That raised a new question: Where did the shrimp come from?
The researchers found that the huge seamount interacts with tides and ocean currents to create a vortex in which water rises along the outside of the mountain and then descends into its interior.
The descending water is likely bringing shrimp to the eels, allowing them to thrive in such large numbers.
Another member of the team, Craig Young, a biologist with the Oregon Institute of Marine Biology in Charleston, is intrigued by how the eels have taken advantage of a quirk of oceanography.
"The eels have selected a place there where the currents are concentrating food," he said.
But the scientists are even more intrigued by a region called the Moat of Death.
Moat of Death
The moat lies between Vailulu'u's encircling crater and the rim of the cone inside it.
It's an extremely toxic environment, Staudigel said, where oxygen levels are dangerously low and volcanic vents fill the water with iron soot "almost like underwater smog."
The volcano is also spewing liquid carbon dioxide, which combines with seawater to make a deadly acidic mix.
And the same currents that bring shrimp to the eels also bring fish into the toxic moat, trapping them.
The result? "We find one fish carcass after another," Staudigel said.
But one species survives within the moat, a type of sea worm that seems to be feeding on the animal carcasses.
It's not clear how the worms manage to live in a region where nothing but bacteria can live.
Jim Barry, a deep-sea biologist with the Monterey Bay Aquarium Research Institute in Moss Landing, California, is impressed by the volcano find.
"This is a new habitat," he said.
And it's potentially important for future study, he added, because by trapping carbon dioxide in the moat, this habitat provides scientists with a way to study what the oceans of the not-so-distant future may be like.
As levels of carbon dioxide continue to rise, due to the combustion of fossil fuels, deforestation, and other causes, much of the gas will inevitably wind up in the sea, he explained.
"We're going to see big changes in the chemistry of the ocean in the next 200 years," Barry said.
"Systems like this will give us a great deal of information about what we might expect in the future."
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