Our Better Nature: Mysteries of the American Eel — Sex, Spawning, and the Sargasso Sea

Only a government committee could have made eel reproduction more complicated.

American Eel (Duane Raver, U.S. Fish and Wildlife Service, Flickr)

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A type of fish that has never been known to spawn and whose adults don’t seem to have sex organs no matter how old or large they get shouldn’t last very long as a group. “Survival of the fittest” means who can live long enough to make babies, so if a fish can’t procreate, it isn’t fit to survive. Seems like a bad way to run a species.

For over 2,000 years, scientists and sages were unable to figure out how eels reproduce. Even Sigmund Freud, a man obsessed with objects longer than they are wide, sliced open about 400 eels in 1876 as he vainly searched for a penis. An eel penis, that is.

Aristotle claimed eels spontaneously appeared from deep in the Earth. Pliny the Elder, revered as a scientist and respected as a Roman army commander, said aquatic dandruff that sloughed from the skin of mature eels turned into baby eels.

It wasn’t until 1896 that biologists found a couple of eels with the right complement of ovaries and testes. It turns out that when an eel becomes sexually mature, it soon leaves its customary haunts and heads downstream for an epic journey.

Native to the eastern half of the United States, the American eel (Anguilla rostrata) is catadromous, meaning it spawns at sea but lives most of its life in fresh water. Contrast this with salmon, which are anadromous, meaning they can write with either hand. Or it means they’re ocean-dwellers, but spawn in fresh water.

American eel ( Clinton & Charles Robertson via the Creative Commons Attribution 2.0 Generic license, Wikimedia Commons)

As a kid whose summers were spent in the Thousand Islands region of New York, I was fascinated by the stringers laden with eels that returning anglers pulled from their boats at the public docks. Given the abundance of eels of the 1970s, I was astonished when the American eel was declared “at very high risk of extinction in the wild” by the International Union for Conservation of Nature in 2014.

Biologists tracking this eel decline were aware of factors that probably contributed to the situation, like water pollution, hydro dams, agricultural runoff, and overfishing. But much less was known about conditions in their spawning grounds.

Mature eels make an arduous trip – thousands of miles, in some cases – from inland waterways to the Sargasso Sea. The Sargasso is an egg-shaped, eternally gyrating patch of plastic garbage in the northwest Atlantic, roughly 2,000 miles long east-west, and 680 miles wide north-south. Its “west coast” ends just off Bermuda. Although it wobbles a bit, it is corralled by a confluence of ocean currents which maintain its spin.

The Sargasso Sea (Shutterstock)

I should probably mention that in addition to trash, the Sargasso has seaweed. Actually, it’s mostly seaweed, named after the free-floating, brownish-yellowish, stringy plants in the genus Sargassum which form a tangled mat on its surface. In terms of biology, the Sargasso is to marine environments what rainforests are to ecosystems on land: productive and diverse, yet largely an enigma. And it is the go-to breeding ground for perhaps 800 eel species from every continent save Antarctica.

Sargassum in the Sargasso Sea (Wikimedia Commons)

The eels’ sex act itself takes place about 450 to 600 feet below the surface – no wonder we thought they never spawned. Eels die after this, presumably in a happy frame of mind. Having just one site on the planet in which to breed is peculiar. Especially since it is weed-choked, gyrating, and inconveniently located. Nature is usually elegant, but only a government committee could’ve made eel spawning more complicated.

A freshly hatched eel is gossamer and transparent; a willow leaf made of glass. Loitering beneath the tangle of Sargassum for several weeks, it feeds on “marine snow,” a sort of dandruff (maybe Pliny the Elder was onto something) that peppers down from the rich community above. After supping on this meager vichyssoise, the paper-thin newborn retraces the thousand-mile-plus journey home its parents recently made.

However, the fact an ethereal waif can complete such an odyssey isn’t the wild part of the story. It’s that a baby American eel somehow knows it belongs here. Not just generally here; it understands which stream, lake or river its “people” came from, and that’s where it goes. Millions of fragile babies representing some 800 eel species are mixed together in a giant spaghetti-plate of swirling seaweed, yet each heads for its respective place of origin. Now, that’s wild.

The glassy willow-leaf eels morph into the glass eel phase, where they look more like eels but you can still see their innards. As they grow larger and gain pigment, they become elvers. Then they beef-up and graduate to the yellow eel stage, where they stay for up to 25 years. Eventually, eel puberty hits. No body hair, but males and females develop testicles and ovaries, respectively. When their gonads are good and ripe, they get an urge to visit the Sargasso.

Baby eels in the “glass eel” phase (Shutterstock)

Starting 30 years ago, the American eel population began to plummet, dropping by about 50 percent. Reasons may include historic issues such as pollution and dams, as well as fishing pressure due to a 2011 tsunami that devastated Japanese eels and the closure of the European eel market in 2012. It looks like the population has stabilized as of the early 2000s, although their numbers are not coming back up. In spite of much research, marine biologists still don’t know what caused the population to decline and then mysteriously stabilize, or if such events could happen again.

We’re also in the dark as to what makes baby eels suddenly crave fresh water, and how they navigate. Though climate change will certainly impact the Sargasso, we can’t say what the likely effects will be without more studies. Funding is a major impediment to unraveling eels’ mysteries – to hire a research vessel to the Sargasso can cost $30,000 or more per day, which might explain why no one books a cruise there. Or maybe it’s because the only entertainment on the ship is watching eels make babies, and we already heard the ending to that story.

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