The reproduction of freshwater eels—specifically European and American species—is one of biology's most remarkable and long-standing mysteries. Until the end of the 19th century, scientists were unable to determine where eels originated, as no one had ever observed their eggs or fry in rivers. Aristotle, for instance, famously believed they were spontaneously generated from the mud.
Here is how this process actually unfolds, step by step:
1. Preparation for the Journey (Transformation)
Eels inhabit the freshwater rivers and lakes of Europe or America for the majority of their lives, typically for 5 to 15 years or more. When it is time to breed, the fish undergo an incredible metamorphosis:
* They stop feeding.
* Their color changes from yellow-green with a light belly to a metallic silver with a dark back and a white underside.
* Their eyes increase in size to see better in the darkness of the ocean depths.
* The digestive system atrophies while the sex glands begin to develop rapidly.
Such eels are known as "silver eels."
2. The Great Migration
Silver eels migrate from rivers into the ocean to begin a journey spanning thousands of kilometers.
European eels swim toward the **Sargasso Sea** (a region in the Atlantic Ocean known for its seaweed, between Bermuda and the West Indies).
* The journey takes approximately 6–8 months.
3. Spawning
This is the most secretive stage; eels gather in the Sargasso Sea at great depths of about 400 to 700 meters, though some data suggests they descend as far as 1,500 meters.
* There they spawn and release their milt.
* Important fact: After spawning, every adult eel dies, as they reproduce only once in their lifetime.
4. Larval Stage (Leptocephali)
Larvae hatch from the eggs and look completely different from adult eels.
They are called **leptocephali**.
* They are transparent, flat, and shaped like a willow leaf.
* They lack red blood cells and feed on "marine snow" (organic debris).
Rather than swimming actively, these larvae drift with the Gulf Stream toward Europe. This drift lasts from 1 to 3 years. During this time, they grow to a length of several centimeters.
5. The Return (Glass Eel)
As it approaches the shores of Europe, the larva undergoes a new transformation:
It becomes narrow and cylindrical but remains transparent, at which point it is called a **glass eel**.
* They enter the mouths of rivers and begin moving against the current.
6. Maturity (Yellow Eel)
As they move upriver, the eels gain pigmentation—with the back darkening and the belly lightening—and transform into yellow eels. In this stage, they live, grow, and feed for many years until the cycle finally begins again.
Brief Summary: Eels are born in salt water (the Sargasso Sea), grow in fresh water (the rivers of Europe), and return to the ocean to give life to a new generation and die.
