0:00:07.661,0:00:09.378
They're long and slithery,

0:00:09.378,0:00:11.045
and they're not very colorful.

0:00:11.045,0:00:14.252
But they do have a strange beauty of their own.

0:00:14.252,0:00:16.003
Their sinuous, nocturnal movements

0:00:16.003,0:00:16.961
through the water

0:00:16.961,0:00:18.878
are mesmerizing to watch.

0:00:18.878,0:00:21.176
And though they may resemble underwater snakes,

0:00:21.176,0:00:25.293
eels are, in fact, a very interesting type of fish.

0:00:25.293,0:00:26.763
There are several things about eels

0:00:26.763,0:00:28.127
that make them unique

0:00:28.127,0:00:31.294
besides their elongated shape and limbless bodies.

0:00:31.294,0:00:32.211
For one thing,

0:00:32.211,0:00:35.731
eels have the ability to breathe through their skin.

0:00:35.731,0:00:37.360
Some can even leave the water

0:00:37.360,0:00:40.069
and move over land for short periods.

0:00:40.069,0:00:42.044
And, unlike most migratory fish,

0:00:42.044,0:00:43.127
such as salmon,

0:00:43.127,0:00:44.574
which spawn in fresh water

0:00:44.574,0:00:46.658
but live their adult lives in salt water,

0:00:46.658,0:00:48.794
eels of the genus Anguilla

0:00:48.794,0:00:50.746
migrate in the opposite direction,

0:00:50.746,0:00:53.258
spawning and breeding in oceans and seas,

0:00:53.258,0:00:55.127
while spending most of their intervening time

0:00:55.127,0:00:56.961
in fresh water.

0:00:56.961,0:00:59.173
If we were to take one such fresh water eel

0:00:59.173,0:01:00.961
and follow its life story,

0:01:00.961,0:01:03.759
it would be born in the middle of the North Atlantic Ocean,

0:01:03.759,0:01:06.461
about a thousand miles east of Bermuda.

0:01:06.461,0:01:08.795
This area, called the Sargasso Sea,

0:01:08.795,0:01:10.350
forms the western part

0:01:10.350,0:01:12.044
of a subtropical gyre,

0:01:12.044,0:01:15.199
a giant whirlpool in the middle of the ocean.

0:01:15.199,0:01:17.564
Our eel, let's call it Eli,

0:01:17.564,0:01:21.235
would begin as one of ten to twenty million tiny eggs,

0:01:21.235,0:01:23.238
carried by a female eel,

0:01:23.238,0:01:25.711
hatching into a transparent leaf-shaped thing

0:01:25.711,0:01:29.211
that looks nothing like an adult eel.

0:01:29.211,0:01:31.294
Eli starts to drift in ocean currents,

0:01:31.294,0:01:33.247
predominantly the Gulf Stream

0:01:33.247,0:01:35.259
towards either Europe or North America,

0:01:35.259,0:01:38.461
depending on its particular species.

0:01:38.461,0:01:40.045
Upon reaching the coast,

0:01:40.045,0:01:41.876
Eli is about two inches long,

0:01:41.876,0:01:44.510
looking more eely but still transparent,

0:01:44.510,0:01:47.262
known at this stage as a glass eel.

0:01:47.262,0:01:49.044
But within a couple of days in fresh water,

0:01:49.044,0:01:52.100
Eli's skin becomes pigmented a brownish-black,

0:01:52.100,0:01:55.354
now looking more like that of an adult eel.

0:01:55.354,0:01:56.395
You might notice

0:01:56.395,0:01:57.648
that we haven't mentioned anything

0:01:57.648,0:01:59.462
about Eli's gender yet.

0:01:59.462,0:02:01.294
That's because this is only determined

0:02:01.294,0:02:03.109
once an eel enters fresh water,

0:02:03.109,0:02:06.323
though nobody is sure exactly how that happens.

0:02:06.323,0:02:08.907
Most of the eels that stay in the estuaries

0:02:08.907,0:02:10.826
and brackish water become males,

0:02:10.826,0:02:13.163
while those that go upstream become females,

0:02:13.163,0:02:15.211
growing up to two to three times bigger than

0:02:15.211,0:02:17.465
their future mates.

0:02:17.465,0:02:18.417
In this case,

0:02:18.417,0:02:22.339
it turns out that Eli was actually short for Elaine.

0:02:22.339,0:02:23.424
As a female eel,

0:02:23.424,0:02:25.008
Elaine will be quite solitary

0:02:25.008,0:02:26.794
for most of her life in the stream,

0:02:26.794,0:02:28.845
eating whatever falls in the water:

0:02:28.845,0:02:31.349
grasshoppers, crickets, small fish,

0:02:31.349,0:02:34.236
insect larvae, frogs, baby birds,

0:02:34.236,0:02:37.128
almost anything she can get her mouth around.

0:02:37.128,0:02:38.688
And she will grow quite big,

0:02:38.688,0:02:40.230
up to four feet long

0:02:40.230,0:02:43.377
and weighing as much as thirteen pounds.

0:02:43.377,0:02:44.877
We don't know exactly how

0:02:44.877,0:02:46.277
fresh water eels know when it's time

0:02:46.277,0:02:47.912
to return to the ocean,

0:02:47.912,0:02:49.617
but something calls to them.

0:02:49.617,0:02:50.908
And their fall migration

0:02:50.908,0:02:52.744
is one of the largest unseen migrations

0:02:52.744,0:02:54.579
on the planet.

0:02:54.579,0:02:56.497
As Elaine leaves fresh water for the ocean,

0:02:56.497,0:02:59.626
she undergoes a shocking metamorphosis.

0:02:59.626,0:03:02.127
Her eyes enlarge by about ten times,

0:03:02.127,0:03:03.711
her skin gets thicker,

0:03:03.711,0:03:06.044
and her fins get larger.

0:03:06.044,0:03:07.759
These are most likely adaptations

0:03:07.759,0:03:09.593
for their upcoming ocean travel,

0:03:09.593,0:03:11.680
and Elaine seamlessly makes the transition

0:03:11.680,0:03:13.627
from fresh to salt water,

0:03:13.627,0:03:16.711
which would be toxic for most other fish.

0:03:16.711,0:03:18.936
Once Elaine leaves the mouth of the fresh water streams,

0:03:18.936,0:03:22.189
she will disappear completely from human view.

0:03:22.189,0:03:23.732
No one has witnessed,

0:03:23.732,0:03:25.151
or been able to follow,

0:03:25.151,0:03:27.111
an adult eel on their migration,

0:03:27.111,0:03:29.905
nor do we know how deep they spawn.

0:03:29.905,0:03:32.659
But it's assumed that they can follow some signs

0:03:32.659,0:03:34.162
that they can detect,

0:03:34.162,0:03:36.453
such as a thermal barrier between ocean currents

0:03:36.453,0:03:37.953
or a salinity front,

0:03:37.953,0:03:40.309
in order to return to the same area of the ocean

0:03:40.309,0:03:42.178
where they were born.

0:03:42.178,0:03:44.084
Because we don't even know exactly

0:03:44.084,0:03:46.006
what happens during an eel's migration,

0:03:46.006,0:03:49.126
we can only imagine what the actual breeding looks like.

0:03:49.126,0:03:50.711
But the common hypothesis

0:03:50.711,0:03:52.462
is that Elaine and thousands,

0:03:52.462,0:03:53.763
or hundreds of thousands,

0:03:53.763,0:03:54.711
of other eels

0:03:54.711,0:03:57.127
gather in large, intertwined masses

0:03:57.127,0:03:58.794
and release their eggs and sperm

0:03:58.794,0:04:01.896
in a giant orgy known as panmixia.

0:04:01.896,0:04:03.905
A couple of days after the eggs are laid,

0:04:03.905,0:04:04.811
they hatch,

0:04:04.811,0:04:06.608
and the cycle begins again.

0:04:06.608,0:04:08.044
And because we've never seen

0:04:08.044,0:04:10.655
the adult eels returning up the fresh water rivers,

0:04:10.655,0:04:11.878
we must assume that,

0:04:11.878,0:04:14.408
having completed their long and roundabout journey,

0:04:14.408,0:04:16.702
these amazing and mysterious creatures

0:04:16.702,0:04:18.295
finally die there,

0:04:18.295,0:04:20.572
in the same place where they were born.

0:04:20.572,0:04:22.000
Goodbye, Elaine!

0:04:22.000,0:04:23.709
It was a pleasure knowing you.