You wouldn’t think hellbenders would be hard to find: The huge salamanders, the biggest amphibians in North America, can grow up to 30 inches long. Yet hellbenders make themselves scarce, living on the bottoms of mountain streams, lurking under massive rocks.

As a result, locating hellbenders takes a crew of scientists. First, some of them must wedge a long pole under a rock to hoist it up, and then their colleagues must plunge into the chilly water to catch their quarry.

A couple of years ago, Stephen Spear, a conservation scientist at the Orianne Society in Athens, Ga., heard about a possible alternative. Instead of finding rare animals, some experts were gathering animal DNA from their habitats. That way, they didn’t have to track down a species to be sure it was there.

Dr. Spear decided to try. He traveled to rivers in the Southeast where he and his colleagues had found hellbenders, and scooped water into jugs.