No one can figure out how eels have sex | Lucy Cooke

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From Ancient Greece to the 20th century, Aristotle, Freud, and numerous other scholars were all looking for the same thing: eel testicles. Freshwater eels could be found in rivers across Europe, but no one had ever seen them mate and no researcher could find eel eggs or identify their reproductive organs. So how do eels reproduce, and where do they do it? Lucy Cooke digs into the ancient mystery. [Directed by Anton Bogaty, narrated by Adrian Dannatt, music by Jarrett Farkas].

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00:07

From

Ancient Greece

to the 20th century,

Aristotle

Sigmund Freud

and numerous other scholars, we're all looking for the same thing.

Eel

testicles, freshwater

eels

Anguilla anguilla

could be found in rivers across

Europe

, but no one had ever seen them mate. And despite countless dissections, no researcher could find

eel

eggs or identify their reproductive organs.

00:38

Devoid of data naturalists proposed various

eel

origin stories.

Aristotle

suggested that heals spontaneously emerged from mu. Cleaning the elder argued

eels

rub themselves against rocks and the subsequent scrapings came to life.

Eels

were said to hatch on rooftops, manifest from the gills of other fish and even emerge from the bodies of

Beetles

. But the true story

eel

reproduction is even more difficult to imagine. And to solve this slippery mystery, scholars would have to rethink centuries of research.

01:19

Today, we know the freshwater

eel

lifecycle has five distinct stages. Larva, leptocephaly, miniscule glass

eels

, adolescent elvers, older yellow

eels

and adult silver

eels

. Given the radical physical differences between these phases, you'd be forgiven for assuming these are different animals. In fact, that's exactly what European naturalists thought.

01:49

Researchers were aware of leptocephaly and glass

eels

, but no one guessed they were related to the elders and yellow

eels

living hundreds of kilometers upstream, confusing matters more.

Eels

don't develop sex organs until late in life, and the entirety of their time in the rivers of

Europe

is essentially yield adolescence. So when do

eels

reproduce? And where do they do it?

02:18

Despite its name, the life of a freshwater

eel

actually begins in the salty waters of the

Bermuda Triangle

at the height of the annual

Cyclone

season, thousands of three millimeter

eel

larvae drift out of

the Sargasso Sea

. From here, they follow migration paths to

North America

and

Europe.

Continents that were much closer when

eels

established these routes 40 million years ago.

02:46

Over the next 300 days,

Anguilla anguilla

larvae ride the ocean currents 6,500 kilometers to the coast of

Europe

, making one of the longest known marine migrations. By the time they arrive, they've grown approximately 45 millimeters and transformed into semi transparent glass

eels

03:10

It's not just their appearance has changed. If most marine fish entered brackish coastal waters, their cells would swell with fresh water in a lethal explosion. But when glass

eels

reached the coast, their kidneys shift to retain more salt and maintain their blood salinity levels.

03:29

Swarms of these newly freshwater fish migrate up streams and rivers, sometimes piling on top of each other to clear obstacles and predators Those that make it upstream develop into opaque elvers, having finally arrived in their hunting grounds. Elvers begin to eat everything they can fit into their mouths. These omnivores grow in proportion to their diets.

03:55

And over the next decade, they develop into larger yellow

eels

in this stage, they grow to be roughly 80 centimeters and finally developed sexual organs. But the last phase of

eel

life and the secret of their reproduction remains mysterious. In 1896, researchers identified leptocephaly as larval

eels

and deduced that they had come to

Europe

from somewhere in the

Atlantic

04:26

However, to find this mysterious breeding ground, someone would have to perform an unthinkable survey of the ocean for larvl no larger than 30 millimeters. Enter

Johannes Schmidt

, for the next 18 years, this Danish oceanographer trawled the coasts of four continents, hunting down increasingly tiny leptocephaly. Finally, in 1921, he found the smallest lava yet on the southern edge of

the Sargasso Sea

04:58

Despite knowledge of their round trip migration, scientists still haven't observed mating in the wild or found a single eel. Leading theory suggests that

eels

reproduced in a flurry of external fertilization, in which clouds of sperm fertilize free floating eggs. But the powerful currents and tangling seaweed of

the Sargasso Sea

have made this theory difficult to confirm.

05:25

Researchers don't even know where to look, since they have yet to successfully track and field over the course of its return migration. Until these challenges can be met, the

eels

, ancient secret will continue to slip through our fingers.

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