Speaker 1: Welcome to brain Stuff, a production of I Heart Radio. Hey,
Speaker 1: brain Stuff, Lauren vogel Bomb here online. It's become a
Speaker 1: frequently asked question. How do eels, those long, writhing fish
Speaker 1: you might have seen an aquarium or on a menu,
Speaker 1: go about making eel babies? How to eels reproduce for real?
Speaker 1: People have studied these creatures for literally thousands of years,
Speaker 1: but even after all this time, some aspects of their
Speaker 1: breeding lives are still shrouded in mystery. And to get
Speaker 1: these skinny on eel reproduction for the articles, episode is
Speaker 1: based on how Stuff Works. Spoke with an expert, Caroline Derriff,
Speaker 1: an ecologist at the Norwegian Institute of Marine Research who
Speaker 1: studies the habits of these incredible fish. Okay, first off,
Speaker 1: calling something an eel doesn't make it one necessarily, no
Speaker 1: doubt you've heard of the South American electric eel. Despite
Speaker 1: common name, that dramatic fish isn't really an eel. It's
Speaker 1: a Bizarro carp relative that's classified as a knife fish.
Speaker 1: The order Anguilliforms contains all of the world's true eels.
Speaker 1: There are more than eight hundred species from Mora's you know,
Speaker 1: Ursula's sidekicks from the little Mermaid to the aptly named spaghetti.
Speaker 1: Eels usually slender and elongated a true eels do not
Speaker 1: have pelvic fins, which are twin appendages often found on
Speaker 1: the underbellies of other fish. Eels can be scaleless, Many
Speaker 1: species feel slippery to the touch, and salt water is
Speaker 1: the preferred habitat for the majority of eels. However, there
Speaker 1: are nineteen species of so called freshwater eels or anglid,
Speaker 1: which spend some stages of their lives in fresh water.
Speaker 1: They spawn in the ocean, that is, the adults produce
Speaker 1: eggs and sperm, and then the resulting babies hatched there,
Speaker 1: but they grow in freshwater before turning to the sea.
Speaker 1: Saying that these fish have a complex life cycle is
Speaker 1: a little bit of an understatement. If it survives to
Speaker 1: reach sexual maturity, a freshwater eel will have gone through
Speaker 1: five distinct stages. With each new phase, the animal experiences
Speaker 1: both a physical transformation and shift in lifestyle. Derec explained.
Speaker 1: The first stage is called the leptocephalus larva. They're called
Speaker 1: leptocephalus because lepta means leaf and cephalis means head. True
Speaker 1: to the name, the newborn larva have leaf shaped bodies
Speaker 1: that appear broad and flattened in profile in biological jargon.
Speaker 1: Their bodies are laterally compressed, being nearly transparent. They're also
Speaker 1: very well camouflaged. Imagine trying to recover a lost contact
Speaker 1: lens from the bottom of a swimming pool. Leptocephalus larva
Speaker 1: are ocean going animals. Eventually, though instinct pushes them defined
Speaker 1: change in scenery, which is where the change into the
Speaker 1: second phase of the life cycle occurs. Derriff said, they
Speaker 1: migrate for great distances. They drift through the gulf stream,
Speaker 1: and then when they reach the continental shelf, they metamorphos
Speaker 1: into glass eels. A glass eels are still more or
Speaker 1: less transparent, but they're longer and skinnier by comparison, and
Speaker 1: they're attracted to fresh water, so they head inland by
Speaker 1: traveling up rivers, and that brings us to life stage
Speaker 1: number three, a yellow eels. Unlike the transparent larva and
Speaker 1: glass eels, these guys have body pigment with a yellowish
Speaker 1: overall complexion. But it's not the last color change that
Speaker 1: the fish will go through. A Derff said, oh, when
Speaker 1: they're ready, they become silver eels, which is like puberty.
Speaker 1: We often call them silver because they have a silver
Speaker 1: belly and a black dorsal area. It's an adaptation to predation.
Speaker 1: Many fish have the space of color pattern. When from below,
Speaker 1: their silver bellies blend in with the brighter light coming
Speaker 1: from the surface, and when viewed from above, their darker
Speaker 1: backs blend in with the dimmer water below. But this
Speaker 1: change in eels doesn't happen overnight. Transitioning from a yellow
Speaker 1: eel to a silver eel can take twenty or thirty years.
Speaker 1: Once the process finally ends, they return to their roots
Speaker 1: and head seaward. Only then can the eels attain sexual maturity,
Speaker 1: the fifth and final stage in their life cycle. However,
Speaker 1: we don't know much about the reproductive stage of angle
Speaker 1: at eels because no one has ever caught a sexually
Speaker 1: mature eel alive in the wild. Likewise, no one has
Speaker 1: observed these eels spawning in their natural habitat. The scientists
Speaker 1: have yet to catch wild angleids in the act, if
Speaker 1: you will, whatever happens out there. Experts think freshwater eels
Speaker 1: die shortly after mating. The laboratory researchers have managed to
Speaker 1: sexual maturity in silver eels by injecting them with hormones.
Speaker 1: But after the transition their health declines, and Dereff said
Speaker 1: the bones become decalcified like a woman during menopause. It's
Speaker 1: super interesting, actually, and then their digestive tract their gut regresses.
Speaker 1: Maybe that's just as well a breeding age. Freshwater eels
Speaker 1: get together in places where their usual food options like
Speaker 1: insects and small fish are probably rare or non existent.
Speaker 1: Out in the Atlantic Ocean, there's a region called the
Speaker 1: Sargasso Sea. Unlike the Mediterranean, the Red and most other seas,
Speaker 1: this one is not bordered by any land masses. Instead,
Speaker 1: its borders are formed by strong ocean currents. Both American
Speaker 1: and European species of freshwater eels come here to reproduce.
Speaker 1: A research suggests that they might use magnetic fields as
Speaker 1: a navigation tool. The European eel has the longest path
Speaker 1: to travel. Some individ jewels transverse around five thousand miles
Speaker 1: or eight thousand kilometers to get there from Norway, half
Speaker 1: a world away. The freshwater eels that live in and
Speaker 1: around the Pacific Ocean have spawning areas of their own.
Speaker 1: The Japanese eel is thought to breed at a site
Speaker 1: west of the Mariana Islands. Other species could be procreating
Speaker 1: somewhere between New Caledonia and Fiji. Eels release their eggs
Speaker 1: underwater to be fertilized by clouds of expelled sperm that
Speaker 1: goes for both freshwater eels and the non freshwater species
Speaker 1: such as the aforementioned moras and conger eels, speaking of
Speaker 1: which a derec explained that we know even less about
Speaker 1: conger eels than anglids when it comes to reproduction. She said,
Speaker 1: but we think there is at least a spawning area
Speaker 1: in the Mediterranean Baby. Hopefully future research will shed some
Speaker 1: light on their private lives. Today's episode is based on
Speaker 1: the article how do eels reproduce? On how stuff works
Speaker 1: dot Com, written by Mark Mancini. Brain Stuff is production
Speaker 1: of I Heart Radio in partnership with how stuff works
Speaker 1: dot Com and is produced by Tyler Clang. Four more
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