You’ve heard of the endemic salamanders that live in Barton Springs, but did you know that Austin is also home to nocturnal eels?

Swimmers who frequent the springs by moonlight sometimes catch a glimpse of sinuous yellow eels moving slowly through strands of algae and blades of aquatic grass on nightly excursions in search of food. These are American eels, Anguilla rostrata, which plumb the depths of Barton Springs and Lady Bird Lake, hiding in dense vegetation or rocky crags during the day to avoid detection.

The eels have a wide range that extends from the Gulf of Mexico and along the Atlantic Coast all the way up to Canada and even Greenland. This turns out to be especially impressive, because American eels all mate in the same place, regardless of where they live.

Every year, they swim out to sea in one of the greatest and least known migrations on Earth. They swim across currents and gyres, some traveling thousands of miles across the perilous waters of the open ocean, moving up and down the water column in daily cycles that scientists think helps them avoid predators. Some get fished from the water by anglers before they’ve even made it out to sea, and others get snapped up by roving sharks.

Those that survive find their journey’s end in the nutrient-rich waters of the great Sargasso Sea. It’s there that these eels, after having lived decades in streams and rivers across the American continents, come to spawn, senesce and die. Females can each lay several million buoyant eggs, which float to the surface, where males aggregate to fertilize them.

The Sargasso is thus both cradle and grave for the American eel, and although the adults will never again leave, a complex story of metamorphosis has just begun for their progeny. The eggs, incubating in the autumn sun, slowly begin to hatch. Were you to compare the baby eels to their parents, there would hardly be any resemblance at all. The creature newly hatched in the ocean looks more like a glass leaf than an eel.

These are called leptocephali, and they’ve just entered the first of five distinct phases of a long and intricate life cycle. The eel larva then make their first migration west

With see-through organs and fins to help them evade predators, the eels are carried by currents toward the continental shelves of North and South America. Most hitch a ride in the warm waters of the Gulf Stream, which carries them north. Along the way, large groups exit the main procession as they find inviting estuaries.

The first ones to leave head to the freshwater rivers of Florida, then Georgia and so on successively farther and farther north until they run out of rivers to infiltrate. But there are no freshwater bodies connecting Texas to the Atlantic, which must mean that some eels get around Florida to reach the Gulf, as well.

Scientists at the University of Texas are currently at work trying to figure out how these eels are able to navigate this particular stretch of water.

“The path to Texas and other Gulf Coast rivers is much more complex than catching the Gulf Stream from the Sargasso straight up the eastern seaboard,” said Dean Hendrickson, curator of ichthyology at UT. “If they’re at the mercy of the currents, it’s probably very few eels that that actually make it to the Gulf Coast.”

Hendrickson said evidence suggests juvenile eels can actually swim to some extent, but data regarding the migration of leptocephali is scarce.

“We’re totally in the dark as to how long it might take them to get here,” he said. “The currents in the Gulf are pretty well known to change from year to year, and it could be that the currents are bringing eels to the Texas coast only occasionally, maybe one out of every five years, or ... maybe even longer intervals.”

A short sojourn

Regardless of how long it takes them to arrive or how frequently they do so, they are getting here somehow: American eels live in every major Texas river.

By the time they’ve reached an estuary, where marine and freshwater meet, their bodies have elongated and their fins contracted. So although they’re still transparent, they’ve begun to assume more of the physical characteristics they’ll have as adults.

There the glass eels gather, waiting for a signal imperceptible to us that will beckon them upstream. When lunar phases and ocean tides match up in just the right way, they mobilize once more, beginning the second sprint of their migration.

“They start pigmenting quite quickly as they go up the rivers, and as they gain pigment, they become known as elvers, which is the next stage in their life cycle,” Hendrickson said.

Their numbers dwindle even further as individuals either decide to continue upstream or develop a preference for the more brackish waters of estuarine environments. Those that do continue onward face giant hurdles — dams.

“Most major dams block migrations of eels, so they lose that whole upper watershed part of their former distributions,” Hendrickson said, meaning more eels are trapped in a much smaller area than they’ve enjoyed historically, which might lead to increased competition for food and lower rates of survival.

Yet, there are documented accounts of eels in Barton Springs and Lady Bird Lake. All of them must have somehow found a way to breach the formidable Longhorn Dam.

In some of the New England states where American eels are more common, there’s photographic evidence showing that small mill dams don’t pose much of a problem to glass eels and elvers, Hendrickson said: “These tiny creatures just wiggle their way up through little crooks and crannies and cracks in old dams and get up past them.”

As to how they might be getting past Longhorn Dam, the jury’s still out. The eels typically perform this feat while in the glass eel or elver stage, and so far, eels in those stages have proven extraordinarily hard to spot in Texas.

“Nobody has ever seen a glass eel in Texas, or if they have, they certainly haven’t put it in a museum or told any scientist about it,” Hendrickson said. This is either because American eels are generally rare in Texas due to the difficulty in crossing the Gulf or because they’re being illegally hunted downstream.

Which brings us to the largest threat American eels face during their migration: human appetites. Fresh eel doesn’t fetch a particularly high price in the U.S., but it is in high demand in Asia, where “unagi” has been an integral part of Japanese cuisine for hundreds of years.

Unagi, made from adult eels, is often prepared as sushi, and it's touted to have a number of health benefits. But why do the Japanese rely on the American fish market for their eels?

A taste for endangered eels

The Japanese have had a fondness for eel at least since the 17th century, when the consumption of eel was purported to ward off heat during the summer months. Every year on the “midsummer day of the ox,” sales of unagi soar as Japanese ritually partake of the dish — or, at least, those who can afford it.

Already a pricey dish, the cost of unagi has risen dramatically within the past few decades. That’s because Japan has hunted its version of the American eel to the brink of extinction.

The Japanese eel, Anguilla japonica, spawns in the Pacific Ocean, just west of Guam, after which the larva migrate northwest toward China, Taiwan, Korea and Japan, as well as south toward the Philippines and Indonesia. But a burgeoning Japanese population means fewer and fewer eels to go around, and eel populations there have plummeted to the extent that in 2013, Japan added the species to its endangered list.

The simplest solution would have been to grow stock in captivity that could then be sold for food, but this turned out to be more difficult than expected. Since Japanese eels spawn at sea but mature in estuaries, they’re notoriously hard to breed in captivity. Instead, fishers catch wild glass eels at sea in droves, which are then grown in enclosed spaces until mature.

As their numbers crashed in Asia, Japan increasingly turned toward the closely related European eel, Anguilla anguilla, to sate its appetite with dispiritingly predictable results: Since 1985, there has been about a 90% decrease in European eels, which are now listed as critically endangered by the International Union for the Conservation of Nature.

This prompted the European Union to temporarily ban eel imports and exports in an attempt to stave off the decline, but this did nothing to abate demand. Rather than try to shift away from eel consumption, Japan again shifted its hungry gaze, this time to American shores.

This sudden surge in demand meant that fisheries in the U.S. could make as much as $2,000 for a pound of glass eels in 2012. If you knew what you were doing, you could rake in millions practically overnight.

A murky future

The International Union for the Conservation of Nature has recently listed the American eel as endangered, and while there isn’t much of a demand for eel in North America, the U.S. isn’t entirely free of blame. Canada declared the species endangered in 2008, which prompted fishing restrictions and other measures to ensure their continued survival.

But the U.S. Fish and Wildlife Service has so far declined to list American eels as endangered, which would negatively affect U.S. fishers and require costly mitigation efforts.

“People started to recognize the impact it would have on economics,” Hendrickson said. “If you list it as endangered, you have to manage for its continued persistence. And what that often entails is removing barriers.”

In the absence of federal oversight, several coastal states have taken the initiative by listing American eels as endangered on their own, he said: “In the New England states, there have been a number of dams taken out to provide access again to eels and other fishes to upstream parts of river basins where they’d been excluded.”

It’s also illegal to catch glass eels in every state except for South Carolina, Maine and Florida. But illegal eel fisheries appear to be thriving along the East Coast. In response to the high Japanese demand, shady deals were made in which fishers would illegally catch eels in certain states, then ship them to be mixed with legally caught eels elsewhere, after which they could be shipped overseas.

As to how eels might fare in the future, Hendrickson and other researchers remain unsure.

“We're never going to be able to do anything as far as managing them until we know how they're getting here and what dams they’re able to get around or not,” Hendrickson said.

Hendrickson and his colleagues are currently studying the natural history of American eels by using a combination of physiology and genetics in collaboration with and with funding from the Texas Parks and Wildlife Department. By using structures called otoliths, they can get a better idea of the eels’ migration routes.

Otoliths are small, circular deposits of calcium in the ear that help fish and other vertebrates sense gravity, allowing them to distinguish between up and down. In fish, a single layer of otolith is formed each year.

“They lay down annual marks,” said Hendrickson. “It's like reading tree rings. (They) can tell us about not only the ages of the specimens we've caught, but also the marine and freshwater history. The chemistry of those stones in their ears reflects the water they were in.”

Researchers also are studying slight variations in the eels’ DNA to determine how much genetic diversity is left in the group, information that might ultimately be useful in negotiating for better protection for one of North America’s true biological wonders.

This story has been corrected to note that the Texas Parks and Wildlife Department, not the U.S. Fish and Wildlife Service, is funding research by Hendrickson and his colleagues into the natural history of the eels.