Eels are long, serpentine fish belonging to the order Anguilliformes, a group with over 800 species. Many species are strictly marine and spend their entire lives in saltwater. However, the habitat story is complex because a distinct group of eels migrates between freshwater and the open sea. This migratory behavior is the reason for the common confusion about where eels truly belong.
Eels That Live Only in Saltwater
The ocean is home to the vast majority of eel species, which complete their entire life cycle in marine environments. Moray eels (family Muraenidae) are perhaps the most recognizable group of these ocean-dwelling fish. They are found across all tropical and subtropical seas, often inhabiting coral reefs and rocky shorelines where they hide in crevices and caves. Morays possess thick, scaleless skin and jaws lined with sharp teeth suited for seizing prey.
Another significant group is the Conger eels (family Congridae). Conger eels generally inhabit deeper waters compared to Moray eels, often living on the continental shelves and slopes. These species reproduce in the sea, and their offspring remain in the marine environment, establishing them as permanent ocean residents.
The Migratory Freshwater Eels
A distinct group of eels, including the American eel (Anguilla rostrata) and the European eel (Anguilla anguilla), exhibit catadromy. This means they spend the majority of their adult lives in freshwater rivers and streams but must migrate back to the ocean to reproduce. After hatching in the sea, the young eels travel toward coastal areas where they transform into transparent, cylindrical glass eels.
As they enter estuaries and move upstream, the eels develop pigmentation and are known as elvers. They spend many years, sometimes decades, living and growing in rivers, lakes, and streams as yellow eels. This yellow eel phase is the longest of their lives, during which they feed and mature. Upon reaching sexual maturity, the eels undergo a final transformation into silver eels, developing enlarged eyes and darker coloration, preparing for the long return migration to the oceanic spawning grounds.
The Mysterious Spawning Grounds
The final destination for migrating silver eels from North America and Europe is the Sargasso Sea, a vast region of the Atlantic Ocean bounded by powerful ocean currents rather than landmasses. Here, at depths estimated to be between 400 and 700 meters, the silver eels spawn and then die, completing their reproductive journey. The mechanism that guides them to this distant location remains a mystery of marine biology.
The eggs hatch into the first larval stage, known as the leptocephalus, a name meaning “thin-head.” These larvae are tiny, transparent, and laterally compressed, resembling a leaf-like shape. This specialized form allows the larvae to passively drift on major ocean currents, like the Gulf Stream, for thousands of kilometers. The larval journey can take a year for the American eel and up to three years for the European eel as they are transported back towards their respective continents. Upon approaching coastal waters, the leptocephali metamorphose into the glass eels, ready to begin the freshwater phase.