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Biodiversity Blog


Sigmund and His Eels

Freud FINAL merged
 Illustration: Nicole Elmer 

If someone asks you to imagine Sigmund Freud, what do you see? An older gent with a well-trimmed white beard, cigar in hand? Is he perhaps listening to a patient who talks freely about personal issues while laying on a couch?

This is the Freud most know as the famous neurologist and founder of psychoanalysis. But well before Freud developed theories such as the “Oedipus complex” or “Penis envy,” he studied zoology, and had quite a time with European eel (Anguilla anguilla) sex specifically.

Until somewhat recently, the reproductive life of the European eel was a great mystery. No one had seen them laying eggs, giving birth, or mating. To add to the mystery, eels lack genitalia and determining their sex requires dissection and careful examination of their gonads. The surprise to early scientists was that males appeared to not exist.  

Without two sexes, how did these things reproduce? This was a question tossed around for millennia, quite literally. Aristotle reasoned that they must generate spontaneously, out of non-living matter, from the interaction of elements in the right conditions. This “spontaneous generation” theory was quite popular for centuries. Ideas on how eels reproduced came from everyone from Pliny the Elder to Hildegard of Bingen, explanations involving eels coiling around each other, foaming, spitting seeds, or scales falling from their bodies to grow into baby eels.

Larva and glass eels
 Top: larva (photo: Sönke Johnsen) Bottom: glass eels

It wasn’t until the Renaissance that folks began to question spontaneous generation, and started to make connections that would explain a creature’s inception in ways we recognize today. The invention of the microscope and the new insights it gave to scientists would help put spontaneous generation to rest.

In the beginning of the 18th century, an Italian doctor had managed to locate ovaries in a female. But without the discovery of male organs, reproduction was still elusive. Freud would change this.

At the ripe age of 17 in 1873, Freud entered the University of Vienna. His intention was to study law, but he instead gravitated towards medicine. His studies would include philosophy, physiology, and zoology, the latter subject under Darwinist professor, Carl Claus. Claus had the intention to prove that eels produce sexually, and set out to do this by locating a male eel.

The year 1876 would find the young Freud assisting with this mission at Claus’ zoological research station in Trieste, a seaport city in northeastern Italy. Claus gave young Freud a rather monotonous task. For four weeks in what felt like an exercise in futility, Freud dissected hundreds of eels to try to locate testes. “All the eels I have cut open are of the tenderer sex,” he would report until around eel number 400 when finally, he found the prize: gonads, buried in the abdominal cavity.

Sigmund Freud eels web

But why did so many of these eels not have male parts? To understand this requires knowing two things: how their sex is determined and their life cycle.

Both the American Eel (Anguilla rostrata) and the European Eel start out as eggs drifting about in the Sargasso Sea, in the middle of the Atlantic Ocean. They then hatch into larva, or “leptocephalus.” In this stage, they look like transparent willow leaves. They are not strong swimmers at this point, so the ocean currents will carry them. As they get closer to freshwater near land, they transform into what is called a “glass eel.” At this stage, they look more like an eel but are still quite transparent. They begin en masse to swim upstream into freshwater sources. Here, they transform into “elvers,” losing their transparency to look more like their adult form. They can travel very far upstream in this stage. It will take about 20 years for them to reach maturity. They then synchronously return to the Sargasso Sea to mate and then die. This cycle happens for both the American Eel as well as the European Eel. For the latter, they have a slightly longer migration to reach Europe.

What Freud, his instructor, and the scientists before them did not know is that the sex of an eel is not determined by genetics as it is in human beings. When an eel first hatches, it is neither male or female. The sex will later be determined by their environment, but the details around this are still unknown. Males do not become obvious until they are migrating or have reached the Sargasso Sea. This is the point in their life cycle when they start developing testes.

So Freud was dissecting many eels that would have become males later in their journey, had they not met the knife. The male that Freud found was possibly one developing early, which was something of a fluke, or one that came from the ocean. Either way, this was an incredible stroke of luck for Freud. Even if he was later missive about it, this discovery answered a question that had stumped so many before him.

Thanks to Adam Cohen, Icthyology Collection Manager, for his edits to this article.


“The Real Eel!” Presentation by Adam Cohen, Ichthyology Collection Manager for the Biodiversity Center at UT Austin. Barton Springs University 2020 Day. (https://www.youtube.com/watch?v=TYYjza0WH9I&feature=emb_logo)

Klein, Christopher. “10 Things You May Not Know About Sigmund Freud.” August, 22, 2018. History. (Accessed online: https://www.history.com/news/10-things-you-may-not-know-about-sigmund-freud)

Lee, Alexander. “Sexual Eeling. The slippery subject of eel reproduction evaded human understanding for millennia” History Today. Vol 70, issue 30. March 3, 2020. (accessed online: https://www.historytoday.com/archive/natural-histories/sexual-eeling)

McManus, Melanie Radzicki. “How Sigmund Freud Worked.” May 3, 2013.  HowStuffWorks.com. ( accessed online: https://health.howstuffworks.com/mental-health/psychologists/sigmund-freud-worked.htm)

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