The century-old question
In the middle of the Atlantic Ocean lies a sea without a shore. The Sargasso Sea is unique, defined not by land but by four circulating ocean currents. It is the destination for every single European eel (Anguilla anguilla) and American eel (Anguilla rostrata) that is ready to reproduce. The mystery of where eels came from puzzled naturalists for millennia; the philosopher Aristotle speculated they emerged spontaneously from mud. It wasn't until the early 20th century that a Danish biologist, Johannes Schmidt, solved part of the puzzle.
Between 1920 and 1922, Schmidt led a series of expeditions on the research vessel Dana. By systematically trawling the Atlantic and mapping the size of eel larvae, he discovered that the smallest, most recently hatched specimens were always found within the Sargasso Sea. This confirmed the area as the spawning ground, but the adult eels themselves remained elusive. To this day, no one has ever found adult eels or their eggs in the Sargasso Sea, nor has anyone witnessed the act of spawning in the wild.
An incredible transformation
The eel life cycle is a complex cycle. The process begins when eggs hatch in the Sargasso Sea, releasing leaf-shaped, transparent larvae called leptocephali. These larvae are so transparent they were once mistaken for a separate species of "leaf-fish". They drift on the Gulf Stream and other currents for one to three years, traveling thousands of kilometers toward the coasts of North America and Europe.
As they near the continental shelf, the leptocephali metamorphose into "glass eels," which are cylindrical and still transparent. Moving into freshwater rivers and estuaries, they develop pigment and become elvers. The next stage, the "yellow eel," is the primary growth phase, where they can spend anywhere from 5 to 40 years maturing in freshwater.
The final transformation is for the return trip. The eel becomes a "silver eel." Its eyes enlarge, its skin thickens and turns a metallic silver, and its digestive system degenerates as it stops feeding entirely. Driven by instinct, it begins its 6,000 kilometer migration back to the Sargasso Sea to spawn once and then die.
The search continues with new tools
Modern science has deployed new technology to finally observe the eels' complete migration. Researchers have attached pop-up satellite archival tags (PSATs) to silver eels to track their route. The endeavor is difficult; the tags are large for the eels and can increase the energy cost of swimming by more than double. Many tagged eels are eaten by predators shortly after release.
Despite the challenges, a 2022 study successfully tracked several European eels all the way to the Sargasso Sea, providing the first direct evidence of their complete migration to the spawning grounds. The data confirmed that the journey can take over a year. While the exact location and conditions for spawning remain a secret, these tracking efforts are providing important data to help protect this critically endangered species, whose population has declined by over 90% since the 1980s.