Eels are an unusual group of fish known for their ability to live in both freshwater and saltwater environments, a life history trait called catadromy. For millennia, the process by which these serpentine creatures reproduced remained one of the greatest biological riddles. The mystery stemmed from the fact that no sexually mature eels were ever found in the rivers and lakes where they spent most of their lives.
The Centuries-Old Question
The ancient Greek philosopher Aristotle, in the fourth century BCE, believed eels simply sprang from the mud, a theory of spontaneous generation accepted for over a thousand years. This idea took root because dissecting an eel caught in a river or pond revealed no signs of reproductive organs. This led to the assumption they reproduced asexually or emerged from the earth. By the 18th century, the Italian physician Carlo Mondini discovered ovaries in a female eel, proving they were fish that reproduced sexually.
The question of the male lingered. In 1876, Sigmund Freud spent weeks dissecting hundreds of eels in a fruitless search for the male testes. Freud’s failure was later understood: eels only develop their gonads when they are about to embark on their final, long-distance migration. The vast distance and timing of this journey meant that no one could observe the eels in their final, sexually mature state.
Unlocking the Secret: The Spawning Grounds
The definitive breakthrough came from the systematic work of Danish marine biologist Johannes Schmidt, who dedicated nearly two decades to solving the riddle in the early 20th century. Schmidt’s method involved meticulously trawling the North Atlantic for the smallest forms of eel larvae, known as leptocephali. He reasoned that the smallest larvae would be closest to the place where they had hatched, thus pinpointing the spawning location.
Schmidt’s expeditions, beginning in 1904, spanned from Europe westward across the Atlantic Ocean. He observed that the larvae became progressively smaller the further west he traveled, eventually collecting the smallest specimens (7 to 10 millimeters long) in the Sargasso Sea near the Bahamas. This systematic mapping, published in 1922, established the Sargasso Sea as the communal spawning ground for both the European eel (Anguilla anguilla) and the American eel (Anguilla rostrata).
The identification of this remote oceanic region solved the location puzzle, even though Schmidt never witnessed the actual act of spawning or found a mature adult eel there. His findings were based entirely on the size distribution of the eel’s distinct larval form. A century later, satellite tagging technology provided direct evidence, with researchers tracking migrating European eels to the Sargasso Sea, confirming Schmidt’s deduction.
The Incredible Journey and Life Cycle
The life of an eel is a multi-stage, transoceanic odyssey that begins when the adults spawn in the deep, warm waters of the Sargasso Sea. Following fertilization, the eggs hatch into the first larval stage, the leptocephalus, a transparent, flattened, and leaf-shaped organism. These larvae drift passively for up to two years, using ocean currents like the Gulf Stream to carry them across the Atlantic toward North America and Europe.
As the leptocephali approach the coasts, they undergo metamorphosis, transforming into the glass eel stage. This form is still transparent but is cylindrical, allowing them to swim actively rather than just drift. Glass eels then enter coastal estuaries and rivers, where they gain pigmentation and become known as elvers, beginning their upstream migration into freshwater habitats.
Once established, elvers mature into the yellow eel stage, characterized by a yellowish belly and a long residency in freshwater lasting five to over twenty years. When yellow eels reach sexual maturity, they undergo a final transformation into silver eels, developing larger eyes, a silvery-white belly, and thickened skin for the deep-sea journey. Silver eels cease feeding, their digestive tract shuts down, and they begin their migration back to the Sargasso Sea to spawn and complete the cycle, after which they die.
Remaining Mysteries and Conservation Concerns
While the general location of the spawning grounds and the full life cycle sequence are understood, many details about the reproductive process remain elusive. Scientists have never observed eels mating in the wild, so the exact mechanism of spawning and the behavior of adults in the deep ocean are still unknown. It is also unclear how migrating silver eels navigate the vast distances of the open ocean to reach the precise spawning location.
These knowledge gaps complicate efforts to protect the species, as many eel populations are classified as critically endangered by the International Union for Conservation of Nature (IUCN). The complex life cycle means eels face numerous threats, including habitat fragmentation from dams and weirs that block upstream migration. Overfishing and the illegal trade of glass eels, which are caught in coastal waters for aquaculture, also deplete the wild population, making conservation an urgent global concern.