Mysterious Eel Sex in the Bermuda Triangle

Aesthetics aside, freshwater eels represent one of the few great biological mysteries left on the planet. Though these strange fishes have been around for more than 40 million years and contributed significantly to human history, culture, cuisine, and mythology, key aspects of their life cycle and reproductive behavior remain unknown.

The first famous thinker to grapple with eel sex was Aristotle. According to the fantastic book, “The Truth about Animals,” by Lucy Cooke, “Aristotle was obsessed with eel genitals.”(Full disclosure, the idea for this article and much of its contents are based on a chapter in that book. It’s an exceptional read, and I highly recommend it.) In the fourth century BC, Aristotle sliced open countless eels in his laboratory on the island of Lesbos, searching for evidence of genitalia. He found nothing resembling gonads in any of the eels he dissected, which prompted his problematic theory of spontaneous generation. Since Aristotle couldn’t find their sex parts, he concluded (wrongly) that baby eels emerged as fully formed wriggling beings from wet mud, no sticky parental entanglement required.

Aristotle was far from the last person to invent an elaborate story to explain the genesis of infant eels. Try as they might, early naturalists could never figure out how or where eels reproduced, or even find any trace of sex organs. In the absence of real explanations, they came up with some very creative hypotheses.

Some examples include: Eels emerge from the gills of other fishes, from the fresh morning dew during certain months, from electrical disturbances, from the thatching of roofs after a rainstorm. They reproduce by rubbing themselves against rocks and that “the scrapings come to life.” One medieval theory suggested that eels were the transmogrified forms of the bastard children of priests and married women, fleeing persecution through shapeshifting. Another claimed that baby eels were birthed by small beetles, while another proffered that they fell from the tails of horses into water where they then came to life.

Post-enlightenment eel science, like all other science, began to apply empirical data to the age-old eel conundrum. French researchers in the 1930s gathered thousands of young eels (called elvers) each about 3 inches long, and kept them in a tank together to finally unlock the secrets of their origins. Sadly, that didn’t happen. The researchers did, however, find out just how impressive freshwater eels are at consuming, well, everything. Despite daily feedings, only 71 eels remained after one year due to rampant cannibalism. Three months after that, only one eel remained, a female who was still alive four years later when the Nazis invaded. We have no record of what happened to her post occupation.

In addition to being the objects of ichthyological fetish, eels are also a famously favorite food fish. At least, they are outside of North America. Roman author Marcus Gavius Apicius wrote in the first century AD that 6,000 eels were served at feasts celebrating a victory by Julius Caesar. Da Vinci painted eels onto plates at his depiction of the Last Supper, though this was more a representation of his personal tastes than any attempt at historical accuracy. While some of Da Vinci’s archived shopping lists included eels, I found no evidence that Jesus would have eaten them around modern-day Israel. In Japan, and elsewhere, they’re a sushi staple. An estimated 100,000 tons of eel are barbecued into unagi every year.

Humanity’s obsession with eels, both as regional cuisine and sexual freakshow, continues, but even modern science doesn’t have these critters fully figured out. Here’s what we do know. Or, at least, think we know.

Common European freshwater eels, Anguilla anguilla, and American freshwater eels Anguilla rostrate, start their life as an egg no bigger than a grain of rice. These tiny eggs suspend in the depths of an underwater forest in the Sargasso Sea—the deepest, saltiest slice of the Atlantic Ocean, located smack in the middle of the Bermuda Triangle. Over the next seven months to three years, they undergo two complete metamorphoses and travel nearly 4,000 miles to the rivers of North America and Europe, where they wriggle upstream and bury themselves in substrate, eating absolutely everything they can gather. Anywhere from six to 30 years later, individuals make the return trip back to the Sargasso, undergoing yet another metamorphosis and developing sexual organs somewhere along the way. Some theories hold that groups of randy eels intertwine in giant, wriggling balls of mass copulation in the darkest depths of the ocean, but as disturbingly catchy as that image may be, we don’t actually know.

Researchers have spent billions of dollars trying to understand how these critters perpetuate themselves. And yet, no eel has ever been tracked all the way to its mating grounds and no wild fertilized eggs have ever been found, despite the use of sonar, satellite tags, and traps full of females artificially juiced with extra hormones to attract migrating males.

So why all the effort and expense? Why has humanity spent so many generations trying to figure out the mysteries of eel sex, and why are we still working so hard on it today?

In her book, Cooke describes a “global eel crisis.” Populations are plummeting, in some places by as much as 99%, due to overfishing, pollution and, of course, dams. European and Japanese eels are listed as critically endangered and American eels have twice come up for consideration under the Endangered Species Act. Global supplies are shrinking while demand remains high.

Americans’ historic disinterest in our eels has given way to exploitation. People have long caught immature eels migrating up into the rivers and sold them as bait, often for stripers. Today, however,  fishing for elvers and glass eels (the stage of metamorphosis before elver) has become a $40 million industry in the U.S. These baby eels are sold to eel farms, which grow them to maturity and then sell them around the world.

This mostly unregulated market has many of the hallmarks of every other kind of “boom.” Be it gold, oil, or eels, whenever a natural resource is readily available, highly valuable, and lightly managed, unscrupulous characters swoop in to snatch the profits. Cooke reports this has led to, “shady eel dealers handing millions in cash over to middlemen in motel car parks and fishermen brandishing AK-47s during armed standoffs over the best fishing spots.”

Humanity, in general, loves a good mystery, and these creatures represent one of the few left on this planet, but there’s a more pressing reason for our eel voyeurism. Better understanding these fish might help us prevent their extinction. We don’t know how many young an individual eel can produce, or what percentage of them survive that colossal migration, so we don’t really know how close they might be to collapse.

If we better understood eel sex we might be able to effectively manage our stocks, or breed them in captivity to take some pressure off wild fisheries. If we don’t figure out this millennia-old mystery soon, we may never get the chance.