybe the guys know a science experiment when they see one and are just not gonna tell. I don't know which of those three.

LUCY COOKE: Or they are actually breeding on the moon and falling out of the sky, raining on the Sargasso. We can't—we can't rule that out.

CHRIS BOUSER: We don't know where exactly. We don't know how exactly. But somewhere in the Sargasso Sea, there's the miracle of a fertilized eel egg.

ROBERT: Miracle because no one's ever seen it?

CHRIS BOUSER: Correct. Ever.

ROBERT: In the wild.

CHRIS BOUSER: There is a secret sauce with eels that we have not solved. Forget the Coca-Cola recipe, eel sex is the real mystery of our generation. And we think we know roughly where in the Sargasso Sea it happens, but that's a think not a know. We have never witnessed in the wild eel egg fertilization.

ROBERT: Well, I know you're not an eel. So let me ask you like, have you imagined where all the—let's assume for the sake of argument that there is a place that Atlantic eels go to, and it is a place where they have sex and it's a place where they have babies and it's a place where they die. In your imagination, what does that place look like? Speaking as a human.

CHRIS BOUSER: In my imagination, and make sure that line makes it in [laughs], in my imagination—and again, this is without any proof but in my imagination, I like to think of some deeper waters of the Sargasso Sea, somewhere between Bermuda and Puerto Rico. It's a quiet place. It's a dark place. It's beyond the reach of all but the barest of wavelengths of light up top. And I think that with these eels, you know, I've often wondered, do they pair off discreetly? Is there sort of a massive orgy of eels that happen? And I like to think of eels as once they get there, there's gotta be some sort of primitive ichthyological celebration, that feeling of, "My gosh! I have finally accomplished a 30-year, 3,000-mile journey! All of us have!" But I'm gonna go back to what we were talking about, how much I love the mystery. I don't like to overthink it.

ROBERT: Oh, you don't want to even think about this.

CHRIS BOUSER: I don't want to overthink it. Because again, I almost respect and love the mystery so much that I see, like, in the eel movie, right? Fade to black. [laughs] Cue the music, the fireside lighting, you know? Slow dissolve. Fade to black. Tah-dah!

ROBERT: This piece was produced by Becca Bressler and Matt Kielty. Special thanks to the men and women of the Hudson River Estuary Program who helped us with this project and brought their equipment and their bodies and—and their strange clothing down to the creek. And to Clay Hiles of the Hudson River Foundation who set us up with these folks, and to Kim Airstrup.

JAD: And to all eels that are on the cusp ...

ROBERT: Yes.

JAD: ... of that great journey into the unknown.

ROBERT: Who still won't tell us what they know.

JAD: Deep—deepwater nursery ...

ROBERT: Yeah. Thank you, eels.

JAD: ... of mystery.

ROBERT: Yes.

JAD: Keep doing it, eels. Keep doing it.

ROBERT: And they're doing it, by the way, right around now. So it's in the—in hurricane season.

JAD: [laughs]

ROBERT: So for a variety of reasons, these animals are just not telling.

JAD: Okay.

ROBERT: We gotta say goodbye now.

JAD: Okay. I'm Jad Abumrad.

ROBERT: I'm Robert Krulwich. Thanks for listening.

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[LUCY COOKE: Hello, hello, hello. This is Lucy Cooke, author of The Truth About Animals and expert on eels. I'm giving you your credits. Radiolab was created by Jad Abumrad, and is produced by Soren Wheeler. Dylan Keefe is our Director of Sound Design. Suzie Lechtenberg is our Executive Producer. Out staff includes Simon Adler, Becca Bressler, Rachael Cusick, David Gebel, Bethel Habte, Tracie Hunte, Nora Keller, Matt Kielty, Robert Krulwich, Annie McEwen, Latif Nasser, Sarah Qari, Arianne Wack, Pat Walters and Molly Webster. With help from Shima Oliaee, W. Harry Fortuna, Sarah Sandbach, Malissa O'Donnell, Neel Dhanesha, Marion Renault and Paloma Moreno Jimenez. Our fact-checker is Michelle Harris. Thank you very much. Good-bye.]

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