Phenomenal Migration and Transformation of American Eels

Spring in the Northeast is a time of change: longer sunnier days, colorful ephemerals bursting into bloom, birdsong filling the air. It is also the time of year when freshwater creeks and streams all along the Philadelphia region are swarming with native, migratory eels — and it’s just one stage of their wildly complex life story. 

The small dark American eels (Anguilla rostrata) that are currently making their way through our local freshwater rivers and streams actually had their start out in the Atlantic Ocean’s deep saltwater. Over a year ago, these eels hatch from their eggs in large breeding grounds located somewhere in the warmer climes of the Sargasso Sea, southeast of the United States.  

After hatching, they begin to undergo the first of many radical physical transformations. In their early days, they float around on the ocean currents as a flat, transparent and rather small larval form, also known as leptocephali. Their leaflike shape helps the eels catch more flow from the currents, as they are transported by the Gulf Stream toward their destination: the Atlantic coast. As they are tossed about the waves, the leptocephali spend their time feeding on organic ocean detritus. 

In time they grow, and they begin their next intermediate phase of life with a transformation into glass eels. While still transparent, they now bear a more eel-like shape, armed with gills, distinctive eyes and noticeable skeletal structures. As glass eels, they will start their famous migration up the coast, seeking freshwater habitats.  

After drifting across the Atlantic Ocean and reaching the coast, the eels then head up stream and begin yet another transformation. About a year old now and considered juveniles, or elvers, these eels gain darker pigmentation, strength and size as they feed and grow over the course of another year or so. These elvers will be seen flowing up the waters of our rivers and streams throughout spring as they search for a place to live all around the Northeast.  

Time passes and suddenly they make yet another transformation, with their color changing into a more golden hue. Having gained knowledge and strength, these yellow eels can be seen climbing over wet grasses or burrowing through sandbanks to test their boundaries and seek out other places to live throughout the country’s waterways. This stage is perhaps the longest of their cycle, as they will remain in this adult form, steadily growing and predating on small aquatic invertebrates and insects, for over the course of a decade. 

Then, mother nature calls, or their biological alarm clock goes off, and the eels begin their long migration back to the ocean to spawn. As they reach the ocean’s coast once more — older, larger and heading in a different direction — they begin their final transformation.

Much like some salmon, their bodies mutate in strange ways, often to help camouflage them from predators in the wide-open ocean waters. Their coloration changes into a shiny silvery dappling, their eyes enlarge to optimize for dark, saltwater observation and their organs disintegrate so they no longer feed. Now known as silver eels, they begin their mysterious journey through the open ocean to their place of origin, which may take an additional year of nonstop traveling.  

Once they reach their birthplace, the destined spawning grounds, and mate, the eels will die. Their eggs hatch, with instinctive knowledge of freshwater locations, and this phenomenal story begins again.  

Academy scientists have been studying eels for over 15 years to better understand how human infrastructure, such as dams, impact their complex developmental life cycles and long migration journeys.  

“Eels are vitally important to our local ecosystems,” says Daniel Morrill, staff scientist in the Fisheries Section of the Academy. “Not only are eels a common food source for other fishes, birds and mammals, but also a fascinating and energetic link between marine and freshwater environments.”