A freshwater snail has been rediscovered on the Cahaba River in Alabama, 12 years after it was declared extinct.

Nathan Whelan, a graduate student in biology at the University of Alabama, spotted the snail — called the oblong rocksnail, or Leptoxis compacta — on a small stretch of the river.

The rocksnail, about the size of a nickel, with a yellow body and a black band on its head, once had a range of about 50 miles along the Cahaba, but it was in major decline by 1935. In 2000, the International Union for Conservation of Nature declared it extinct.

But Mr. Whelan, whose doctoral research is on snails, had a hunch that it might still be out there.

“It never really made sense to me, why this particular species went extinct when all these other species found in the same stretch of the river are still there,” he said.