Which eel species migrates from European rivers to the Sargasso Sea to spawn?

The European eel is the eel species that migrates from European rivers to the Sargasso Sea to spawn.

Its life cycle is unusually divided between freshwater and the open ocean. Young larvae hatch in the Sargasso Sea and drift toward Europe with Atlantic currents. Near the coast, they transform into transparent glass eels, enter estuaries, and may travel far upstream. They then grow for years as yellow eels in freshwater or brackish habitats.

When mature, they become silver eels, developing larger eyes, silvery sides, and physiological changes suited to ocean travel. They leave rivers and begin a journey of roughly 6,000–7,000 kilometres back across the Atlantic. They apparently spawn in the Sargasso Sea and die afterward, although the spawning event itself has been difficult to observe directly.

The Danish biologist Johannes Schmidt identified the Sargasso Sea as the likely spawning ground during expeditions in the early twentieth century, locating the smallest larvae there in 1921. The American eel also uses the Sargasso Sea, but it is a separate species with a primarily North American freshwater distribution.