ABOVE: Pufferfish (family Tetraodontidae) use their stomachs to drastically increase their size, not to digest food. © ISTOCK.COM, FtLaudGirl

It’s hard to imagine living without a stomach. Besides being one of our noisiest organs, gurgling and churning to let us know it’s time to eat, the stomach contains strong acids and enzymes that help us break down dense proteins and tough-to-digest foods. But tell that to the pufferfish (family Tetraodontidae), which lives quite happily without a working version of the organ.

“Evolutionarily, [the stomach] has brought us so many advantages,” explains Patrícia Ferreira, a PhD student in Jonathan Wilson’s lab at the Waterloo, Canada campus of Wilfrid Laurier University (WLU). “It’s brought us the capacity . . . to break down the protein right away in the stomach before it reaches the main site of digestion. . . . So what’s the point in losing it?”

A functional stomach- is typically defined as an acid-secreting sack enclosed by two sphincters. Somewhere in evolutionary history, pufferfish stomachs lost one of their sphincters and the ability to produce gastric acids—but gained another function. When pufferfish get scared, their stomachs fill with a gush of water, and, like a rapidly filling balloon, they triple in size into something that resembles a spiky bowling ball. As notoriously poor swimmers, they rely on this sudden change in size, along with their poisonous insides and skin, to deter predators. 

Ferreira started looking into this behavior in 2014, after starting a master’s in Wilson’s lab, back when he was at the University of Porto in Portugal. (Wilson moved his group to WLU a few years later, and Ferreira returned there for some of her PhD work.) Wilson had completed a project on the genetics underlying the loss of stomach function in pufferfish and a handful of other animals. The researchers began wondering, Ferreira says: “Is this kind of a replacement of function? Does this mean that for an animal to be able to use its stomach to inflate, they need to lose their capacity to digest with it?” Ferreira suspected it did. For starters, it would certainly seem difficult to maintain both functions simultaneously: saltwater is alkaline, so when fish swallow their weight in water to inflate, they could run the risk of neutralizing stomach acids. 

The pufferfish lives quite happily without a working version of the organ.

Ferreira knew pufferfishes weren’t the only fish that inflate. Sargassum fish (Histrio histrio) and the swell shark (Cephaloscyllium ventriosum) do also, and by combing the literature, she found evidence that bearded gobies (Sufflogobius bibarbatus) and pygmy leatherjackets (Brachaleuteres jacksonianus) also balloon in response to predators. She decided to test whether each species had, like the pufferfish, lost a functional stomach. She made internal pH measurements, then searched for the proteins and genes involved in acid secretion and protein digestion. She was particularly interested in the gastric proton pump Atp4a and the genes involved in making it (ATP4A and ATP4B), as well as genes coding for pepsinogens, enzymes whose presence Ferreira’s colleagues had previously found to correlate with a functional stomach.

Pygmy leatherjacket in tray.
This pygmy leatherjacket (Brachaleuteres jacksonianus) specimen was sent to Patrícia Ferreira from the Australian Museum in New South Wales.
Courtesy of Patricia Ferreira

Ferreira was able to get live sargassum fish and swell sharks into the lab relatively easily. But when the pandemic hit a couple of years ago, she knew that the bearded goby and pygmy leatherjacket, natives of Southern Africa and Australia respectively, would be tricky to source. Traveling, let alone venturing out into the open ocean to search for an elusive inflating fish, was impossible. In the end, she obtained whole pygmy leatherjacket specimens from the Australian Museum in New South Wales. For the goby, she found a scientist in Norway who had been on a research cruise in Namibia a few years back and still had small tissue samples on tiny film slides. It would be enough to do a genetic analysis, but she still needed an intact stomach to perform histology on. Luckily, a museum in Japan had some  preserved specimens—though they were almost half a century old. Ferreira describes them as “mummies wrapped in gauze.”

Specimens secured, the team got to work. To Ferreira’s surprise, pH analyses of the live fish she could get in the lab—the swell sharks and the sargassum fish—suggested that these animals did, in fact, produce gastric acids. A swell shark puffed up while one of her colleagues was making pH measurements inside its stomach in the lab, she says, and as it did so, the researchers saw the pH inside its stomach increase—i.e. become less acidic—confirming that saltwater has pH-altering properties. They also found that, while pygmy leatherjackets lacked the genes for a functional stomach, bearded gobies had them. For most of the species she studied, it seemed, inflation didn’t come at the price of a working stomach. It’s still likely that inflating fish with functional stomachs can’t digest as well after inflating themselves, Ferreira notes, but this is probably temporary.

Swellshark (Cephaloscyllium ventriosum) off Flame Reef, Santa Cruz, Channel Islands, California
Researchers found that swell sharks (genus Cephaloscyllium), unlike pufferfish, still have a functional stomach.
© ISTOCK.COM, Natalie Ruffing

The findings leave pufferfishes’ loss of stomach function a mystery. It’s possible that pufferfish inflate more often than other species, making the selective pressure greater for pufferfish to lose their stomach function, says Ferreira. Or perhaps clues can be gleaned from other animals that don’t inflate but have also forgone a functional stomach. A decade ago, Ferreira’s colleagues estimated that 20 percent of teleosts—a phylogenetic group that contains most living fish—and some other vertebrates, including the platypus, lack a functional stomach. Marine biologist Michael Horn, an emeritus professor at California State University, Fullerton, explains to The Scientist that the stomach might just not be that important. “I don’t know how expensive it is to maintain a stomach. But apparently, it’s not necessary.”

Pufferfish have found good workarounds for their digestion. Kainã Fagundes, a marine biologist at São Paulo State University in Brazil who first characterized the pufferfish’s digestive system in 2016, found that to compensate for not having a functional stomach, pufferfish rely heavily on a wide digestive tract and a fatty liver, as well as the pancreatic enzymes that all vertebrates use to break down food in the intestines. The fish also secrete acidic mucus all along the intestines, including in the rectum. “Basically all the digestive tract is a stomach,” says Fagundes. “It shows how biology is beautiful because they can conquer different . . . environments.”

Illustration of puffer fish with mouth as a water tap. 

He goes on to say that pufferfish do extremely well in their environments, despite their lack of a functional stomach. They can exist in extremely high or low salinity, eat just about anything, and can survive in extremely toxic, polluted oceans, likely because of their unique digestive and endocrine functions. “The digestive tract is not only for digestion but also for the puffing response and [to] protect against contamination,” says Fagundes. “Pufferfish should be studied more. They can answer a lot of curiosities of evolution and biology.”

Correction (July 6): The sixth paragraph of this story has been amended to note that the Australian Museum in New South Wales supplied multiple pygmy leatherjacket specimens, not one, and that specimens provided by the Japanese museum were almost half a century old, not half a decade old. The Scientist regrets the errors.