Illustration of Eel metamorphosis

1922 Eel metamorphosis

The book of science

Tom Sharp

Johannes Schmidt ichthyology Illustration of Eel metamorphosis

Eel metamorphosis

It took a long while for people to realize that the small transparent leaf-shaped saltwater fish, Leptocephalus brevirostris, was the larval stage of the long and bony freshwater eel. From 1904 to 1922, Johannes Schmidt conducted a series of oceanic research voyages to find where eels come from. Assuming that the leptocephali and the eel had common ancestors and finding smaller and smaller leptocephali the further he sailed from European shores, Schmidt finally found, south of Bermuda, the smallest ever and concluded that eels breed in the Sargasso Sea even though no one had caught an adult eel there or in any other salt water, and it had not been shown that leptocephali metamorphosed into young eels after they reach inland waters.

Sexual maturity

Young eels, elvers, are not sexually differentiated. These slowly migrate from coastal waters up rivers and creeks. They grow into sexually immature adults called yellow eels. In 1876 the young Sigmund Freud dissected hundreds of those oh so phallic eels in a frustrated search for male sex organs. Yellow eels metamorphose into silver eels. These lose their digestive tracts, have larger pectoral fins and bigger eyes, and become sexually mature by the time they have migrated back to the Sargasso Sea to spawn.


Who funded Johannes Schmidt’s research trips? —The Carlsberg Foundation. Who was married to the daughter of the director of the Old Carlsberg Brewery in Copenhagen? —Johannes Schmidt.


During its metamorphosis from fish to elver, the glass eel is unable to eat. Humans don’t metamorphose after the womb, though analogs are easy. A teenager unable to think rationally and a woman in menopause are subject to hormonal makeovers.

The metamorphoses of the eel are driven by hormones. The gonadotropin hormone is important during metamorphosis into a silver eel. The hormone prolactin helps the eel adapt to fresh water during its metamorphosis from glass eel to elver, and the hormone cortisol helps the silver eel adapt to salt water.

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