The two-foot-long salamanders that live in Missouri go by a lot of different names.

Scientifically they’re known as Ozark hellbenders (Cryptobranchus alleganiensis bishopi).

More colloquially these massive amphibians have a few more colorful sobriquets, including “mud devil,” “snot otter,” “Allegheny alligator” and even “old lasagna sides.”

But if they could talk, some of the Ozark hellbenders living at Saint Louis Zoo might call each other by different names: Mom and Dad.

Saint Louis Zoo is the only institution in the world that’s breeding Ozark hellbenders, and they’re doing it well. Since 2011 their program’s parent hellbenders have laid more than 6,500 eggs that have resulted in the births of more than 5,100 tiny hellbender hatchlings.

That’s critically important, because the species isn’t doing well outside the zoo walls. Ozark hellbenders are admittedly hard to count in the wild—they’re nocturnal and live under big rocks in remote rivers—but the most recent estimates suggest that the adult population has fallen from about 27,000 just a few decades ago to around 600 today. That’s due to a combination of threats, including habitat loss, pollution, pesticides, disease, river sedimentation from construction and the illegal pet trade.

The zoo is helping to boost that number. Over the past several years many of the salamanders born in the zoo—along with more hatched from eggs collected in Missouri’s rivers—have been returned to their native habitats as juveniles. As of last October the zoo has released 5,792 juvenile Ozark hellbenders, along with an additional 319 eastern hellbenders (a separate subspecies).

Hellbender at two weeks of age. Credit: Mark Wanner St. Louis Zoo

Caring for these hellbenders is a heavenly responsibility. “To be part of a program that works with a relic species that’s been around for millions of years, and it’s right in our backyard here in Missouri, that’s just very special,” says Mark Wanner, the zoo’s manager of herpetology and aquatic species.

“I am still mind-blown every time I walk into this center,” adds Lauren Augustine, the zoo’s curator of herpetology. “They’re a really incredible species and a great way to connect us with the people in Missouri. Personally, I like how they eat like crocodiles.” Hellbenders, which have notoriously big mouths, eat mostly crayfish in the wild, but they’ve been known to chow down on just about anything that they can swallow whole.

The eating may be cool, but it’s the breeding that matters. Like many species brought into captivity, hellbenders require very specific conditions in their artificial habitats in order to live and thrive. At the zoo, adult hellbenders live in two outdoor artificial streams, each 40 feet long and 6 feet deep with plenty of rocks for hiding, plus another 32-foot, indoor stream housed in one of four climate-controlled rooms.

The indoor facilities are carefully adjusted to copy the ebb and flow of the hellbenders’ natural rivers. “We try to mimic the natural conditions as close as we can,” says Wanner. “We have chillers and boilers, and all year long the temperature is tweaked according to historical wild river temperatures.” They can also simulate rain events, adjust the water speed, and raise or lower the river at any given time.

But the real trick for successfully keeping hellbenders in captivity is the water itself—or more specifically, what’s in it.

“We think the golden ticket was paying closer attention to the total dissolved solids and ion concentrations in the water,” Wanner says. The addition of nitrates, nitrites, nitrogen ammonia and phosphates makes the water in the zoo’s streams much more like that of real rivers and that, along with the efforts to mimic other annual river conditions, has contributed to the breeding success. The number of fertilized and hatched eggs took off after the zoo came up with what Wanner calls “our recipe” in 2011.

It turned out the water used before that may have been a little too clean. “We’re still not 100 percent sure, but what we believed was happening was that the hellbender sperm was being damaged by the hard water,” Wanner says.

Hellbenders, it turns out, breed more like fish than other salamanders. The female lays a clutch of eggs and the male comes by and releases sperm (“or ‘milt,’ as we call it in the hellbender world,” he says). The sperm enters the water column on its way to the clutch, but if the water damaged the sperm it couldn’t penetrate the egg. Changing the composition of the water appears to have solved this problem.

Hellbenders developing inside eggs. Credit: Mark Wanner St. Louis Zoo

Another important tool in the program is a series of artificial nesting boxes, which the males use to solicit a nest site. After the eggs are fertilized, the males stick around and guard them. “If another male tries to enter the next box, the male guardian will sometimes flight to the death to protect the nest,” Wanner says. “Usually it doesn’t end up that way. Mostly it’s a good bite on the head or the front arm and the other male goes in the other direction.”

The water and the nest boxes work well with each other. “We think that’s the combination of things that allowed us to have this success,” he says.

Of course, the hellbenders don’t do it all on their own. “The incredibly dedicated staff that meticulously care for these animals is also in the recipe,” Augustine points out. “There’s an amazing attention to detail. They have very specific husbandry procedures and being able to track and keep data on these animals over the years has obviously been very beneficial to the success of the program.” The team includes three full-time hellbender keepers, one more part-timer, a seasonal keeper and “a plethora” of interns every semester. It has partners at the U.S. Fish and Wildlife Service and the Missouri Department of Conservation, not to mention the support of zoo veterinarians, nutritionists, volunteers and other aides.

“It does take an army to manage the program,” Wanner says.

Most recently that army has been witness to the program’s latest success: This past October a 7-year-old hellbender male—one of the first of his kind born at Saint Louis Zoo—became the proud papa of 39 little hatchlings. It was the first time a group of second-generation Ozark hellbenders had been produced by a first-generation, captive-born parent.

Wanner recounts the period leading up to the hatching. “It was a really rainy day, and Jeff Briggler, the state herpetologist—who is hands down probably the best hellbender biologist in the world—just happened to be here at the zoo checking eggs and we found a fertile clutch. It was what Jeff called a ‘yahoo’ moment. It was definitely a great day.”

Again, that’s important, because Ozark hellbenders face increasing pressures in the wild—climate change may be the next looming threat—and proving that captive-raised animals can go on to produce the next generation may be a key to the subspecies’ survival, both at the zoo and in the wild.

“I think that this program can safeguard us from any kind of tragedies,” says Augustine before she brings up a painful example. One of the hellbenders’ native river systems recently experienced a major flood, and surveys after the event did not turn up any remaining salamanders, meaning an entire genetic line could have been wiped out in the wild. “Luckily, we have animals from that river system in our collection,” she says. “That gives me the confidence that this program is going to save hellbenders.”

Meanwhile, the brood at Saint Louis Zoo is expected to keep growing. Hellbenders take six to eight years to reach sexual maturity, so more of the animals born at the zoo will soon start entering the dating pool. The younger females won’t produce many eggs at first—Augustine says that’s typical with many species of reptiles and amphibians—but the clutch size will increase as they get older, meaning the number of captive hellbenders could soon expand exponentially—as could the number of juveniles eventually eligible for release back into the wild.

Of course, the real proof of the program’s impact will come in a few years when we learn how the juveniles have done after release. Each animal is tagged so it can be identified as coming from the zoo if it’s later recaptured. Following those released hellbenders will reveal whether all the hard work has paid off.

“The first time we find a male that’s been raised here, released to the wild, guarding a nest with fertile eggs,” Wanner says. “That would be, I think, the culmination of success for us.”

This post first appeared on The Revelator on February 8, 2019.