Of the Tenderer Sex

In the spring of 1876, a nineteen-year-old medical student from Vienna was sent to the zoological station in Trieste to find the testicles of an eel. He spent about a month there, dissecting more than four hundred of them, and in a letter he summed up his progress: “All the eels I have cut open are of the tenderer sex.”

The student was Sigmund Freud.

There’s an obvious joke here, and nearly everyone who tells this story makes it, so I’m going to leave it on the table where it belongs. The more interesting fact is that Freud wasn’t bad at his job. He was given a question that had beaten Aristotle, and it beat him too. Somewhere around the four-hundredth eel he found a pair of lobed organs that might have been testes, wrote a careful, hedged paper saying so, and later left that paper off his list of publications. He moved on to the human mind, which he presumably found more forthcoming.

The eel question was this: where do eels come from? It’s a simpler question than most of what science has answered since. It is still not fully answered.

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Mud

Here’s what anyone could see, for most of recorded history. Eels turned up in rivers, lakes and ponds, sometimes ponds with no connection to anything. They grew fat for years. Then at some point they swam away downstream and were gone. Nobody had ever seen an eel egg. Nobody had seen eel sperm, an eel spawning, an eel carrying young, or a baby eel that looked like an eel.

Aristotle, who dissected a great many animals and was right about a surprising number of them, looked at all this and concluded that eels weren’t born at all. In his History of Animals he wrote that they come from “the so-called ‘earth’s guts’ that grow spontaneously in mud and in humid ground,” and added that eels had been seen wriggling out of earthworms.

It’s easy to laugh at that. I’d argue it was the best available reading of the evidence. If an animal shows up with no sign of ever having been born, “it isn’t” is the most economical theory. Aristotle’s mistake wasn’t bad reasoning. It was that the eel had hidden the evidence.

How it hid the evidence is the key to this whole story, so let’s set it down now. The European eel lives as five different-looking animals, one after the other:

1. A leptocephalus, a flat, transparent, leaf-shaped larva that drifts in the open ocean.
2. A glass eel, still see-through but now eel-shaped, which arrives at the coast.
3. An elver, the same animal once it gains color and heads upriver.
4. A yellow eel, the long, brownish adult most people picture. It can spend a decade or more in fresh water, growing to 60–80 centimeters.
5. A silver eel, the final form. Its sides turn silver, its eyes get large, it stops eating, and it swims for the sea.

Each change is drastic enough that you wouldn’t guess the stages were the same species. And for a long time, people didn’t.

The knife

The dissection era started where the eels were: in Italy. Comacchio, on the Adriatic, was a lagoon town that had made its living catching eels for centuries. In the early 1700s a doctor there found ovaries in an unusually large eel, the first clear sign eels had sexes at all. In 1777 Carlo Mondini described the ovaries properly and settled that eels were ordinary fish rather than mud spirits.

That left the males. In 1874 a zoologist named Szymon Syrski announced he’d found them: a lobed organ that came to be called the Syrski organ. Freud’s month in Trieste two years later was an attempt to confirm it. He couldn’t, not with confidence.

We now know why. Eels in rivers aren’t sexually mature. Their gonads don’t really develop until the silver stage, when the eel is already leaving for its spawning grounds. Freud was searching for reproductive organs in animals that hadn’t grown them yet. Everyone was. The eel did its maturing offstage.

The net

The next clue came from the first disguise.

A small, transparent, leaf-shaped fish had been sitting in the scientific literature for decades as its own species, Leptocephalus brevirostris. In 1886 a French zoologist, Yves Delage, kept some alive in an aquarium at Roscoff and watched them turn into eels. A decade later the Italian Giovanni Battista Grassi worked through the whole transformation, using larvae from the Strait of Messina, where upwelling currents bring deep-water creatures to the surface in good condition. The leaf was the baby. The eel’s infancy had been given its own Latin name.

Since the larvae were turning up in the Mediterranean, the natural guess was that eels spawned there. Then, in 1904, the Danish research ship Thor caught an eel larva in the Atlantic, west of the Faroe Islands, far from anything Mediterranean.

The biologist on board, Johannes Schmidt, came up with an idea that was simple and brutally laborious. Larvae get bigger as they drift away from where they hatched. So if you trawl the Atlantic and measure every larva you catch, you can follow the sizes down, like a slope, until you reach the place where they’re smallest. That’s the birthplace.

He spent nearly twenty years doing it, sampling the ocean with nets, financed in large part by the Carlsberg Foundation. Not everyone was impressed: in 1912 the Royal Society rejected one of his papers on the grounds that Grassi had already sorted the matter out. Schmidt kept going. On the Dana expedition of 1921–22 he caught the smallest eel larvae anyone had ever seen, in the Sargasso Sea.

The Sargasso is an odd place to be born. It’s a sea without coastlines, a slow gyre in the western Atlantic defined by the currents that circle it, thick in places with floating seaweed. Every European eel, from Morocco to Norway, appears to start life there. The glass eels in a Dutch canal drifted across an ocean to get to it.

Schmidt’s paper, “The Breeding Places of the Eel,” appeared in the early 1920s and became textbook fact. It was also, strictly speaking, an inference. He had found the smallest larvae. He had not found an egg. He had not found an adult eel anywhere near the place.

The tag

Neither has anyone else. For a century after Schmidt, expeditions went to the Sargasso and came back without a spawning adult or an egg.

So in 2018 and 2019, a team led by scientists at Cefas, the UK’s fisheries science agency, tried another tool. They caught 26 large silver eels (mature, migrating, presumed female) and fitted them with satellite tags, then released them off the Azores, roughly a third of the way there. The tags were programmed to let go of the eel after a set period, float to the surface, and transmit what they’d recorded.

The results came out in 2022. Some eels were tracked more than 2,000 kilometers. They cruised at roughly 3 to 12 kilometers a day, slow and steady. Five of the tags surfaced inside the Sargasso Sea. One came up inside the area Schmidt had marked as the spawning ground.

Four of the tags released early for a reason I find extremely satisfying: the eels dove deeper than the tags could handle, and a safety mechanism cut them loose. Even with the best tracking technology anyone had ever attached to one, the eel still had a move left: go somewhere the machine can’t follow.

The paper is careful about what it proved. It’s the first direct evidence that European eels make the journey. It is not evidence of spawning. In its own words, “surveys have failed to locate either adult eels or eggs” in the Sargasso Sea. Nobody has ever watched a European eel reproduce in the wild.

The bill

Most unsolved mysteries are free. This one isn’t.

The European eel is listed as Critically Endangered. Compared with 1960–79, the number of glass eels arriving on Europe’s coasts is down about 98.6% in the North Sea and about 94% elsewhere. There are lots of causes: dams that block migration, hydropower turbines, pollution, parasites, overfishing. But one of them grows directly out of the eel question.

You can’t breed an eel.

Eel farming exists, in Europe and on an enormous scale in East Asia, but every farm runs the same way. Catch wild glass eels and fatten them up. The full life cycle has never been closed in captivity. Researchers can hormonally push captive eels to spawn, and in 2023 a Dutch company kept captive-bred larvae alive for around 200 days, a record. None of them became glass eels. Nobody knows what cues the eels need to mature properly, and nobody knows exactly what the larvae eat in the open ocean. The best guess is “marine snow,” the drifting particles of organic debris. That’s very hard to manufacture.

So the whole eel industry depends on catching babies, and babies are worth a fortune. In East Asian markets, European glass eels can sell for up to about €6,000 a kilogram. Exporting them out of the EU has been banned since 2010, which created exactly what you’d expect: a smuggling trade. Europol’s long-running Operation LAKE has made more than 850 arrests since 2015, and Europol estimates that up to 100 tonnes of glass eels a year have been trafficked out of Europe, often by couriers carrying them in oxygenated plastic bags packed into suitcases. They’re raised on Asian farms and then sold legally, as farmed fish, which in a narrow sense they are.

That’s the cost of the gap in our knowledge. If we knew how eels reproduce, we could probably farm them, and if we could farm them, the price of a wild baby eel would collapse and the smugglers would go home. What Aristotle didn’t know about eels is being paid for in suitcases.

The well

In 1859, in the Swedish fishing village of Brantevik, an eight-year-old boy named Samuel Nilsson dropped an eel into his family’s well. Swedes used to do this. An eel in the well ate insects and kept the water clean.

The eel stayed. The boy grew up, and so did his descendants, and the house changed hands. The eel became a local celebrity, said to have grown enormous eyes from living in the dark. On August 7, 2014, the house’s owner found it dead. If the story is accurate, it was 155 years old.

Experts are skeptical. One called 155 “quite a stretch.” The age could in principle be checked by counting rings in the eel’s ear stones, the way you count rings in a tree. As far as I can find, nobody has published that result. But eels kept from the sea do seem to live remarkably long lives. An eel called Putte lived to around 85 in a Swedish aquarium, while wild eels mostly live to about twenty. The most likely explanation is that an eel that never gets to begin its final journey never starts the process of maturing, so it never gets to the end of its life. It stays a yellow eel. It waits.

The Brantevik well had a second eel. It was put in sometime around 1904, the same year the Thor netted that first larva off the Faroes and Schmidt started his long walk west across the ocean. The last report I can find says it was still down there in 2023.

It’s never been to the Sargasso. Something in it is presumably still waiting for the signal to go. Of everyone who has worked on the eel question, it might be the only one who knows how close we are.

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