One glance at the creatures was unlikely to assuage nervous onlookers. The Eastern hellbender, the largest salamander in the Western Hemisphere, looks as though someone yanked out a giant's esophagus, gave it legs and taught it to swim. The two-foot-long amphibian has slime-covered skin, beady eyes and a paddle-like tail. Its ruffled torso resembles the edge of a lasagna noodle, inspiring one of the creature's many colorful nicknames, “old lasagna sides.” Other monikers are equally undignified: “snot otter,” “mud devil,” “grampus.”

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“They're pretty odd-looking creatures,” Boyer acknowledged. “Nocturnal and aquatic and secretive and strange. … Otherwordly.”

But they're also threatened. Which is why scientists at the Bronx Zoo have been working to raise the giant salamanders in captivity and then reintroduce healthy adults into the wild.

Hellbenders and their relatives have been around far longer than humans. They are “living fossils” — some of the last surviving members of an ancient lineage. The ancestors of Hellbenders and their cousins, the giant salamanders of China and Japan, evolved when dinosaurs still roamed Earth. The lineage has persisted almost unchanged for some 65 million years.

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Hellbenders' name is likely inspired by their bizarre appearance, herpetologists Tom R. Johnson and Jeff Briggler wrote in a pamphlet for the Conservation Commission of Missouri. Early American settlers thought the salamander looked like “a creature from hell where it's bent on returning” or a reminder of the “horrible tortures of the infernal regions.” In reality, though giant and undeniably freaky, hellbenders are harmless.

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Besides, the casual observer is unlikely to spot them. Hellbenders are most active at night, and their mottled brown coloring means they are easily missed or mistaken for river-bottom detritus. You could spend your whole life in the hellbender's native habitat (which follows the Appalachian mountain range from northern Georgia to southern New York) and never realize you share a home with an animal that looks like a monster of legend.

Yet hellbender numbers have dwindled in recent years. The International Union for Conservation of Nature deems them “near threatened.” In New York, hellbenders are found in only two watersheds and are considered “a species of special concern” by the state. They breathe through their mucus-covered skin, so they require clean, well-oxygenated and fast-moving water — an increasingly scarce resource in U.S. waterways. Young animals are especially vulnerable; Boyer said he has seen adults swimming around and laying eggs but rarely spots any newborns or juveniles.

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So in 2010, the Bronx Zoo teamed up with the Buffalo Zoo, the U.S. Fish and Wildlife Service and several local universities to collect hellbender eggs from New York streams and raise them in the relative safety of the zoo's “biosecure” facility. When the animals reached adulthood, the zoo would rerelease them into their native waterways.

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“The idea is they would be less vulnerable to predation and better able to utilize the habitat,” Boyer said. “If we could put back hellbenders that are larger, they may have a better chance for survival.”

But hatching a clutch of hellbender eggs is no simple task. In the wild, male hellbenders collect eggs by “luring” females into their underwater dens and persuading them to lay eggs, which they then fertilize. The males then care for the eggs as they incubate, flushing them with fresh water and moving them around to ensure that they develop properly.

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With no adult hellbenders available, zoo herpetologists had to take on these tasks themselves. Once the eggs hatched, they fed them first with tiny black worms, then upgraded to shrimp and krill. By 2013, the hellbenders were ready to return home. Thirty-eight of the creatures were driven up to the Allegheny watershed in a pair of picnic coolers, then plopped back into the stream from which they came.

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The zoo is now raising its second batch of hellbenders — 103 animals taken from the Susquehanna watershed near Binghamton. It also just opened a hellbender exhibit, where several of the wrinkly creatures can be seen swimming around.

“This is not like, 'wow, we’ve saved hellbenders,' " Boyer said. There are several other institutions working to raise and reintroduce Eastern hellbenders and their subspecies, the Ozark hellbenders of Arkansas and Missouri. “This is focused on one watershed.”

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Scientists aren't even sure why so many young hellbenders are having trouble growing. Boyer noted that the task of protecting them is made more difficult by the many interconnected threats they face: pollution, dam construction, boat traffic, riverbank erosion caused by human activity. Fungal disease outbreaks also pose a threat.

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“A lot of us are really concerned about the decline,” he said.

Because hellbenders are so vulnerable to changes in their habitats, they are an important indicator species — the “canary in the coal mine” whose ill health presages broader and more devastating changes in the overall ecosystem. Weird and obscure they may be, Boyer said, but “these strange creatures are important parts of our living world.”

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Correction: An earlier version of this article misstated when hellbenders evolved. Though their ancestors arose in the time of the dinosaurs and the lineage has persisted for tens of millions of years, the Eastern hellbender species evolved during the Pleistocene.