does a platypus have a stomach?

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How the Platypus and a Quarter of Fishes Lost Their Stomachs

The platypus is an anthology of weirdness. It has a leathery duck-like bill, a flattened tail and webbed feet. The males have a venomous claw on their hind feet, and the females lay eggs. And if you look inside a platypus, you’ll find another weird feature: its gullet connects directly to its intestines. There’s no sac in the middle that secrete powerful acids and digestive enzymes.

In other words, the platypus has no stomach.

The stomach, defined as an acid-producing part of the gut, first evolved around 450 million years ago, and it’s unique to back-boned animals (vertebrates). It allowed our ancestors to digest bigger proteins, since acidic environments deform these large molecules and boost the actions of enzymes that break them apart.

But over the last 200 years, scientists have shown that many vertebrates have lost their stomachs. The platypus doesn’t have one, nor do its closest relatives, the spiny echidnas. Lungfish, a group of slender freshwater fish that can breathe in air, don’t have stomachs; nor do the chimeras, bizarre-looking relatives of sharks and rays.

And the teleosts—the group that includes most living fishes—have taken stomach loss to extremes. Of the almost 30,000 species, it seems that around a quarter have abandoned their stomachs, including groups like wrasse, carp, cowfish, pufferfish, zebrafish and more. (It’s commonly said that pufferfish puff by expanding their stomachs, but while they have a sac in the right place, it’s not a glandular, acid-secreting one, so it doesn’t really count.)

On at least 18 separate occasions, vertebrates have abandoned their stomachs. And we now know that several of these losses were accompanied by disappearing genes.

Xose Puente from the University of Oviedo first discovered that the platypus has lost its main stomach genes, back in 2008. Now, Filipe Castro and Jonathan Wilson from the University of Porto have found the same pattern in other stomach-less vertebrates, like the zebrafish, pufferfish, medaka, platyfish, and Australian ghostshark.

They scoured the full genomes of these species and showed that they’re all missing the genes for the gastric proton pump—the enzyme that acidifies the stomach. They’ve also lost many of the genes for pepsinogens—digestive enzymes that break down proteins. The pufferfish was the sole exception—like the platypus, it has kept a single pepsinogen gene, which it uses for non-digestive purposes. “It’s a clear-cut pattern of gene loss and stomach loss across all of these species,” says Wilson.

the genes associated with that feature. But that’s not always the case.

Blind cavefish still have the right genes for making eyes, and if you cross-breed populations from different caves, you can actually make sighted individuals. Toothless mammals still have genes for making enamel—they just don’t work anymore. And birds also have tooth-making genes—relics from their dinosaur ancestors. “You can go to the chicken genome and find that most genes involved in the formation of the enamel are still there, just where you would expect to find them,” says Puente. They’ve been inactivated, but not lost. With the right genetic tweak, you can switch on these dormant programmes and produce chickens with teeth.

Why the Platypus Will Never Have a Stomach

Bizarrely, many species of animals, such as the carp and platypus, lost their stomachs in the evolutionary past, and new research suggests they may never evolve the organs back.

The stomach is the part of the gut where the main part of digestion takes place. Glands in this organ secrete enzymes known as pepsins, which break down proteins, and strong acids that soften food and help the enzymes work. The glands first appeared about 450 million years ago, and they represent an evolutionary innovation found exclusively in jawed creatures with backbones.

Surprisingly, the gastric glands that define the stomach are missing in a number of jawed vertebrates. In 1805, the French zoologist Georges Cuvier discovered that many teleosts, or the largest living group of fish, such as the carp family, lack stomachs. The past 200 years of research suggests that up to 27 percent, speaking conservatively, of all teleost species may lack stomachs. Primitive bony fish such as lungfish and some cartilaginous fish such as chimeras lost the organs as well.

Scientists wondered if all of these examples of stomach loss had anything in common. Moreover, the researchers wanted to know if these animals might reinvent the stomach one day. There are a number of evolutionary instances of species redeveloping complex traits; for instance, a number of stick insects apparently reinvented the wing.

Stomach loss specifics

Since many animals have now had their genomes sequenced, researchers investigated 14 species with and without stomachs to see what genes they all might be missing. The scientists found that in all species examined, stomach loss was clearly linked with the complete loss of the genes responsible for pepsin and acid digestion.

The researchers suggest the ancestors of these stomach-free species grew to depend on diets in which digestion via pepsins and acids was not likely or even possible. For instance, diets rich in chalky shells or bottom muck can neutralize stomach acids. If these species adapted to survive without the need for a stomach, the genes for its function could then be lost by mutation over time without ill effect. These genes can be energetically costly to maintain, which could hasten their loss if they were rendered superfluous.

The investigators noted the loss of these genes suggests the reinvention of the stomach in these species is highly unlikely. Although species can redevelop complex traits, past research found the ancestors of those species retained the genes for those characteristics, and their descendants merely reactivated the genes. In contrast, the stomachless species the researchers analyzed have apparently lost the complex genes for gastric digestion beyond the point of recovery.

“It appears that once the stomach is lost, that’s all folks,” study author Jonathan Wilson, a comparative physiologist at the University of Porto’s Interdisciplinary Center for Marine and Environmental Research in Portugal, told LiveScience.

Regaining a stomach

Still, it might be possible for these species to regain stomachs in the distant future if they adapt genes similar to ones for acid and pepsin digestion. These similar genes “could, in theory, evolve similar functions” to those necessary for stomachs, study lead author Filipe Castro, an evolutionary biologist at the University of Porto’s Interdisciplinary Center for Marine and Environmental Research,told LiveScience.

Future research can look for a missing link in the evolution of stomach loss — “animals lacking a stomach but having retained the genes,” Wilson said. However, since there are at least 5,000 vertebrate species without stomachs, any such work could be like searching for “a needle in a haystack,” he added.

Scientists can also investigate why the stomach emerged and persisted in evolution. “That will help to understand the phenomenon of loss,” Castro said.

And researchers may be interested to find out what would happen if the genes for acid and pepsin digestion were inserted back into stomachless species.