Our journey through the realm of European eels concludes with our modern biological understanding of them.
As we’ve seen, eels have been a mystery to humans for basically as long as natural history has been interesting to us. Despite tremendous amounts of research, it took Western scientists until the end of the 19th century to find an eel with testicles.
How is it possible that eels escaped our scientific scrutiny for so long?
The answer lies in their bizarre life history.
You likely know about the concept of metamorphosis, the process through which animals drastically change forms throughout their lives. Larvae turning into flies is a common example of this, as are caterpillars turning into butterflies.
Well, eels do the same thing.
They start life as eggs in the ocean.
When they hatch, they become what’s called leptocephalus (literally “thin-headed”) larval eels. These are tiny, transparent, wriggly little eels that are a few millimeters in length. They spend their time eating phytoplankton and other bits of matter that drift in the eternal currents.
As they mature, they swim towards the European coast, and become glass eels. They look a bit like cooked rice noodles at this stage.
Here’s the first weird part: glass eels live in rivers, which are freshwater. Generally speaking, animals either live in saltwater or freshwater, because those two environments are so different that few animals have the adaptations necessary to survive both of them. Eels are one of the exceptions — one metamorphosis and they’re good to switch from the ocean to rivers.
A little while later, they turn into elvers, which is a very pretty name for a young eel.
Two years after that, they become yellow eels. When you picture an eel, this is probably what you’re imagining. They live in freshwater and spend their time mostly eating and swimming around. This stage can last up to twenty years, and during this time they’re sexually immature.
Then, at some point within these twenty years, they decide to mature into silver eels. It’s not clear how they do this; there’s some secret signal they obey, and suddenly paf! They have functional ovaries and testicles, and are ready to reproduce.
Because all these life stages are so different, it took us a long time to figure out they were all the same species. Which didn’t help anything when it came to studying their reproductive behavior. But even after identifying all these different eels as the same European eel, we still had trouble understanding them.
They don’t reproduce in their rivers — that would be too easy for scientists to observe. Instead, they migrate to the Sargasso Sea, a part of the Atlantic Ocean not far from Florida.
How do we know this? Excellent question.
See, for years we knew they were reproducing somewhere, but had no idea where. So one day in 1904, Danish zoologist Johannes Schmidt decided to spend his time sailing around the Atlantic looking for eels. His logic was that if he followed smaller and smaller eels, going from the glass eels that lived on the European coast to the tiniest of leptocephalus larva, he could deduce where the spawning grounds were. So he sailed away for twelve years, following the eels until he found the smallest ones, and declared this to be where eels spawned.
He was right.
With tracking technology, we’ve been able to double check his work, and European eels do in fact migrate across the Atlantic Ocean to reproduce.
Why do they do this? That remains a mystery.
But there’s something kind of cool in the story of the eel. After thousands of years, we still don’t know anything about how they mate in the wild, or why they choose that specific part of the world to do it, or how they even know how to get there. And for some reason, despite all the wars and disasters and heartache the world has suffered since the first human observed the first eel, the mystery that lies behind it has remained an important one for us to solve. We want to know what this eel does; we want to know why it does it.
Why do we want to know?
I don’t know.
But I’m glad we do.
For my sources this week, click here and here.
P.S. I recently moved, and in so doing lost access to my scanner. Apologies for any lower-quality images as a result!