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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.

A family tree of vertebrates, showing those that do and don't have stomachs.
A family tree of vertebrates, showing those that do and don’t have stomachs.

It might seem intuitive that animals which lose a certain feature might lose 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.

But in the case of the stomach-less species, “the genes are just gone,” says Puente. “No trace of them can be found.”

This means that the stomach-less species could only regain their lost organ by reinventing it from the ground-up—a feat that Castro and Wilson deem unlikely. This fits with Dollo’s principle, which says that complex traits that have been lost through evolution cannot be regained.

But why lose a stomach at all?

Castro and Wilson suspect that diet is part of the answer. We know that animals evolve very different sets of pepsinogen genes to cope with the proteins in their specific diets. Perhaps the ancestors of stomach-less species shifted to a different diet that made these enzymes worthless. Over time, they built up debilitating mutations, and were eventually lost.

You can see the first hints of this process at work in animals that still have stomachs. Many newborn mammals use a gene called Cym to digest proteins in their milk, but our version of Cym is inactive because our milk is relatively poor in proteins.

Pepsinogens work best in acidic environments, so if they disappear, you don’t need an acidic chamber any more. Gastric pumps need a good deal of energy to keep the stomach acidic, so if they are no longer needed, they would eventually be lost too.

This is all just speculation; here’s another plausible idea. Some animals eat lots of shellfish and corals, whose shells are rich in calcium carbonate—a substance that neutralises the acid in a stomach. Bottom-feeding fish like wrasses get similar mouthfuls when they suck up large quantities of muck. These species are all effectively gorging on antacids.

So, why bother acidifying your stomach if your food immediately undoes all that work? The gastric pumps are superfluous, so they are soon lost. And without an acidic environment, the pepsinogen genes are also useless, so they follow suit. “Diet most likely has a predominant role, but we’re still working out what that role is,” says Wilson. He notes that all the stomach-less species live in the water (or, like the echidna, had aquatic ancestors). “My gut feeling is that it’s something related to that,” he says.

For now, one thing is clear: many animals cope quite well without a stomach. There are many possible workarounds. The intestine has its own protein-busting enzymes. The throats of some fish have an extra set of teeth that help to break down what they swallow. “You can have a shift of function to other areas of the gut,” says Wilson. “Every which way you turn, there are species that do perfectly fine without a stomach. They aren’t aberrations; they’re quite common.”

PS: The lungfish in the image at the top has a little story. Wilson bought him from a pet store in the UK and posted him to his lab in Portugal. He got lost in the post, and it took him a week to arrive. “I was a little worried when I opened up the package,” says Wilson. “But he’s a lungfish, so he was fine.” For no particular reason, Wilson named him Horatio.  

Reference: Castro, Goncalves, Mazan, Tay, Venkatesh & Wilson. 2013. Proceedings of the Royal Society B. http://dx.doi.org/10.1098/rspb.2013.2669

33 thoughts on “How The Platypus And A Quarter Of Fishes Lost Their Stomachs

  1. This strengthens my conviction that the platypus is the weirdest animal known. Well, the weirdest vertebrate, at least 😉

    Seriously, though, this is really interesting–thanks for this post.

  2. 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.

    More precisely, acidic environments uncoil proteins, making them more accessible to enzymes, and they break down proteins on their own even without enzymes (just more slowly).

    Alkalic environments, like guts, do the same.

    So, why bother acidifying your stomach if your food immediately undoes all that work? The gastric pumps are superfluous

    Worse: they’re a massive waste of energy, so losing them is an advantage.

  3. My father used to work at the Natural History Museum in London, in the fish department. He had a lungfish called O’Shaughnessy.

  4. Fascinating. So there are no known cases of stomach loss in amphibians, reptiles, feathered reptiles (“birds”), or furry reptiles (“mammals”) except the two kinds of furry reptiles which still lay eggs? And the fish cases seem to be rather hit-or-miss. There’s speculation about eating coral, but I don’t see parrot fish on the chart (am I missing it?), and what eats more coral than they do? And all of these cases are about equally thoroughly lacking in stomachs and key genes? Are there no cases of critters showing highly degenerate stomachs, as there are birds that seem to be quite recently flightless? Do any living examples seem to be without stomachs because their ancestors were also without them? It doesn’t appear so, unless something regained a stomach, which is indeed unlikely. Or in other words, nothing without a stomach seems to have given rise to any new evolutionary branches/innovations. Very curious.

  5. Great article Ed! In response to David Bump’s comments, the parrot fish (Scaridae or Scarinae) are indeed an agastric group anatomically and functionally. They are not represented in any of the genome data bases so were not available for our study where we were looking at gene loss. However, there is an on-going project to sequence 10 000 animal genomes so one day we’ll have a pretty good idea if they’ve lost their gastric genes too. There are plenty of examples of animals that rely less on gastric acidification but have not lost their gastric glands completely. We have been looking at the armored catfish (plecostomus) that uses its stomach for air-breathing (like a lung). Even though it fills it stomach with air rather than food, it still has some gastric glands and the genes too. To our knowledge there are still no examples of reinvention/regaining of the stomach in groups with stomachless ancestors but our search continues. However, our results would indicate that this is highly unlikely if the gastric genes are lost. If you’d like a copy of our paper let me know and I can send a reprint.

  6. “The pufferfish was the sole exception—like the platypus, it has kept a single pepsinogen gene, which it uses for non-digestive purposes. ”

    Am I correct in thinking that this “other” purpose is making poison? If so, does the female platypus carry the gene even though she does not create a poison?

  7. Hi Denise. The original paper that found pepsinogen expression in the skin of pufferfish was by Kurokawa in 2005. They found expression in the mucous secreting cells in the skin and hypothesized that it was involved in biodefense (non-specific defense against pathogens). Cathepsins (a different type of protease) have been found in mucus and to have this biodefensive role. In platypus, the only pepsinogen they’ve kept is chymosin but in the original paper by Ordonez in 2008 they didn’t detect expression in any of the tissues they looked at. They only report its presence in the platypus genome and I don’t believe its on a sex related chromosome so would be found in both males and females.

  8. Well, it’s simple. Producing hydrochloric acid is expensive from an energetic point of view.

    If acid is not needed, platypuses with no stomach have an evolutionary advantage over those who have a stomach.

    I don’t believe that you cannot regain a given feature if the environment changes enough.

  9. This animal is such an interesting hybrid, I was thinking that if it has bird-like features then it must have something in common with these evolved reptiles. On the other hand, it’s a mammal so maybe it could help us study how to genetically work out something to infuse useful characteristics from reptiles into mammals through genetic therapy. I’m mostly thinking of the ability to grow back their teeth. I don’t kow if it’s possible, but it sounds like an interesting idea.

  10. So, why bother acidifying your stomach if your food immediately undoes all that work?
    but if the food is not neutralized at all then, it will stay alkaline and won’t get digested. It will also cause some side effects.

  11. I once read that anteaters do not secrete stomach acid, but rather rely on the ants’ formic acid. Has anybody looked at their pepsinogens?

  12. Has the possibility that those animals without stomachs have never had them – they never developed stomachs through the evolutionary process?

    1. @ Ken — see my earlier comment — if you look at the chart, all the stomach-less animals appear to be descendants of animals that had stomachs, so …
      @ Jay Edwards — when a mutation causes the loss of something that an organism survives well without, it’s not unusual for that loss to become predominant in the population, and the gene to become permanently lost. Examples include blindness in cave fish and flightlessness in birds, where the genes may remain for some time. Early on, they can sometimes be recovered; there is some recent research indicating that epigenetic factors can turn off genes or even repair genes that had been damaged or lost in a previous generation. However, as time goes by the loss becomes permanent and recovery quickly becomes so unlikely it would probably never happen. (just my 2 bits from amateur reading)

  13. Ostriches are related to dinosaurs. They may have remained flightless because they retained a muscular stomach of sorts. Ostriches have to swallow pebbles to aid their digestive system.

  14. Is abandoning a gene a normal evolutionary process & if so would they be able to survive if their condition needed them to reinstate the gene? I also wonder if it’s a process exclusive to species that have a knack for mutation/evolutionary adaptability?

  15. What about the concept that the stomach acid knocks down the bacterial population, allowing the animal to eat a larger quantity of food and then retreat to a safer location to digest the meal over a period of time. While waiting for the digestive system to process the meal, the bacteria would be expected to replicate wildly, endangering the host. Acid keeps the bacterial population in check.

    1. Hi Rich, you are correct that stomach acid can act as a barrier to foreign bacteria (and other pathogens) before they enter the intestine. Of course the intestine has its own population of bacteria (flora) that makes its own important contribution to digestion.

  16. What is evidence that platypus lost its stomach? as they miss gene for stomach,if they had stomach in the past they should have incessive gene for stomach.

    1. Hi Michal, if you look at the family tree of the platypus (and echidna) you see that their ancestors had a stomach and that they therefore lost theirs. The platypus is a mammal and all other mammals have a stomach. When you examine their genome (nuclear DNA) the stomach genes for acid secretion or making pepsin are mutated, sometimes beyond the point of recognition. Not sure what you mean by “incessive”. typo?

  17. I read an article about flabby whalefish.Males are born with a stomach,but as they mature lose not only their stomach but their esophagus as well.Females continue to feed as adults.

    1. Hi Robert. I found your comment interesting and had to took up the original paper. Very interesting. It’s not uncommon in terminally spawning fishes (e.g. Pacific salmon, lamprey) that stop feeding on their spawning migration to have gut degeneration but loss is something else.

  18. This a most interesting animal. I wish I could have one as a pet. Very odd indeed I never knew that they did not have a stomach. Are there any other animals like this one?

  19. Things like this amaze me. I was watching a show the other night and they were talking about the evolution of man. Hiccups don’t serve a useful function anymore, but they were useful to our very ancient ancestors. To early fish and amphibians, the motion of a hiccup was vital to continued survival. Humans’ ever-so-great-grandcestors were just beginning to use lungs to crawl out of the water. When roaming the land, they needed to suck air down into their lungs. How cool is that!!!

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