For thousands of years, eels have been a mystery. People captured them by the thousands. They ate them. They dissected them. But they could not find their reproductive organs. Where were the ovaries? The testes? How did these snakelike creatures reproduce?
We do know all the American eels in the entire world swim back to one place to breed — the Sargasso Sea, located east of the Carolinas and north of the Caribbean. Each year thousands of American eels from rivers all over the eastern Atlantic—from Brazil, from Massachusetts, from Iceland—travel back to the Sargasso Sea. Once there, the female eels release millions of eggs that the males externally fertilize. Then the eels die. (Or at least that’s what scientists think happens because no one has actually seen an adult eel or its spawn in the Sargasso Sea.)
The Sargasso Sea is unusual in its own right. It is bounded by four shifting ocean currents that circulate clockwise to form the North Atlantic gyre, or vortice. It is the only sea without land boundaries or a consistent shape. The sea is named because of the brown sargassum seaweed that accumulates in the vortice. (Another, somewhat infamous marine vortice, dubbed the Great Pacific Garbage Patch, is known for the microplastics and sea debris that accumulates there.)
People developed many theories about where eels came from. Ancient Egyptians associated the eels with the sun god Atum, because the eels seemed to spring up out of nowhere when the sun warmed the Nile River. In about 350 BCE, the great philosopher Aristotle reasoned that eels, since they had no sex organs and therefore could not procreate, must “originate both in the sea and in rivers wherein putrid matter is abundant.” In the first century A.D., Roman naturalist Pliny the Elder thought that eels rub against rocks, and the scrapings came to life as new eels. The English thought that eels were born when hairs from horse tails fell into the water and transformed into eels.
In the 1870s in Trieste, Italy, Sigmund Freud dissected 400 eels, hoping to be the first person to identify the testes of an eel (someone had recently found a mature female eel with ovaries). Freud did not succeed. On his deathbed in 1874, the German biologist Max Schultze said that all the important scientific questions had been settled, “except the eel question.”
Eels change so much during their life and travel so far that it is not surprising people had trouble putting all the pieces together. Eels hatch from eggs into transparent leaflike larvae that drift on currents. These larvae metamorphose into tiny transparent ribbonlike creatures called glass eels. Then they change into dark elvers and later into yellow eels. During their final life stage, they metamorphose once again to become sexually mature silver eels. Altogether eels have five life stages, but it is only in the last stage, which can take 25 years to reach, that eels become sexually mature. At this point they leave freshwater rivers and lakes and migrate back to the ocean, traveling thousands of miles to the Sargasso Sea.
In search of eel breeding grounds
In 1886 the French zoologist Yves Delage kept eel larva alive in a laboratory tank, where he observed the larvae metamorphosing into glass eels. In the 1890s, scientists observed captive eels that metamorphosed into adult sexually mature silver eels (and they had sex organs!). Eels undergo metamorphosis three times: from egg to leaflike larvae, from larvae to glass eels, and from yellow eels to sexually mature silver eels. The changes that occur at other life stages are just normal developmental changes.
In early 1900s, a Danish zoologist, Johannes Schmidt, was determined to find out where eels originated. Schmidt trawled the ocean to find eel larvae, which he believed would tell him where eels were born. He discovered that the largest larvae lived in coastal waters. As he moved west from Europe into the Atlantic, he found smaller larvae. He continued this meticulous process until, 19 years after starting the project, he found larvae so small that they must have been hatched from eggs. However, Schmidt never found spawning eels or eel eggs. He proposed that eels migrated to the Sargasso Sea to breed.
It wasn’t until 2018 that scientists confirmed Schmidt’s idea. They tagged 26 mature female eels off the coast of the Azores. A year later, five tagged eels arrived in the Sargasso Sea. The eels had traveled thousands of miles across the Atlantic Ocean. Still scientists have not seen mating eels or eel eggs in the Sargasso Sea.
The American eel (Anguilla rostrata) and the European eel (Anguilla anguilla) are members of the same family, and both travel to the Sargasso Sea to reproduce. For many years, scientists thought that American eels and European eels were the same species, and that ocean currents scattered them either to the Americas or to Europe. Genetic studies, however, indicate that the two groups are separate species. What’s even more puzzling is that the two species sometimes interbreed. These hybrid eels do not go to North America or to Europe. Instead, they go to Iceland! The hybrid eels all have a European mother and an American father.
Eels are so interesting that I will have much more to share about them next week, when I discuss American eels in our Massachusetts rivers.
A stronger
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