Hunting with a pH sensor
Japanese sea catfish find their prey with the help of special sensors - a very handy strategy for finding a meal in the dark seawater. These fish can sense both electrical impulses and pH changes.
Scientists investigating the Plotosus japonicus discovered that the fish reacted very strongly to a specific group of amino acids. The reason for this reaction was not obvious until someone realised that these particular amino acids all affect the pH of water. Because seawater has a fairly stable pH of 8.1 (mildly basic), at first it was not apparent why such sensitive pH sensors – able to pick up a change of 0.1 to 0.2 units – would be necessary. The Japanese sea catfish, however, searches for its prey in the dark, and its favourite food is the bristle worms that hide in the mud among the coral. The carbon dioxide breathed out by these worms reacts with the seawater, making the water directly around it a tiny bit more acid. The Japanese sea catfish can detect this minute, very local difference in pH, so it knows that a worm is hiding right there.
In order to know for sure that the pH was causing this effect, a tube was used to introduce water with a pH of 7.9 into an aquarium with the sea catfish. The fishes immediately demonstrated food-searching behaviour, even biting the tube.
The photo shows a related species of eeltail catfish.