CORYDON, Ind. — With a long, slimy body and beady eyes, North America’s largest salamander wouldn’t top any cutest animal lists. The hellbender’s alien appearance and mysterious ways have earned the big amphibian a bad reputation and unflattering nicknames ranging from snot otter to devil dog.

But hellbenders, which can grow two or more feet long, are facing troubles bigger than an image problem. The aquatic creatures found only in swift-flowing, rocky rivers and streams are disappearing from large parts of the 16 states they inhabit.

The rare amphibians breathe almost entirely through their skin, making them a living barometer of water quality because of their sensitivity to silt and pollution, said Rod Williams, a Purdue University associate professor of herpetology who’s tracked Indiana’s hellbenders for nearly a decade.

“These are animals that live up to 30 years in the wild, so if you have populations declining, that alerts us that there could be a problem with the water quality,” he said.

The U.S. Fish and Wildlife Service is conducting an assessment of the eastern hellbender — one of two subspecies — to determine if it should be added to the federal endangered species list. The other subspecies, the Ozark hellbender, found only in Missouri and Arkansas, was declared endangered in 2011 after a 75 percent decline.