North America’s biggest salamander is a reclusive crayfish-eater with a compressed body and some rather unflattering nicknames.

1. MOST OF THEIR LIVES ARE SPENT UNDER ROCKS.

Hellbenders have exacting real estate needs. Suited for a very specific habitat, they can only be found in clear, fast-moving streams with large, flat rocks at the bottom. An adult male hellbender will usually defend a territory of about 1000 square feet that is centered around its favorite rock—under which the animal sleeps.

2. HELLBENDERS ARE CLOSELY AKIN TO ASIA’S GIANT SALAMANDERS.

There’s just one recognized species of hellbender, which scientists have dubbed Cryptobranchus alleganiensis. Wild ones may be encountered as far north as New York State, as far south as Alabama, and as far west as Missouri.

The hellbender grows up to 29 inches long, making it earth’s third-largest salamander. Number one is the appropriately-named Chinese giant salamander (Andrias davidianus). Exceeding even some humans in size, this Asian monster can reach 5.9 feet in length and weigh 110 pounds. Right across the Sea of Japan lives the Japanese giant salamander (Andrias japonicus), which grows to be about 5 feet long and maxes out at around 55 pounds.

Together, these three juggernauts form the Cryptobranchidae family. Fossils reveal that the group once invaded Europe and western North America. The hellbender’s ancestors most likely evolved in Asia before migrating to the U.S. via land bridge.

3. THE SPECIES GOES BY MANY ALIASES.

“Hellbender” is an intense name for such a harmless amphibian. How did this word come about? Nobody knows. Perhaps—as herpetologist C.M. Bogert once wrote—early settlers thought that the animal looked like “a creature from hell where [it was] bent on returning.” Or maybe its wrinkled skin reminded someone of the tortures said to take place in Satan’s domain. Both theories seem plausible.

Hellbenders have gone by other nicknames as well, including devil dogs, mud-devils, lasagna lizards, and Allegheny alligators. Yet another nickname refers to their texture: Grasping a hellbender is quite difficult because of the slimy mucus that coats its skin, so they're sometimes known as snot otters.

4. HELLBENDERS MAINLY HUNT CRAYFISH.

Though they also feed on insects, earthworms, and small fish, crayfish represent 90 percent of a hellbender’s natural diet. Upon gulping one down, the salamander uses sharp, tiny teeth to pierce its shell. (These chompers can also break human skin.)

5. CONTRARY TO POPULAR BELIEF, THEY DON’T AFFECT GAME FISH.

USFWSmidwest, Flickr // CC BY 2.0

There are some unfortunate hellbender myths floating around out there. For instance, fishermen have long accused them of driving away bass and other game fish by eating their eggs—or even the fish themselves. However, scientists have yet to find any such foodstuff inside of a hellbender’s stomach [PDF].

Another common misconception is that the salamanders have venomous fangs. People who believe this often kill them on sight—even though this accusation has no merit. In fact, no one has yet discovered any amphibian with such a bite.

6. HELLBENDERS CAN DO WITHOUT THEIR LUNGS.

Like many amphibians, these stream-dwellers primarily breathe through their skin, extracting oxygen from water. This process is made easier thanks to folds running along their sides, which increase the skin’s surface area.

Hellbenders do have lungs, but they definitely aren’t vital organs. Consider this: As part of a 1967 experiment, both lungs were surgically removed from one individual [PDF]. The animal survived, and its ability to process oxygen was unaffected by the ordeal. So does this mean that a hellbender’s lungs are useless? Not quite. They may not be used for respiration, but the organs probably help regulate buoyancy underwater.

7. THEY’RE PREDOMINANTLY NOCTURNAL.

Nighttime is when these creatures do most of their hunting. Between dawn and dusk, hellbenders can usually be found hiding under rocks. On cloudy days, however, they tend to get a bit more active and may leave their haunts well before nightfall [PDF].

Come mating season, the amphibians get particularly bold. Most hellbenders reproduce in either August or September. During those months, they’re far more likely to be active in broad daylight—especially before noon.

8. AFTER MATING, IT’S THE MALE WHO GUARDS THE EGGS.

A male prepares to breed by digging a tunnel under some nearby rock. Once finished, he curls up inside and pokes his head out. Ideally, a passing female will spot him and swim on over. The male then guides her into the hole, where she’ll release anywhere from 150 to 450 eggs. As she lays them, he sprays semen all over the clutch, fertilizing it.

Following this, the male chases his mate away and proceeds to spend the next few weeks protecting their unborn offspring. Ordinarily, hatching takes place somewhere between 64 and 80 days later. By then, there’s a good chance that the father will have eaten a few of the eggs—though never more than 20 or 30.

9. THEY’RE FULLY AQUATIC.

Brian Gratwicke, Flickr // CC BY 2.0

If placed ashore by some disruptive human, hellbenders can crawl back into the river, but unlike many amphibians, they almost never leave the water voluntarily. In the water, the salamanders mainly get around by crawling over submerged rocks—though the animals are decent swimmers as well.

10. SADLY, HELLBENDER NUMBERS ARE FALLING FAST.

Two subspecies are out there—and they’re both in trouble. The Ozark hellbender (Cryptobranchus alleganiensis bishopi), native to southern Missouri and northern Arkansas, has grown frighteningly scarce. Recent estimates indicate that there may be as few as 590 individuals left in the wild. Since the 1980s, the Ozark hellbender population has gone down by roughly 75 percent.

Elsewhere, the eastern hellbender (Cryptobranchus alleganiensis alleganiensis) isn’t faring much better. Previously widespread throughout their New York State range, they now exist in only a few streams and rivers. Similar reports have been made about the animal’s fading presence in West Virginia, Tennessee, Missouri, and Georgia [PDF].

Why are these creatures dying out? Siltation is the main culprit. Whenever a forest is torn down, huge amounts of soil and sand are disturbed. These later get washed into nearby waterways—including the streams that hellbenders call home. Unwelcome sediments muddy up their habitat, bury their tunnels, and suffocate their eggs.

Still, hope remains. Zoos from Toledo to St. Louis have launched captive breeding programs designed to lend snot otters a helping hand. If all goes well, these efforts will rejuvenate our ailing hellbender populations with a surge of young, healthy sub-adults. Keep your fingers crossed.