Japanese sea catfish have an unusual advantage when hunting in pitch-dark waters, a new study reports: Their whiskers can detect minute changes in the water’s acidity.

John T. Caprio, a physiologist at Louisiana State University, was studying how chemical stimuli were encoded by a catfish’s taste system when he noticed a strong reaction from the whiskers. Further study revealed that previously undetected sensors on the whiskers were responding not to the chemical itself, but rather to the effect it was having on the water.

“It was changing the pH of the water,” he said, “and not by much.”

The researchers then placed the catfish in aquariums with hidden polychaete worms, their preferred prey. The worms release tiny amounts of carbon dioxide and hydrogen when they breathe, raising the water’s acidity (and thereby lowering its pH). Even in the dark, the catfish were drawn to the area with the worms. The researchers, who published their work in the journal Science, also observed that the fish located the decreased pH areas even when no worms were present.

“This fish is a swimming pH meter, and its accuracy was as good as pH meters I was using in the lab,” Dr. Caprio said. “If you drop the pH less than one-tenth of a pH unit,” he added, the fish’s sensors “start firing like machine guns.”