Name: 【TED-Ed】Eli the eel: A mysterious migration - James Prosek
Uploaded: 2015-01-03T08:48:38.000Z
Duration: 4 min 39 s
Description: Thousands of YouTube videos with English-Chinese subtitles! Now you can learn to understand native speakers, expand your vocabulary, and improve your pronunciation...

They're long and slithery, and they're not very colorful.

But they do have a strange beauty of their own.

Their sinuous, nocturnal movements through the water are mesmerizing to watch.

And though they may resemble underwater snakes, eels are, in fact, a very interesting type of fish.

There are several things about eels that make them unique besides their elongated shape and limbless bodies.

For one thing, eels have the ability to breathe through their skin.

Some can even leave the water and move over land for short periods.

And, unlike most migratory fish, such as salmon, which spawn in fresh water.

but live their adult lives in salt water,

eels of the genus Anguilla migrate in the opposite direction, spawning and breeding in oceans and seas, while spending most of their intervening time in fresh water.

If we were to take one such fresh water eel and follow its life story, it would be born in the middle of the North Atlantic Ocean, about a thousand miles east of Bermuda.

This area, called the Sargasso Sea, forms the western part of a subtropical gyre, a giant whirlpool in the middle of the ocean.

Our eel, let's call it Eli, would begin as one of ten to twenty million tiny eggs, carried by a female eel, hatching into a transparent leaf-shaped thing that looks nothing like an adult eel.

Eli starts to drift in ocean currents, predominantly the Gulf Stream towards either Europe or North America, depending on its particular species.

Upon reaching the coast, Eli is about two inches long, looking more eely but still transparent, known at this stage as a glass eel.

But within a couple of days in fresh water, Eli's skin becomes pigmented a brownish-black, now looking more like that of an adult eel.

You might notice that we haven't mentioned anything about Eli's gender yet.

That's because this is only determined once an eel enters fresh water, though nobody is sure exactly how that happens.

Most of the eels that stay in the estuaries and brackish water become males, while those that go upstream become females, growing up to two to three times bigger than their future mates.

In this case, it turns out that Eli was actually short for Elaine.

As a female eel, Elaine will be quite solitary for most of her life in the stream,

eating whatever falls in the water: grasshoppers, crickets, small fish, insect larvae, frogs, baby birds, almost anything she can get her mouth around.

And she will grow quite big, up to four feet long and weighing as much as thirteen pounds.

We don't know exactly how fresh water eels know when it's time to return to the ocean, but something calls to them.

And their fall migration is one of the largest unseen migrations on the planet.

As Elaine leaves fresh water for the ocean, she undergoes a shocking metamorphosis.

Her eyes enlarge by about ten times, her skin gets thicker, and her fins get larger.

These are most likely adaptations for their upcoming ocean travel, and Elaine seamlessly makes the transition from fresh to salt water, which would be toxic for most other fish.

Once Elaine leaves the mouth of the fresh water streams, she will disappear completely from human view.

No one has witnessed, or been able to follow, an adult eel on their migration, nor do we know how deep they spawn.

But it's assumed that they can follow some signs that they can detect, such as a thermal barrier between ocean currents or a salinity front, in order to return to the same area of the ocean where they were born.

Because we don't even know exactly what happens during an eel's migration, we can only imagine what the actual breeding looks like.

But the common hypothesis is that Elaine and thousands, or hundreds of thousands, of other eels gather in large, intertwined masses and release their eggs and sperm in a giant orgy known as panmixia.

A couple of days after the eggs are laid, they hatch, and the cycle begins again.

And because we've never seen the adult eels returning up the fresh water rivers,

we must assume that, having completed their long and roundabout journey, these amazing and mysterious creatures finally die there, in the same place where they were born.

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【TED-Ed】Eli the eel: A mysterious migration - James Prosek

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