A bristle worm, buried in the sand at the bottom of the ocean, might seem safe. Nothing can see it. The overlying sediment masks its scent. It doesn’t disturb the surrounding water. It does, however, still need to breathe. As it does, it releases spurts of carbon dioxide, which makes the water above its burrow ever so slightly more acidic. The change is tiny, fleeting and restricted to a 5 millimetres zone around the burrow’s entrance.
It’s enough for the Japanese sea catfish.
Like all catfishes, this species has long whiskers or barbels sticking out of its face. They house tastebuds that allow the animal to detect chemicals in the water around it. But John Caprio from Louisiana State University has discovered that the barbels are also pH meters. They are so sensitive that they can pick up the tiny changes in acidity produced by a breathing worm. When this predator swims ahead, a simple exhalation gives its prey away.
Caprio’s discovery, published today, is the culmination of around 25 years of on-and-off work. He has long been fascinated how the nervous system encodes information about taste and smell. “Why are there these two chemical senses, when you have just one visual one and one auditory one?” he says. “These systems evolved in vertebrates in the water, so you have to go and ask the fish.”
By 1984, Caprio travelled to Japan to work with marine catfish. He had already worked with similar animals in the Gulf of Mexico, and he wanted to know if their Japanese counterparts taste the world in a similar way. But as he exposed the fish to various chemicals, he noticed something odd. One particular group of amino acids—the building blocks of proteins—triggered a powerful reaction in a nerve within the fish’s barbels.
At first, it didn’t make sense. There didn’t seem to be any common thread to the amino acids that sent the nerve into overdrive. Then, Caprio worked it out. “I suddenly realised that all of them, even though they were very different, would change the pH of a solution,” he says.
The pH scale typically runs from 0 (extremely acidic; red on litmus paper) to 14 (extremely alkaline or basic; blue on litmus paper). Hydrochloric acid has a pH of 0; drain cleaner is 14. Distilled water is perfectly neutral at 7. Seawater is slightly basic at around 8.2.
Detecting pH changes isn’t a weird ability; you’re doing it right now. Sensors in your brainstem detect the pH of your blood, which reflects how much carbon dioxide is dissolved in it. If the pH falls too much, you automatically start breathing more quickly. If it goes up, your breaths slow down. But these sensors are internal ones; by contrast, the catfish’s barbels are the first known animal sensors that detect pH changes in the surrounding world.
And that’s very odd, because pH values in the ocean are incredibly stable. Between the 18th and 20th centuries, the pH of the surface ocean has fallen by just 0.1 of a unit, and it took all the carbon dioxide released by all human activity around the entire world to pull that off. In this world of constancy, why would a fish need such an exquisite pH sensor? It’d be like having an altitude meter in a world that’s completely flat.
Caprio batted some ideas around with his Japanese colleagues, and they reasoned that the pH sensors could help the fish to find its prey. Japanese sea catfish eat bristle worms—we know that because fishermen have found loads of the worms inside their stomachs. The worms hide in U- or Y-shaped burrows, and the catfish search for them by cruising along the ocean floor at night.
When Caprio’s team placed the worms in beakers, they found that the pH of the water just above the burrows falls by 0.1 to 0.2 units. That’s well within the range that the barbels can spot. Indeed, when the team placed artificial worm-filled tubes in tanks containing catfish, the fish would always swim over and suck the worms out—even in pitch darkness.
The team repeated the same experiment without any worms; they just hooked up a pipe to the artificial tubes, and released a small squirt of sea water with a pH of 7.9—slightly more acidic than the tank water at 8.1. “Immediately, the fish’s behaviour changed. It went straight into food searching behaviour,” says Caprio. They would even bite the end of the tube. “That was very consistent; we never saw that when we pumped in water with the same pH [as the tank].”
But why has the catfish evolved this astonishing sense, when it has so many others at its disposal? It has taste, smell, sight, and the ability to sense pressure changes in the water.
Caprio thinks that a pH sense offers several advantages. Taste and smell can react to chemicals in the water that come from rotting flesh, but pH changes always mean the presence of live prey. And close prey too. An extra burst of acidity doesn’t last long in the sea, so if the catfish sense a drop in pH, it means that food is right there. “It doesn’t have to search; it goes into feeding mode,” he says.
But wait: the sea catfish can also detect the minute electrical signals given off by its prey. Sharks, rays, and platypuses have the same ability, and they use it to uncover hidden meals just like the sea catfish does. “Why does it need an extra sense in addition to electroreception?” asks Caprio. “I can’t answer that.”
Also, what are the pH sensors? Are they the same as the ones that help our brainstem to control our breathing? Where are they? They’re definitely in the barbel and the lip, so perhaps they’re all over the fish’s head. What carries information from these sensors to the brain? And what will happen to the catfish’s ability as the world’s oceans become more acidic? Will its other senses keep it well-fed, or will it suffer?
Unfortunately, Caprio probably won’t be the one to answer these questions. “All the authors of this paper are at the end of our scientific careers,” he says. “Three have already retired. One will. I’ve been at LSU for 38 years. We’re hoping this report alerts the young folks in the field to follow the question, because we probably won’t.”
Reference: Caprio, Shimohara, Marui, Harada & Kiyohara. 2014. Marine teleost locates live prey through pH sensing. Science http://dx.doi.org/10.1126/science.1252697