We humans have been weirdly obsessed with eel reproduction for a really long time.

You’d think that the ancient Egyptians, Greeks, and Romans had enough to worry about, but no, they were all very concerned about how eels came to be. Their explanations ranged from the sun shining on the Nile River to spontaneous generation to being reanimated from bits of dead eel skin.

And whether or not eels even had the necessary equipment for mating was also up for debate. Their apparent lack made eels a favorite dish for medieval Europeans looking to avoid so-called “unclean” animals during Christian holidays with dietary restrictions. And they were not actually wrong about eels lacking reproductive organs, at least, some of the time.

That fact, that eels only have reproductive organs some of the time, is bound up in a bigger mystery about the lives and life cycles of eels. So, where do eels come from? [♪♪ INTRO ♪♪] Anguilla anguilla, more commonly known as the European eel, is a fish that spends most of its adult life in and around the coasts of the north Atlantic and Mediterranean, and the seas and inland waterways that connect to them. They go through five life-stages, and how they look changes at each stage.

As fully mature adults, they’re more silver in color. As young adults, they’re more yellowish-brownish and they’re often referred to as ‘yellow eels.’ Before they make it that far though, the eels can be caught by fishermen on Spain’s Atlantic coast at about 8 centimeters long and translucent-on-their-way-to-opaque. And they look quite a bit like small versions of the adults.

At this stage, they’re called ‘glass eels,’ and they’re considered an expensive regional delicacy, though they apparently don’t have much flavor. And they’re also, just saying it, highly trafficked by organized crime. Feels like a joke saying that one of the main ways they’re smuggled is in literal suitcases full of glass eels, but I guess our relationship with these fish has always been strange.

And for hundreds, if not thousands, of years, we could not tell you what European eels looked like before they turn into glass eels. But now we know that the larval stage of the European eel is very, very weird. The larvae of many eels are called ‘leptocephali,’ which means ‘thin heads,’ because they basically look like the ghosts of flattened, see-through leaves with tiny heads.

They look so little like their adult forms that for something like a century they were thought to be their own species of marine fish. Given how not-eel-like these guys look, and the fact that they are just the right size to avoid both plankton nets and trawl nets, it’s no surprise that it took so long for us to get enough evidence to link the eels to their larva. But where did the larva come from in the first place?

Why were folks finding them out in the open ocean when the other life-forms of these eels are mostly found in coastal estuaries, and inland rivers and streams? The short answer is that European eels are catadromous fish. This means they spend most of their lives in freshwater environments before returning to the ocean to breed as silver eels.

And ‘most of their lives’ can mean up to 25 years, and this whole time, they don’t reproduce, because they don’t actually have reproductive organs, yet. Now the longer answer is that the ocean is a big place and while we had found larval eels in a particular region of the Atlantic called the Sargasso Sea, we had never found adult eels or their eggs there. We don’t really know how the adults got there, either.

Like, they must have, because that’s the only place we’d found their larvae, but the rest of the picture was fuzzy. And we were stuck with this indirect evidence for where eels come from until late 2022, when a team of researchers finally published how they tracked them back to their breeding ground. Now, usually, when you want to track an animal’s migration, the obvious way to go is you stick some kind of GPS tracker on them.

Which seems to have been tricky for these eels. Previous research suggested both that European eel migration takes a long time and that, no matter where they start, they pass by the Azores on their way. So the team went looking for adult European eels in the Azores, assuming that they, too, would leave to spawn in the Sargasso Sea.

And they found some, tagged them, and sent them on their way. Of the 21 eels with tags that recorded data, 5 of them ultimately made it to the Sargasso Sea before their tags popped off. Others lost their tags before they made it there, but at various points between the Azores and the Sargasso Sea, so they seemed to be on their way there, at least.

And the other data from the tags also helped clear up some other mysterious things about eel migration. Like, during the day, they can swim at depths of more than 1,400 meters, so it’s no wonder no one’s ever spotted a migrating European eel. And they swim surprisingly slowly.

Eels from Europe probably take 18 months to get to the Sargasso Sea, enough time for their digestive tracts to degenerate and for their reproductive organs to fully develop, just in time for them to spawn and then die. So, if the change into silver eels from the yellow eel stage is part of what triggers the maturation of their gonads and starts their migration back to the ocean at the same time, this explains why so many different groups of people thought eels just were not equipped to reproduce. There just weren’t that many silver eels around for them to find!

The story of how we figured out where eels come from is, at its core, about our curiosity about the world around us, our desire to understand why things are the way they are, or seem to be. And it’s about our powers of observation, and what happens when we are presented with small pieces of a puzzle that’s bigger than we initially thought, one that we just needed the right timing and technological abilities to solve. The Bizarre Beasts pin club subscription window is open from now until the end of May 15th!

And the eel pins are very cool. Got mine already. And if you want more weird animal facts to amaze your friends and family with, we are on Twitter @BizarreBeasts, and on Instagram and Facebook @BizarreBeastsShow!

And, as always, our profits from the pin club and all of our merch go to support our community’s efforts to decrease maternal mortality in Sierra Leone. [♪♪ OUTRO ♪♪]