From the 4th century B.C., when Aristotle provided the first recorded observations on the life of the eel to the beginning of the 20th century, the location of the eels' spawning grounds and their movements at sea was a mystery. Mysteries still exist, but in 1922 the Danish oceanographer Johannes Schmidt reported the discovery of the eels' spawning grounds near the Sargasso Sea in the southwestern part of the North Atlantic Ocean. The migration routes of both adult and larval eels have been learned and it has been shown that two species of eels inhabit the North Atlantic region. The American eel, Anguilla rosirata, differs only slightly from the European eel, Anguilla anguilla, having fewer vertebrae, a larger size and a shorter larval stage. The American eel is elongate and serpentine, with a single continuous dorsal fin extending posteriorly from a point about one third of the body length behind the head and around to the vent.
Eels can also alter their coloration in response to changes in illumination and background.
The skin is thick and tough and may secrete copious amounts of slimy mucous which acts as a protective cover. Unlike the well developed scales of most other fishes, eel scales are rudimentary and embedded deeply within the skin. The scales develop only after the eel has spent about three years in freshwater. American eels may occasionally be confused with the lamprey, sometimes called the lamprey eel. The lamprey, however, differs notably from the eel in having a prominent sucking disc filled with large, hooked teeth, several gill openings and no pectoral fins.