WEBVTT

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They're long and slithery,

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and they're not very colorful.

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But they do have a strange beauty of their own.

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Their sinuous, nocturnal movements

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through the water

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are mesmerizing to watch.

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And though they may resemble underwater snakes,

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eels are, in fact, a very interesting type of fish.

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There are several things about eels

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that make them unique

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besides their elongated shape and limbless bodies.

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For one thing,

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eels have the ability to breathe through their skin.

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Some can even leave the water

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and move over land for short periods.

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And, unlike most migratory fish,

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such as salmon,

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which spawn in fresh water

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but live their adult lives in salt water,

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eels of the genus Anguilla

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migrate in the opposite direction,

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spawning and breeding in oceans and seas,

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while spending most of their intervening time

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in fresh water.

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If we were to take one such fresh water eel

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and follow its life story,

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it would be born in the middle of the North Atlantic Ocean,

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about a thousand miles east of Bermuda.

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This area, called the Sargasso Sea,

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forms the western part

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of a subtropical gyre,

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a giant whirlpool in the middle of the ocean.

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Our eel, let's call it Eli,

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would begin as one of ten to twenty million tiny eggs,

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carried by a female eel,

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hatching into a transparent leaf-shaped thing

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that looks nothing like an adult eel.

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Eli starts to drift in ocean currents,

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predominantly the Gulf Stream

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towards either Europe or North America,

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depending on its particular species.

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Upon reaching the coast,

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Eli is about two inches long,

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looking more eely but still transparent,

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known at this stage as a glass eel.

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But within a couple of days in fresh water,

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Eli's skin becomes pigmented a brownish-black,

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now looking more like that of an adult eel.

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You might notice

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that we haven't mentioned anything

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about Eli's gender yet.

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That's because this is only determined

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once an eel enters fresh water,

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though nobody is sure exactly how that happens.

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Most of the eels that stay in the estuaries

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and brackish water become males,

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while those that go upstream become females,

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growing up to two to three times bigger than

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their future mates.

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In this case,

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it turns out that Eli was actually short for Elaine.

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As a female eel,

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Elaine will be quite solitary

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for most of her life in the stream,

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eating whatever falls in the water:

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grasshoppers, crickets, small fish,

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insect larvae, frogs, baby birds,

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almost anything she can get her mouth around.

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And she will grow quite big,

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up to four feet long

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and weighing as much as thirteen pounds.

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We don't know exactly how

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fresh water eels know when it's time

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to return to the ocean,

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but something calls to them.

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And their fall migration

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is one of the largest unseen migrations

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on the planet.

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As Elaine leaves fresh water for the ocean,

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she undergoes a shocking metamorphosis.

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Her eyes enlarge by about ten times,

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her skin gets thicker,

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and her fins get larger.

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These are most likely adaptations

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for their upcoming ocean travel,

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and Elaine seamlessly makes the transition

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from fresh to salt water,

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which would be toxic for most other fish.

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Once Elaine leaves the mouth of the fresh water streams,

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she will disappear completely from human view.

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No one has witnessed,

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or been able to follow,

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an adult eel on their migration,

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nor do we know how deep they spawn.

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But it's assumed that they can follow some signs

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that they can detect,

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such as a thermal barrier between ocean currents

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or a salinity front,

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in order to return to the same area of the ocean

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where they were born.

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Because we don't even know exactly

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what happens during an eel's migration,

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we can only imagine what the actual breeding looks like.

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But the common hypothesis

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is that Elaine and thousands,

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or hundreds of thousands,

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of other eels

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gather in large, intertwined masses

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and release their eggs and sperm

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in a giant orgy known as panmixia.

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A couple of days after the eggs are laid,

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they hatch,

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and the cycle begins again.

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And because we've never seen

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the adult eels returning up the fresh water rivers,

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we must assume that,

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having completed their long and roundabout journey,

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these amazing and mysterious creatures

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finally die there,

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in the same place where they were born.

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Goodbye, Elaine!

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It was a pleasure knowing you.