How Do Eels Reproduce?

We know that eels spawn, like many marine mammals -- but spawning can take many forms. So how do eels do it? Learn what we know (and don't know) in this episode of BrainStuff, based on this article: https://animals.howstuffworks.com/marine-life/how-do-eels-reproduce.htm

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Episode Transcript

Speaker 1 (00:01):
Welcome to brain Stuff, a production of I Heart Radio. Hey,
brain Stuff, Lauren vogel Bomb here online. It's become a
frequently asked question. How do eels, those long, writhing fish
you might have seen an aquarium or on a menu,
go about making eel babies? How to eels reproduce for real?

(00:25):
People have studied these creatures for literally thousands of years,
but even after all this time, some aspects of their
breeding lives are still shrouded in mystery. And to get
these skinny on eel reproduction for the articles, episode is
based on how Stuff Works. Spoke with an expert, Caroline Derriff,
an ecologist at the Norwegian Institute of Marine Research who

(00:45):
studies the habits of these incredible fish. Okay, first off,
calling something an eel doesn't make it one necessarily, no
doubt you've heard of the South American electric eel. Despite
common name, that dramatic fish isn't really an eel. It's
a Bizarro carp relative that's classified as a knife fish.

(01:08):
The order Anguilliforms contains all of the world's true eels.
There are more than eight hundred species from Mora's you know,
Ursula's sidekicks from the little Mermaid to the aptly named spaghetti.
Eels usually slender and elongated a true eels do not
have pelvic fins, which are twin appendages often found on

(01:29):
the underbellies of other fish. Eels can be scaleless, Many
species feel slippery to the touch, and salt water is
the preferred habitat for the majority of eels. However, there
are nineteen species of so called freshwater eels or anglid,
which spend some stages of their lives in fresh water.

(01:50):
They spawn in the ocean, that is, the adults produce
eggs and sperm, and then the resulting babies hatched there,
but they grow in freshwater before turning to the sea.
Saying that these fish have a complex life cycle is
a little bit of an understatement. If it survives to
reach sexual maturity, a freshwater eel will have gone through

(02:12):
five distinct stages. With each new phase, the animal experiences
both a physical transformation and shift in lifestyle. Derec explained.
The first stage is called the leptocephalus larva. They're called
leptocephalus because lepta means leaf and cephalis means head. True

(02:33):
to the name, the newborn larva have leaf shaped bodies
that appear broad and flattened in profile in biological jargon.
Their bodies are laterally compressed, being nearly transparent. They're also
very well camouflaged. Imagine trying to recover a lost contact
lens from the bottom of a swimming pool. Leptocephalus larva

(02:55):
are ocean going animals. Eventually, though instinct pushes them defined
change in scenery, which is where the change into the
second phase of the life cycle occurs. Derriff said, they
migrate for great distances. They drift through the gulf stream,
and then when they reach the continental shelf, they metamorphos
into glass eels. A glass eels are still more or

(03:19):
less transparent, but they're longer and skinnier by comparison, and
they're attracted to fresh water, so they head inland by
traveling up rivers, and that brings us to life stage
number three, a yellow eels. Unlike the transparent larva and
glass eels, these guys have body pigment with a yellowish
overall complexion. But it's not the last color change that

(03:42):
the fish will go through. A Derff said, oh, when
they're ready, they become silver eels, which is like puberty.
We often call them silver because they have a silver
belly and a black dorsal area. It's an adaptation to predation.
Many fish have the space of color pattern. When from below,
their silver bellies blend in with the brighter light coming

(04:03):
from the surface, and when viewed from above, their darker
backs blend in with the dimmer water below. But this
change in eels doesn't happen overnight. Transitioning from a yellow
eel to a silver eel can take twenty or thirty years.
Once the process finally ends, they return to their roots
and head seaward. Only then can the eels attain sexual maturity,

(04:28):
the fifth and final stage in their life cycle. However,
we don't know much about the reproductive stage of angle
at eels because no one has ever caught a sexually
mature eel alive in the wild. Likewise, no one has
observed these eels spawning in their natural habitat. The scientists
have yet to catch wild angleids in the act, if

(04:50):
you will, whatever happens out there. Experts think freshwater eels
die shortly after mating. The laboratory researchers have managed to
sexual maturity in silver eels by injecting them with hormones.
But after the transition their health declines, and Dereff said
the bones become decalcified like a woman during menopause. It's

(05:12):
super interesting, actually, and then their digestive tract their gut regresses.
Maybe that's just as well a breeding age. Freshwater eels
get together in places where their usual food options like
insects and small fish are probably rare or non existent.
Out in the Atlantic Ocean, there's a region called the

(05:32):
Sargasso Sea. Unlike the Mediterranean, the Red and most other seas,
this one is not bordered by any land masses. Instead,
its borders are formed by strong ocean currents. Both American
and European species of freshwater eels come here to reproduce.
A research suggests that they might use magnetic fields as

(05:54):
a navigation tool. The European eel has the longest path
to travel. Some individ jewels transverse around five thousand miles
or eight thousand kilometers to get there from Norway, half
a world away. The freshwater eels that live in and
around the Pacific Ocean have spawning areas of their own.
The Japanese eel is thought to breed at a site

(06:16):
west of the Mariana Islands. Other species could be procreating
somewhere between New Caledonia and Fiji. Eels release their eggs
underwater to be fertilized by clouds of expelled sperm that
goes for both freshwater eels and the non freshwater species
such as the aforementioned moras and conger eels, speaking of

(06:38):
which a derec explained that we know even less about
conger eels than anglids when it comes to reproduction. She said,
but we think there is at least a spawning area
in the Mediterranean Baby. Hopefully future research will shed some
light on their private lives. Today's episode is based on

(07:01):
the article how do eels reproduce? On how stuff works
dot Com, written by Mark Mancini. Brain Stuff is production
of I Heart Radio in partnership with how stuff works
dot Com and is produced by Tyler Clang. Four more
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