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Sensors help catfish 'see' in the dark

A Japanese sea catfish (Plotosus japonicus).
[Image courtesy of the Kagoshima Aquarium]

Researchers have discovered that the Japanese sea catfish, Plotosus japonicas, has sensors on the outside of its body that detect slight changes in the water's pH level. In other words, these sensors can help the fish tell if the water they're swimming in becomes a little more acidic or basic—an ability that helps them hunt in dark, murky waters.

John Caprio and colleagues, who identified these previously unrecognized sensors on the fish, suggest that they allow the catfish to sense the respiration, or "breathing," of their prey when other senses, like sight, aren't helpful. The researchers had been studying the taste system of these Japanese sea catfish when they happened to notice the remarkable sensitivity of the fishes' barbels, or "whiskers," to changes in pH. So, the researchers hid polychaete worms—a favorite dish of the catfish—in the fishes' aquariums. These worms release tiny amounts of carbon dioxide and atomic hydrogen, which influence the water's pH very slightly.

As expected, Caprio and his team found that the catfish spent much more time near the worms—even when the tanks were pitch black. Whenever the catfish found pockets of lower pH values in the water, they would become active and search for food, according to the researches.

The catfishes' sensors worked best in natural seawater with a pH of 8.1 or 8.2, but the researchers warn that their ability could be compromised if ocean acidification continues to rise with global temperatures as it has been.