When most people hear the term metamorphosis, they think of the transformation of a caterpillar as it forms its cocoon and then hatches into a beautiful butterfly.
The American eel also goes through a metamorphosis, but there is not a cocoon involved and it does not grow big, colorful wings out of its back.
One thing you can say about the American eel is that it is a very consistent creature.
It follows a very specific migration pattern, which scientists refer to as catadromous, and after it spends its nearly 40 years of life sprinkled along the Atlantic coast of North America, it always returns home to where it began.
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Table of Contents
The Life Cycle of an American Eel
What’s fascinating about the American eel is that it begins its life in one place and migrates far, far away from where it spends most of its life, about 40 years of life.
Yet, somehow, when the time is right, they know exactly what to do and where to go to begin the cycle all over again.
The most important thing to keep in mind about the lifecycle of an American eel is that these eels are born in the Sargasso Sea, and after migrating to freshwaters where they live most of their life, they return to the Sargasso Sea towards the end of their life to spawn more eel and eventually die in that place.
The life cycle of the American eel is essentially like a circle, it starts in one place and no matter where it goes it ultimately ends up back at its place of origin only to start over from the same place and continue on with its cycle.
According to the U.S. Fish and Wildlife Service (FWS), the life cycle of the American eel begins and ends in the Sargasso Sea, which is between the Azores and the West Indies making all American eels very similar genetically.
The life cycle of an eel can be summed up in five phases:
- Leptocephali – eggs float to the surface and drift with the ocean current
- Glass eels – larvae stage, develop fins and features after reaching the coast
- Elvers – develop pigmentation, migrate to brackish waters
- Yellow eels – sexually immature adults, yellow-green to olive-brown color
- Silver eels – become male or female silver eels and return to sea to spawn
In the Leptocephali phase, the eggs float to the surface hatch into small, transparent larvae that are shaped like willow leaves.
They drift along with the ocean current until they reach the Atlantic Coast.
When the American Eel eggs finish floating and hatch, they become glass eels.
Also known as the larvae stage, this is where they will develop fins and the features of adult eels. They have reached the coast at this point, are still transparent, and are two to three inches long.
Once they enter the Elvers stage, the eels develop gray to green-brown pigmentation to cover their previously transparent scales. They also migrate to brackish waters at have grown again to be about four inches long by this point.
The Elvers stage is directly followed by the yellow eels stage, where they become sexually immature adults that are actually yellow-green to olive-brown.
American eels are nocturnal, swimming and feeding at night in this phase.
Over the next 3 to 40 or more years of living in freshwater, brackish waters or marine waters, the yellow eels begin to sexually mature.
Depending on a variety of factors, which can include population density, eel growth rate, and water salinity, the yellow eels will become male or female silver eels with dark coloring, bronze or black backs and silver undersides.
Toward the end of their life cycles, they return to the Sargasso Sea to spawn.
They undergo amazing physical changes enabling this return to the ocean, as they go from shallow water, bottom dwellers to ocean travelers.
American Eel Migration Patterns
As stated before, the life cycle of an American eel begins and ends at the Sargasso Sea.
The eels migrate to freshwater locations along the Atlantic coast and at the end of their life, return to the Sargasso Sea to spawn and eventually die.
The migration pattern of the American Eel:
- The Sargasso Sea
- Drift with the Gulf Stream to reach the Atlantic Coast
- Move inland through tidal rivers
- Return to the Sargasso Sea
Mature eels take to the Sargasso Sea in the winter months to spawn.
According to FWS, this behavior perpetuates a single breeding population, which prevents the distinctions sometimes found in species that live in different geographic areas.
They leave the Sargasso Sea as larvae and drift along the Gulf Stream to the Atlantic Coast.
They migrate to brackish waters and many move inland through tidal rivers to settle down in streams, lakes, ponds, and rivers, especially in places where they can hide under rocks during the day.
The American eel has a long lifespan, and after up to 40 years of living in these freshwater environments, the eel goes through an immense physical change in order to travel back to the Sargasso Sea.
As silver eels preparing to travel back through the ocean, their eyes double in size and change in sensitivity toward blue which enhances the eels’ vision in deep water.
Their blood vessels feeding their swim bladders increase in number and this allows for increased gas deposition and reduced loss of gas.
They even stop eating during this migration until they wind up back at the Sargasso Sea where it all began for them.
Catadromous vs Anadromous
Between freshwater and ocean water, catadromous and anadromous are the two types of migration patterns that fish take.
- Born in the ocean
- Mature in freshwater
- Return to the ocean to spawn
- Born in freshwater
- Mature in ocean water
- Return to freshwater to spawn
American eels are usually considered catadromous fish.
Fish such as salmon and trout are also considered anadromous fish.
Although according to FWS, some American eels swim up freshwater streams to mature and others remain and mature in both estuarine and marine water.
This discovery resulted in American eels being described as catadromous, meaning they can be found in freshwater or saltwater while they are maturing.
In conclusion, American eels follow a life cycle that is consistent across the entire species. Similar to a butterfly, they will go around in a circle and ultimately end up where they started.
Are eels able to move on land or outside of water?
Yes. American eels are able to travel over land by absorbing oxygen through their skin and gills. They will usually slither through wet grass or mud in order to keep some kind of moisture beneath their bodies. However, if you ever come across an eel on land, you wouldn’t be able to pick one up because their scales are covered in a mucous layer.
What other kinds of fish are considered anadromous?
Anadromous fish are born in freshwater, go through their development and maturation stages in ocean water, and then return to freshwater to spawn at the end of their lifetime. Along with salmon and trout, some examples of anadromous fish are smelt, striped bass, sturgeon, and American Shad.
Be sure to check out what I wrote about what American Eel eat.