Their gills aren’t exactly like the feathery, ribbed slits of a fish, however. As they grow up, they lose their gills. Instead, they absorb oxygen out of the water as it flows over their skin.

Their love affair with living under big rocks posed a slight problem, however. Hellbenders are so big they need lots of oxygen, which takes lots of water. But water doesn’t flow as well underneath rocks, making it quite stuffy down there.

To solve the problem, they grew loose folds of skin along their sides. The more skin that’s exposed to the water, the more oxygen they can absorb. Hellbender researcher Dr. Michael Freake calls it a “unique adaptation” — increasing the surface area of their skin to maximize their oxygen intake in otherwise less than desirable conditions. Despite the special skin feature, hellbenders still prefer swifter moving streams where water is more likely to circulate under the rocks.

When creeks warm up on hot summer days, especially where people have cut down trees along the banks, the oxygen molecules evaporate from the water, making it harder to breathe.

“When the water temperature warms up they rock a little bit to keep the water moving across their skin,” Freake said.