The Book of Eels summary

You’ve probably heard of the eel. You may have even had unagi, or eel sushi. But have you ever seen an eel in the wild?

Probably not. Eels are shy, nocturnal creatures. They are so good at hiding that, unless you’re a trained eel-catcher, you’d be lucky to find one. If you did, though, you’d see an animal that is unlike any other. Eels have often been mistaken for snakes or amphibians. This is an easy mistake to make – after all, they do have long, slimy bodies and can wriggle across dry land for hours at a time. 

But that’s just the beginning of the eel’s mystery. To this day, we’ve never seen these animals breed. We’ve never really tracked their migration. And we don’t know why their bodies go through so many radical changes, or metamorphoses. Despite centuries of study, we really know very little about the eel.

These blinks focus on the species Anguilla anguilla, also known as the European eel. Generations of scientists, from Aristotle to Freud, have worked to solve this animal’s riddle; with a bit of luck, some of that fascination with the eel will rub off on you, too.

In these blinks, you’ll learn

• why the nineteenth century witnessed a craze to discover the eel’s testicles;
• why eels likely have a very different experience of time than humans; and
• how the eel’s strange sexual life might have influenced Freud’s psychoanalytic theories.

The human relationship with the eel goes back a long way. Ancient Egyptians, for instance, saw eels as demons related to gods. They even mummified eels and put them into tiny sarcophagi.

But the earliest scientific description of an eel was made by Aristotle, many centuries later. 

The key message here is: Scientific interest in the eel goes all the way back to Aristotle.

Most of us know Aristotle for his contributions to philosophy. But he was also well ahead of his time when it came to studying the natural world. His multi-volume work Historia Animalium is considered the first attempt at systematically categorizing organic life. And, among other things, it includes a curious account of the eel.

Aristotle almost certainly dissected many eels himself. His descriptions of the fish’s anatomy are highly detailed and accurate. 

But some of the claims he made about the eels are nothing short of fanciful. He wrote, for example, that eels feed on grass, and that they can live on dry land for five or six days – perhaps even longer.

One of Aristotle’s claims would influence scientists for centuries to come. He thought that eels didn’t reproduce like other fish, by mating and fertilizing eggs. Instead, he claimed, they simply emerged from the mud at the bottom of waterways.

Consider, he said, a small pond during a drought. The water has evaporated; the mud has caked and dried. What fish could possibly live there? And yet, when the water returns, something remarkable happens: all of a sudden, the pond is full of eels. Thus, said Aristotle, eels must be born of the mud itself.

Not all of Aristotle’s contemporaries agreed with this theory. But his critics didn’t have any better ideas. After all, no one had ever seen eels breed – or discovered their reproductive organs.

The legacy of Aristotle’s examination of the eel is not that he got the answer right – he didn’t – but that he set something in motion. He created an enigma that would puzzle scientists for generations to come. 

Scholars call it “the eel question,” and it’s not an exaggeration to say that it was once considered the most important problem in zoology. More than two thousand years after Aristotle, we still have much to learn about the life cycle of an eel. This elusive fish, it seems, is set on remaining a mystery.

The eel’s reproductive system seemed destined to remain a mystery – until the modern scientific era began. 

In 1668, the Italian physician Francesco Redi put Aristotle’s concepts to the test. He wasn’t sure life could simply appear out of nothing. So Redi ran some experiments. He looked at flies; at that time, many believed they spontaneously spawned out of rotting meat. Redi proved the opposite – you couldn’t have new flies without fertilized eggs.

It would take another century, but we would eventually discover that eels produce eggs too.

The key message here is: After centuries of searching, we now know eels reproduce like other fish.

In 1777, a sexually mature female eel from Comacchio, a town on Italy's east coast, found its way onto the dissecting table of Carlo Mondini, an anatomy professor. After studying the specimen, he published a treatise that is now considered an eel classic. In it, Mondini offered the first description of a female eel’s reproductive organ and eggs.

That cleared up one half of the equation. We now knew about the reproductive organs of the female eel – but what about the male? Where were the testicles?

It would take yet another hundred years to reach a new development. In 1874, researchers in Trieste, Italy, announced that they’d found an eel with “an unidentifiable lobe.” That lobe, they speculated, might be the missing testicle.

A renowned marine zoologist, Carl Claus, dispatched one of his young students to Trieste to investigate. That student’s name, believe it or not, was Sigmund Freud.

For a whole month, Freud sat at his desk in Trieste, slicing eels open and studying their insides with a microscope. He dissected over 400 fish – but to his intense disappointment, he couldn’t identify the lobe Italian researchers had described. Freud returned to Vienna empty-handed.

But Freud’s failure to discover the eel’s testicles can’t be put down to simple bad luck. What he couldn’t have known is that eels have a nifty trick. They don’t grow sex organs until they need them. Most likely, many of the eels Freud dissected were males that simply hadn’t matured yet.

Twenty years later, a sexually mature male silver eel was finally discovered off the coast of Sicily. And that put an end to hundreds of years of searching.

Still, this episode in Freud’s early career might not have been completely unproductive. Perhaps it gave Freud his first taste of just how deeply an organism can conceal its sexuality.

By the twentieth century, the eel question had changed. We now knew that eels bred; we just didn't know exactly where.

It was pretty clear it must be somewhere in the ocean. Every year, mature eels departed Europe in the autumn, and young glass eels returned in the spring.

Some proposed that eels bred in the Mediterranean. At the time, its warm waters were the only place where we’d seen eel larvae. But this theory had a flaw: larvae found in the Mediterranean were already quite large. Too large, in fact, to be freshly hatched.

So the enigma remained unsolved, continuing to attract scientists from around the world. In 1904, the eel brigade was joined by a Danish biologist called Johannes Schmidt. He set sail to discover the origin of this fish – little did he know his quest would take him almost 20 years. 

The key message here is: Johannes Schmidt discovered the eel’s spawning grounds in the Sargasso Sea.

Schmidt’s plan was to trace the eel larvae back to their birthplace. How? He proposed to trawl them out of the ocean at various locations and measure their sizes. The smaller the larvae were, the closer he must be to their spawning ground. The plan was sound – it was just incredibly laborious. 

For seven years Schmidt sailed up and down the coasts of Europe, from the North Sea to the Egyptian coast. But he made little progress. He found plenty of eel larvae, sure, but they were all about as large as the ones we’d previously seen in the Mediterranean. 

But eventually Schmidt turned west and ventured toward the Americas. Now, his luck changed. The further he went away from Europe, the smaller the eel larvae became.

It would take Schmidt nine more years, but in the end he found what he was looking for. After measuring thousands of eel larvae, Schmidt came to a place in the Atlantic called the Sargasso Sea. There, he came across larvae so minute that they simply had to be freshly hatched. Schmidt had discovered the birthplace of the eel. 

Thanks to the tenacity of a Danish biologist, we now know where eels breed. And we also know that they migrate more than five thousand miles from the coasts of Europe to the Sargasso Sea. Few animals undertake such a grueling migration just to breed – and we still don’t really know why the eel do it.

So, we now know where eels breed – the Sargasso Sea. The next question is: How do they get there? Exactly how do eels navigate five thousand miles across the Atlantic to arrive in time for breeding season?

One answer might be that eels rely on some powerful senses. Another possibility is that the memory of the journey across the Atlantic is simply hardwired into the eels’ brains. Or it could even be a bit of both!

The key message here is: Eels probably navigate both instinctively and with the aid of powerful senses.

Eels have a remarkable sense of smell. It’s so acute, in fact, that an eel could detect a single drop of rosewater in a huge lake. It’s very likely that this superpower helps eels find their way across the Atlantic. Maybe they can smell the Sargasso Sea itself, or perhaps they locate other eels and then follow them. 

But smell is not the eels’ only navigational tool. Much like birds, they can sense the Earth’s magnetic field. In a way, eels have a built-in compass, which helps them steer the right course.

So, we’ve established that eels are excellent navigators. But is this enough to explain their uncanny ability to find the Sargasso Sea from thousands of miles away? Could it be that the memory of the journey is simply instinctive – hardwired into the eels by evolution?

Well, in 2016 a team of researchers set out to test this hypothesis. They undertook the most extensive study on eel migration to date. Scientists tagged seven hundred eels with electronic transmitters and released them at various locations across Europe. As the fish traveled west, transmitters fell off and floated to the surface. But by then, the devices were packed with information about the eels’ journeys. The results were surprising.

The eels in the study chose wildly different routes. Most were meandering and complicated; virtually none of the fish swam directly to the Sargasso Sea. But when the eels approached the Azores, halfway through their journey, they suddenly closed formation and made a beeline for the mid-Atlantic hatcheries. 

What conclusion should we draw from this? Well, it seems that eels do have an ability to follow some kind of internal map. But it only kicks in as they get closer to the Sargasso. When they start their journey, back in Europe, they have to find their bearings all by themselves. And they do it by using their senses, not their instincts. 

As we’ve already mentioned, eels die soon after breeding. It seems that once an eel has fulfilled its mission, it no longer has a reason to live. 

But here’s something really strange: if something prevents an eel from returning to the Sargasso Sea to breed, the fish can stave off sexual maturity – and death – for decades.

The key message here is: The eel’s metamorphoses are triggered by environmental clues rather than age.

In 1859, a young boy in the town of Brantevik, Sweden, released an eel into a well. Nearly 150 years later, in 2008, a popular Swedish television show took it out and discovered that not only was the fish still alive, but it hadn’t grown any larger. The only things that had changed were its eyes; they’d become bulbous to adapt to the darkness in the well. 

Eels kept as pets in fish tanks also rarely grow in size, or mature sexually. It seems that when they’re separated from the wild, the eels simply stop developing – life gets put on hold.

For us, it’s a weird concept. Human development is closely tied to age. Puberty, for example, almost universally occurs in our late childhood and teen years. 

But eels seem to be driven by a different mechanism. In the 1980s, scientists in Ireland studied wild-caught silver eels. Their research showed that the ages of eels in the same developmental stage of life ranged from eight to 57 years.

So age doesn’t trigger an eel’s metamorphosis. But what does? What causes the eel to decide that it’s time to return to the Sargasso to breed? 

By this point, you probably won’t be surprised to hear that the answer is not very clear. 

What we can say is this: the eel’s development seems more connected to environmental clues, such as how much weight it’s managed to put on. There may well be other factors, but, for now, we are simply not aware of them. 

Whatever the triggers are, the eel’s sense of time seems to be driven by where it is in its own life story rather than by the passage of years on a calendar.

There’s a reason that eel has largely vanished from our diets. Populations of this fish across Europe have plummeted over the last few decades – and they’re only continuing to decline. According to most research, the eel’s current situation is catastrophic. The number of glass eels arriving in Europe today is only about 5 percent of what it was in the 1970s.

Why is this? Well, that’s yet another riddle – the latest and most urgent installment in the eel question.

The problem is, this is the eel we’re talking about. So, it’s not an easy question to answer. But if there’s one thing we can be certain of, it’s that humans are to blame. 

The key message here is: Human activity is putting the future of the eel at risk.

Of course, the most obvious culprit is overfishing. Catching glass eels is the most devastating since these fish are caught at a very young age and in huge quantities. Glass eels are still considered a delicacy in some parts of Europe, especially in Basque Country and areas of southern France.

But fishing alone can’t explain the eel’s rapid decline. Another problem is that disease and parasites have run rampant through eel populations. We’ve been shipping eels across continents, and that’s enabled pathogens to spread across different species.

Humans have also put physical barriers in the way of migrating eels. Locks, sluices, dams – they can all stop the fish on their way to and from the ocean. Hydroelectric plants are especially dangerous: they can kill up to 70 percent of eels that pass through them. 

The most serious threat to the long-term survival of the eel is climate change. It’s an indisputable truth that global warming is altering the direction of oceanic currents. For millennia, these currents have carried eel larvae east across the Atlantic. But as these currents change course, newly hatched eels may struggle to reach Europe. And that means no feeding, no metamorphosis, no return to the Sargasso to breed. 

As of 2018, the EU has enacted a number of radical measures to save the eel. Countries in Europe now have to build “fish bridges” around dams and power plants. And fisheries must release some of the glass eels they catch back into the wild. 

But, despite these efforts, the future of the eel remains far from secure.