Credits: Awake 1993 edition published by JW, correspondent in Ireland

EELS baffled Aristotle, the ancient Greek philosopher. However much he examined these smooth-skinned, snakelike fish, he found neither sex organs nor eggs. “The eel,” he said, “is neither male nor female, and can engender nothing.” He concluded: “Eels are derived from the so-called ‘earth’s guts’ that grow spontaneously in mud and in humid ground.”

Modern researchers have solved this particular mystery about the eel. Christopher Moriarty of the Department of the Marine in Ireland explains that while most fish show very distinct eggs, the eel shows no sign of even a small egg. “The ovary of the eel,” he says, “is inconspicuous—almost invisible in young specimens, and forming no more than a whitish, frilled ribbon in more mature ones.”

Since no spawning eel has ever been caught, even up to modern times, you can understand why Aristotle was puzzled. Not having a microscope, he had no way of finding out where eels came from.

While scientists have solved this particular mystery about the eel, they have uncovered others that are still puzzling. Take a look, for example, at the life cycle of the European freshwater eel, and see if it does not intrigue you.

The Mystery of Its Origin

Each spring, millions of tiny eels two or three inches long [5-8 cm]—called elvers—arrive near the shores of Western Europe and North Africa. Where do they come from? Until the 1920’s, no one knew.

However, near the end of the 19th century, a startling discovery was made that contributed to a solution of this mystery. It was observed that the eel, like the frog and the butterfly, starts life in a different form. Biologists discerned for the first time that a slim, transparent fish called leptocephalus, with a tiny head and a body shaped like a willow leaf, metamorphosed, or changed form, to become a tiny transparent larva called a glass eel.

Once the connection between the leptocephalus and the glass eel was made, it was possible to track the eel larvae back to their source. In 1922, Danish oceanographer Johannes Schmidt discovered that the spawning ground of all Atlantic eels is the Sargasso sea, a vast, weed-strewed area of ocean in the North Atlantic. Both the American and the European eels spawn there, and therein lies another mystery.

Take Different Routes

American and European eel larvae go their different ways somewhere near Bermuda. “How they know which way to go when neither has ever seen its ‘home’ is an unanswered question,” says The Fresh & Salt Water Fishes of the World. The book goes on: “For the American eels, the trip is about 1,000 miles [1,600 km]; the journey requires about a year. European eels travel 3,000 miles [5,000 km] or more, their trip taking nearly three years. Equally astonishing is the fact that the growth rates of the two eels [which are almost impossible to tell apart] differ so that each has developed to about the same size by the time they reach their destination.”

Some amazing instinct directs the two types of eels to go their separate ways. Of this mysterious event, the book Fishes of Lakes, Rivers & Oceans says: “How and why they manage this astonishing maneuver is as much a puzzle as their origins were in Aristotle’s time.”