Eli the eel: A mysterious migration - James Prosek

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They're slippery. They're slithery. And while they totally look like underwater snakes, eels are, in fact, unique fish that can breathe through their skin and even survive out of water. James Prosek tracks the life journey of Eli the Anguilla eel as she (yes, she) travels her mysterious "backward" migration from the sea to fresh water and back again.

Lesson by James Prosek, animation by Cinematic.

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.

Goodbye, Elaine!

It was a pleasure knowing you.