Eels have a life cycle that long remained a scientific enigma, especially regarding their reproduction and early development. For centuries, their spawning locations and young were unknown, leading to much speculation. It was not until the early 20th century that the spawning grounds of Atlantic eels were identified, revealing a fascinating larval stage. This discovery unveiled a creature unlike its adult form, which undertakes a vast ocean journey before transforming into the familiar eel.
The Leptocephalus Stage
The larval stage of eels is scientifically known as “leptocephalus,” meaning “slim head,” which describes its distinct morphology. These larvae are flat, ribbon-like, and almost entirely transparent, making them nearly invisible in their oceanic environment. Their bodies are composed of a gelatinous substance with a thin layer of muscle, and they initially lack red blood cells. Leptocephali typically range from 5 to 10 centimeters, though some species can exceed 30 centimeters.
The internal organs of a leptocephalus are simple, with a basic tube for a gut. For a long time, the diet of these transparent larvae was a mystery because their guts often appeared empty. Research now indicates that leptocephali primarily feed on “marine snow,” which consists of various organic particles and microorganisms. This feeding strategy allows them to obtain nutrition from materials commonly found in the open ocean.
An Epic Migration
Eel larvae begin their lives in specific, remote oceanic regions, such as the Sargasso Sea in the western North Atlantic, where American and European eels originate. After hatching, these tiny leptocephali embark on a journey of thousands of miles, passively drifting. They are carried by powerful ocean currents, such as the Antilles Current and Florida Current, which transport American eel larvae towards the North American coast. European eel larvae utilize currents like the Subtropical Counter Current for their journey towards Europe.
This passive migration often spans many months or even years. For instance, European eel larvae can take approximately three years from hatching to metamorphosis. The precise mechanisms guiding these larvae to their continental destinations, despite their passive nature, remain an area of ongoing scientific inquiry. The long drift allows them to reach diverse coastal and freshwater habitats across continents.
Transformation into an Eel
Upon reaching the continental shelf, the leptocephalus undergoes a transformation known as metamorphosis. The flat, ribbon-like larva changes into a “glass eel,” losing its compressed shape and becoming cylindrical. While still transparent, its body composition shifts, and it shrinks in length during this process. This stage also marks the development of red blood cells and the loss of the fang-like larval teeth.
The glass eels then move into estuaries and, for many species, into freshwater rivers and streams. As they continue to grow and adapt to their new environments, they begin to develop pigmentation, transitioning into the “elver” stage. Elvers become darker and more muscular, preparing them for the next phase of their lives as juvenile eels in coastal or freshwater habitats.