Japanese sea catfish are at home in the dark. Taste buds pepper their skin, allowing the fish to track their prey before slurping them down. Stripes of electric sensors run along their heads and eel-like fins, detecting pulses emitted by food and foes. And now, scientists have identified another sense in the aquatic arsenal of Plotosus japonicus (above): Their whiskers can detect acidity in seawater. As reported online today in Science, this extra sense helps the animals hunt in the dark. Infrared cameras revealed that catfish living in a pitch-black room could use acidity to find polychaete worms, a favorite snack. To elude predators, the worms dig small burrows into the muddy sea floor or coral. But when the worms exhale carbon dioxide, some of the gas reacts with water—H2O—to form carbonic acid. The wrigglers release only miniscule amounts of carbon dioxide, so the catfish must be within 5 millimeters of a lair to detect a worm. To confirm that acidity was the critical beacon, the team replaced worm burrows with a plastic tube that pumped in seawater with slightly lower—acidic—pH than normal. Individual catfish swarmed around the tube, occasionally biting it, as if the lifeless thing was prey. The study should prompt a search for pH perception in other fish, according to the researchers, though they worry that looming ocean acidification caused by greenhouse gases might ultimately stifle fish with this ability, given that the catfish’s acid meter worked best in average seawater—pH 8.1.