Buried in the sand at the bottom of the sea, a bristle worm might seem safe. Nothing can see it and the sand above it masks its scent. It doesn’t even disturb the surrounding water, but it does, however, still need to breathe. As it does, it releases tiny puffs of carbon dioxide which make the water above its burrow fractionally more acidic. And while the change is tiny and restricted to a 5mm area around the burrow’s entrance, it’s more than enough for the Japanese sea catfish to detect.

Like all catfishes, this species has long barbels around its mouth which detect chemicals in the surrounding water. But scientists in the USA have discovered that the barbels can also detect pH and are so sensitive that they can pick up the minute changes in acidity produced by a breathing worm.

Detecting pH changes isn’t in itself a unique ability – even we humans have sensors that detect the pH of our blood caused by CO2 and adjust our breathing accordingly. But our sensors are internal ones and the catfish’s barbels are the first known sensors in the animal world that detect pH changes in the outside environment.

The really interesting question, however, is why the catfish evolved this amazing ability when it already has taste, smell and sight to rely on as well as the ability to sense pressure changes. Scientists think that the pH sense offers several advantages because while taste and smell can be fooled by chemicals released by decaying flesh, pH changes always mean the presence of live prey. And as an extra burst of acidity is always fleeting, a drop in pH always means that food is very close by. For the unfortunate bristle worm then, it seems the only way to avoid becoming lunch is to stop breathing...