The eel belongs to the order Anguilliformes, an elongated, snake-like fish group containing over 800 species. Determining if an eel is saltwater or freshwater is complex because their habitats vary significantly between species. While the majority of true eels are strictly marine, famous species like the American and European eels undertake vast migrations between both environments. These migratory species spend time in freshwater rivers and streams but are born and reproduce in the ocean, creating a life cycle that defies simple classification.
The Catadromous Life Cycle
The American eel (Anguilla rostrata) and European eel (Anguilla anguilla) are known for their unique life history, involving living primarily in freshwater but migrating to the sea to spawn, a pattern known as catadromy. This journey begins in the Atlantic Ocean’s Sargasso Sea, where the adults reproduce and die. The newly hatched young are transparent, leaf-like larvae called leptocephali, which drift on ocean currents toward continental coastlines. This oceanic drift can take up to two years as the larvae travel thousands of miles before transforming into the next stage. As they approach brackish water near estuaries, they metamorphose into transparent, cylindrical “glass eels.”
Upon entering freshwater systems, the glass eels develop pigmentation and become “elvers,” small, pigmented young eels that actively swim upstream into rivers and lakes. The longest phase of their life is spent in freshwater, where they are known as “yellow eels” due to their coloration. This growth period can last anywhere from five to over twenty years. When they reach sexual maturity, they undergo a final metamorphosis into “silver eels,” developing larger eyes and a silver-white belly, while their digestive tracts shut down. These silver eels then undertake the non-feeding migration back to the deep spawning grounds in the Sargasso Sea to complete the cycle.
Eels That Remain in the Ocean
While the catadromous species are the most widely known, they represent only a small fraction of the total eel population worldwide. The vast majority of the 800-plus species are strictly marine, spending their entire lives in saltwater environments. These eels are not part of the genus Anguilla and do not engage in the freshwater-to-ocean spawning migration.
Two prominent examples of strictly marine eels are the Moray eels (family Muraenidae) and Conger eels (family Congridae). Moray eels, with over 200 species, are commonly found in coral reefs and rocky crevices in tropical and subtropical waters, where they are apex predators. Conger eels, which include the heaviest true eel, the European conger, typically inhabit deeper waters on continental shelves and slopes. These marine species complete their entire life cycle within the ocean, often burrowing into sand or hiding among rocks.
Physiological Adaptations for Transitioning Between Waters
The ability of catadromous eels to move between freshwater and saltwater is enabled by osmoregulation, which manages the balance of water and salt within their bodies. Fish living in saltwater constantly lose water to the environment because their body fluids are less salty than the surrounding ocean. To counteract this, they drink seawater and use specialized cells in their gills to actively excrete excess salt ions.
When the eel enters freshwater, the osmotic challenge reverses; their body is now saltier than the water, causing them to constantly gain water and lose salt. They stop drinking water, and their kidneys begin producing a large volume of dilute urine to eliminate the excess water influx. The cells in the gills also reverse their function, switching from salt excretion to active salt uptake from the highly dilute freshwater. This transformation is not instantaneous and is triggered by environmental changes and hormones as the eel moves through brackish estuaries.