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French farmers in Normandy say that eels will leave rivers on spring nights and find their way to vegetable patches to feed on peas. That's a fable, but eels are one of the only fishes that will emerge from the water to take offerings of food—canned mackerel or dog food—on river­banks. I've observed them doing this at sacred Maori eel-feeding sites in New Zealand. Under normal circumstances, an eel's diet is quite varied—everything from aquatic insects and fish to mussels and other eels.

Adaptability aside, the migrations millions of adult eels make from rivers across oceans must be among the greatest unseen journeys of any creature on the planet, spanning thousands of miles. Along the way they face a long list of dangers: hydroelectric dams, river diversions, pollution, disease, predation (by striped bass, beluga whales, and cormorants, among others), and increasingly, fishing by humans. Now, with climate change, another potential disaster looms: shifts in ocean currents that may confound eels during their migrations. Regrettably, although sublime in the eyes of some, the eel is not likely to be the poster child for a conservation movement anytime soon.

From Aristotle to Pliny the Elder, Izaak Walton to Carl Linnaeus, naturalists put forward various theories as to how eels came to be: that the young emerged from the mud, that eels multiplied by rubbing themselves against rocks, that they were born from a particular dew that falls in May and June, that they bear live young. One problem was that no one could identify sperm or eggs in eels. Over a 40-year period in the late 1700s, at the famous eel fishery at Comacchio, Italy, more than 152 million adult migratory eels were caught and cleaned, not one of which was found carrying eggs. No one could say for sure whether eels even had gender, because no one could identify their reproductive organs. (It turns out that the sex organs of eels become enlarged with eggs and sperm only after the adults leave the mouths of rivers for their oceanic spawning grounds and disappear from sight.)

In the late 1800s in Trieste, Italy, a medical student named Sigmund Freud was assigned to investigate the testes of the male eel, postulated to be loops of white matter festooning the body cavity. (Freud's paper on eels was his first published work.) This was confirmed in 1897, when a sexually mature male eel was caught in the Strait of Messina.

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