Variations exist in coloration, from olive green, brown shading to greenish-yellow and light gray or white on the belly.

[3] The eel lives in fresh water and estuaries and only leaves these habitats to enter the Atlantic Ocean to make its spawning migration to the Sargasso Sea.

[9] The scales are not arranged in overlapping rows as they often are in other fish species but are rather irregular, in some places distributed like "parquet flooring".

The distribution of the American eel encompasses all accessible freshwater (streams and lakes), estuaries and coastal marine waters across a latitudinal range of 5 to 62 N.[12] Their natural range includes the western North Atlantic Ocean coastline from Venezuela to Greenland and including Iceland.

[13] Inland, this species extends into the Great Lakes and the Mississippi River[14] and its tributaries as far upstream as Minnesota and Wisconsin.

[15] Nonindigenous occurrences of this species in the United States were recorded from Lake Mead on the Colorado River and on the Arizona border.

[17] It was also stocked and unintentionally introduced in various states, including Illinois, Indiana,[18] Nebraska, Nevada,[16] North Carolina,[19] Ohio and Pennsylvania.

[8] They are found in a variety of habitats including streams, rivers, and muddy or silt-bottomed lakes during their freshwater stage, as well as oceanic waters, coastal bays and estuaries.

[6][13][14][22] Individuals during the continental stage occasionally migrate between fresh, salt and brackish water habitats and have varying degrees of residence time in each.

[23][24][25] During winter, eels burrow under the mud and enter a state of torpor (or complete inactivity) at temperatures below 5 °C (41 °F).

Karlsson et al. (1984) disagreed with this interpretation and found the final temperature preference of 17.4 ± 2.0 °C with a 95% confidence interval.

Seasonal patterns described by Fletcher and Anderson (1972) generalize annual movements from freshwater to estuaries and coastal bays to feed during spring, then either a return during the fall to overwinter (juvenile and immature adults), or a southward migration to the spawning grounds (silver eels Continental phase eels appear highly plastic in habitat use.

Eels are extremely mobile and may access habitats that appear unavailable to them, using small watercourses or moving through wet grasses.

Small eels (<100 mm total length) are able to climb and may succeed in passing over vertical barriers.

[6] The American eel's complex life history begins far offshore in the Sargasso Sea in a semelparous and panmictic reproduction.

After reaching these freshwater bodies, they feed and mature for approximately 10 to 25 years before migrating back to the Sargasso Sea in order to complete their life cycle.

Wang and Tzeng (2000) proposed, on the basis of otolith back-calculations, that hatching occurs from March to October and peaks in August.

However, Cieri and McCleave (2000) argued that these back-calculated spawning dates do not match collection evidence and may be explained by resorption.

Elver influx is linked to increased temperature and reduced flow early in the migration season, and to tidal cycle influence later on.

From life history traits of four rivers of Maine, Oliveira and McCleave (2000) evaluated that sexual differentiation was completed by 270 mm total length.

The silvering metamorphosis results in morphological and physiological modifications that prepare the animal to migrate back to the Sargasso Sea.

The American eel feeds on a variety of things such as worms, small fish, clams and other mollusks, crustaceans such as soft-shelled crabs and a lot of macroinvertebrate insects.

Based on laboratory experiments on European glass eels, Lecomte-Finiger (1983) reported that they were morphologically and physiologically unable to feed.

Feeding activity decreases or stops during the winter, and food intake ceases as eels physiologically prepare for the spawning migration.

[41] According to the International Union for Conservation of Nature, the American eel is at very high risk of extinction in the wild.

[42][1] Substantial decline in numbers and fishery landings of American eels over their range in eastern Canada and the US was noted, raising concerns over the status of this species.

Because of its complex life cycle, the species face a broad range of threats, some of which are specific to certain growth stage.

Being catadromous, the eels' reproductivity success depends heavily on free downstream passage for spawning migration.

Contaminations of heavy metals, dioxins, chlordane, and polychlorinated biphenyls as well as pollutants from nonpoint source can bioaccumulate within the fat tissues of the eels, causing dangerous toxicity and reduced productivity.

Other natural threats come from interspecific competition with exotic species like the flathead catfish (Pylodictis olivaris) and blue catfish (Ictalurus furcatus), pathogens and parasites, and changes in oceanographic conditions that can alter currents—this potentially changes larval transport and migration of juveniles back to freshwater streams.