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Is it possible for a creature that exclusively relies on liquid consumption? Specifically, considering the absence of solid food, would these creatures have a stomach?

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    $\begingroup$ Define exactly what you mean by stomach. Because sometimes they have a crop instead of a stomach, sometimes they have both. A crop is just an enlarged part to store. $\endgroup$
    – DKNguyen
    Commented Jan 21 at 6:04
  • $\begingroup$ They should have some way to store liquid to be absorbed/digested maybe this "sac" is rare or sought after $\endgroup$
    – Overflight
    Commented Jan 23 at 19:28
  • $\begingroup$ If their world builder said so, they would. How does that work for you? $\endgroup$ Commented Jan 23 at 23:14
  • $\begingroup$ Well done @asteroidbelt for asking a question that had so many views! $\endgroup$
    – JBH
    Commented Jan 24 at 0:12
  • $\begingroup$ Andy Weir's "Project Hail Mary" has , among many neat ideas , a creature without a dedicated stomach. It's weirder than Earth cnidarians. $\endgroup$ Commented Jan 24 at 4:28

8 Answers 8

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Possibly

Let's use water as a baseline. Ignoring some details, water is only a fluid. Now let's compare that to nectar fruit juice, which has a LOT of solids in it. No, it's not like chewing a steak, but there are solids. Stomachs are really good at digesting solids. They are, in fact, good at a number of things.

Let's ask what else a stomach (well... a human stomach) is good for. It's good for helping separate poison from nutrient. It's good for vomiting... which is actually a really important role for it to play. In the human perspective, waiting for something undesirable to get into the intestines and then evacuating it via (e.g.) diarrhea is too late. The body would already be harmed.

And then there's cud-chewing critters like cows that have multiple stomachs to deal with different stages of digestion.

So, do your creatures need a stomach?

The basic answer is, "they need access to the features that a stomach allows," but because they don't chew "solids" (not solid in terms of a cube of steel, but solid in terms of vegetable/animal mass that doesn't drain away when you set it aside) they don't need a large stomach or multiple stomachs... but I think the answer is "yes," they need something like a stomach.

But for the sake of argument, let's call it another intestine. It's the thing that would force something unwanted back out or that would break down the more solid aspects of fluids. I can easily imagine it being long and skinny, similar to human intestines.

So, why not just say it's an intestine? Because it doesn't quite do what intestines do, it just kinda looks like an intestine.

Could you get away with saying that there's no stomach?

Yes, this also is true. Let's say your creatures cannot digest solids of any kinds — only fluids with mineral and protein content. In that case you could say that your creature has no stomach because anything bad or indigestible would be evacuated as diarrhea. However, there's a down-side. Where a stomach could (and does) capture indigestible non-pass-along-to-the-intestines objects (like golf balls...) an intestine-only solution would be unlikely to deal with this, meaning the range of objects that could be eaten (*ahem*) "without consequence" (yeah, I know I'm skipping an entire doctrinal thesis with that statement) is much smaller, increasing the danger of harm or death to the individual.

Is there anything in Real Life you can use to model your creatures?

You bet! Vampire bats drink blood, and their "stomach" is a long kind of intestine-looking thing that's quite complex. Lampreys, which also drink blood, also have a "stomach." What you'll see when you look those up (and I strongly encourage you to do so) is that what we humans are labeling on a diagram as "stomach" is something quite different from what's inside us. And that's good! It'll give you ideas about how to design your creatures.

Worldbuilding Note: As you continue with your worldbuilding, please remember that questions not reflecting the specifics of the real world have answers that are very much up to you. In other words, there isn't a single, definitive answer. What there are, are possibilities and consequences. These are all good things! The temptation of getting the one, right answer so your efforts are easier can be hard to overcome, but those possibilities and consequences are what make stories really good.

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  • $\begingroup$ You could also use the model of an earthworm, where the thing that most of us would call a stomach is actually a gizzard. I think that the only difference is that a gizzard is highly muscular, so that might just be semantic. $\endgroup$ Commented Jan 21 at 6:31
  • $\begingroup$ @RobertRapplean Without looking anything up, I'm thinking a crop is just for storage, a gizzard adds on a mechanical digestion function, and a stomach adds on a chemical digestion function. $\endgroup$
    – DKNguyen
    Commented Jan 21 at 19:15
  • $\begingroup$ @DKNguyen, Yup. chickens swallow rocks, and the gizzard uses them to grind up the things it eats. A stomach soaks the food in acid to break it down after it's been chewed. Both are cases of breaking the food down so that the intestines can absorb nutrients. My question (which I should posit to OP) is whether a gizzard counts as a stomach, even though their mode of functionality is different. $\endgroup$ Commented Jan 22 at 7:23
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It's possible and we have a real example in our world.

The eating way of spiders is very peculiar: once they have captured their prey, they inject it with enzymes which liquefy its organs, then they suck the resulting liquid.

Yet, if you look at a spider's inner anatomy, you see that they have a stomach

enter image description here

As you can see, also by eating only liquids, one creature can have a stomach.

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  • $\begingroup$ Leeches have a crop but no stomach. Crop just being an enlarged gut to store. Not sure if the spider stomach is vestigial or if it has other functions a crop does not. $\endgroup$
    – DKNguyen
    Commented Jan 21 at 6:05
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    $\begingroup$ @DKNguyen From doing some quick searching around, it looks like the so-called "sucking stomach" is actually just how the spider takes in the fluids: rather than sucking them in the way we might drink through a straw, it actually expands its "sucking stomach" through muscular action, causing the fluids to be pulled in like a vacuum cleaner. It doesn't look like it has any actual role in digestion so far as I could tell? It seems like it's not a true stomach, and the name is largely just because of the physical similarity in storing digested food. I could be wrong on that aspect though. $\endgroup$
    – Idran
    Commented Jan 22 at 21:46
  • $\begingroup$ @JustinHilyard Nature need not followed human sandboxes. $\endgroup$
    – DKNguyen
    Commented Jan 22 at 21:49
  • $\begingroup$ @DKNguyen Very true! I just mean that anatomically it seems to be more akin to a crop than a proper stomach, and the name is just a holdover or something. Or maybe it's that "sucking stomach" has nicer assonance than "sucking crop". :) $\endgroup$
    – Idran
    Commented Jan 22 at 21:51
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    $\begingroup$ @JustinHilyard I will look up how mosquitoes do it , but they are a completely different lineage. Might not have anything and just uses blood pressure. EDIT: Mosquitoes have muscular pistons pumps in their heads. Makes sense since I guess they need to drink water and nectar and other stuff too. Not just blood from a live creature. $\endgroup$
    – DKNguyen
    Commented Jan 22 at 21:53
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This Already Exists

enter image description here

Animals that only consume fluids include the butterfly, moths, aphids, mosquito, horsefly, hummingbird, leech, and vampire bat.

They still need stomachs to digest the dissolved solids in the fluids they drink. The butterfly and hummingbird needs a simpler stomach because they mostly drink sugars. The bat and leech need a more complex stomach.

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  • $\begingroup$ + many invertebrates which seem to consume solid food are also actually consuming liquids, by first liquefying their prey by injecting enzymes, and then sucking the resulting liquid. Spiders are one of the most common example. $\endgroup$
    – vsz
    Commented Jan 22 at 6:40
  • $\begingroup$ NB, hummingbirds don't exclusively feed on nectar: Most (all?) hummingbirds also eat insects, spiders, etc., to get the protein & other nutrients they need. They catch a lot of flying insects actually, but you'll need slow mo video to see, b/c the whole process takes about 1/30th of a second. Worth a watch if you have time! $\endgroup$ Commented Jan 23 at 17:44
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I would take filter feeders as an example. they could just rely on raw throughput volume and internal surface area.

See sea sponges for real life examples:

Sponges do not have distinct circulatory, respiratory, digestive, and excretory systems – instead, the water flow system supports all these functions. They filter food particles out of the water flowing through them. Particles larger than 50 micrometers cannot enter the ostia and pinacocytes consume them by phagocytosis (engulfing and intracellular digestion). Particles from 0.5 μm to 50 μm are trapped in the ostia, which taper from the outer to inner ends. These particles are consumed by pinacocytes or by archaeocytes which partially extrude themselves through the walls of the ostia. Bacteria-sized particles, below 0.5 micrometers, pass through the ostia and are caught and consumed by choanocytes.[21] Since the smallest particles are by far the most common, choanocytes typically capture 80% of a sponge's food supply.[34] Archaeocytes transport food packaged in vesicles from cells that directly digest food to those that do not. At least one species of sponge has internal fibers that function as tracks for use by nutrient-carrying archaeocytes,[21] and these tracks also move inert objects.[23]

Source: https://en.wikipedia.org/wiki/Sponge#Respiration,_feeding_and_excretion

In other words: No stomach and exists in our real world. I think we can take this as a definitive "No" to the hard requirement.

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    $\begingroup$ Filters feeders don't feed on liquids; they feed on solids suspended in a liquid. This isn't the same thing. $\endgroup$ Commented Jan 21 at 18:36
  • $\begingroup$ How would you even define "feeding on liquids" then? For starters: you can melt or freeze everything. I could now post a bunch of gotchas, But I don't think that's fair. $\endgroup$
    – Daniel
    Commented Jan 21 at 23:56
  • $\begingroup$ I'd define liquids in the usual fashion. Many real animals (see other answers) feed on liquids. $\endgroup$ Commented Jan 22 at 6:52
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    $\begingroup$ @JackAidley By your definition of "the usual fashion" blood is a liquid. But blood contains solids suspended in a liquid. Which is it? Where is the dividing line between tiny particles floating in the liquid and actual liquids? Is your requirement that nutrients must be reduced to their most minimal water-soluble molecules? $\endgroup$
    – barbecue
    Commented Jan 22 at 13:54
  • $\begingroup$ Filter feeders don’t ingest the liquid which has the solids suspended, that’s the difference. They - wait for it - filter the solids out and ingest those, leaving the liquid outside their alimentary structures. The question is about non-filter feeders who ingest liquid food - or, have it your way, liquid with food suspended or dissolved in it, but not filtering. $\endgroup$
    – Beanluc
    Commented Jan 22 at 21:29
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Yes, there are animals which only take in liquid (see other answers).

Stomach has a few important functions, which kind of define it, and which also can apply even if all food is liquid:

  • It's a place for enzymes which break down food molecules, including proteins. Since these enzymes specifically break down biological tissue, they need a special environment, which they can't break down.

  • It's a place which disinfects the food, killing bacteria, but again must not damage the animal itself.

  • It can do mechanical digestion, mixing the food with the enzymes and the acid by muscle movement.

  • It stores potentially unsafe food, which can be vomited out instead of letting it further in, if body somehow determines it is unsafe.

"Safe" food can then be digested and absorbed more effectively later in the GI tract, optimized for that instead of the above tasks.

There are a few things which make for example a human stomach specially adapted for the above tasks:

  • High acidity, which both kills bacteria effectively, and creates a special environment where the tissue breaking enzymes are optimized to work well.

  • Thick mucus on stomach walls prevents acidity and enzymes from damaging the wall, and also keeps the possibly unsafe contents of the stomach isolated.

  • Ability to close both ends of the stomach, so food does not further in before it is processed enough, and food doesn't spill out either for example while running, which would be more than just inconvenient, as it would let the acid and the enzymes damage less protected tissue of the throat.

A different creature might have different ways to achieve the purposes of the stomach, but an organ with the matching functions is very necessary.

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As user171170 pointed out the reason for a stomach is having a vessel for pepsine to work. This enzyme breaks proteins down to aminoacids.

This enzyme is so bad-ass for the body that it is not produced directly but i a form of pepsinogen - a passivated enzyme - and the acid inside stomach activate it. Also, the stomach inner wall is covered by tissue immune to acid and pepsine. Without this protective system - enzyme activated inside a sack immune to the enzyme - the creature will digest itself from inside. If your stomach ruptures you are dead - pepsine will spread in your body and digest you.

The reason for having such a reactor, a Pandorra's box, inside a body is simple: The body is based on different proteins that are synthesized from individual aminoacids.

So if we want to have creature that does not need a stomach we are either looking for creature that does not need aminoacids to build itself on but on something else. That creature would probably need to process more complex structures to simple ones... Oh snap, it needs a stomach!

Or we are looking for creature that does this outside its body!

So we end up with spider-like double-venom creature that:

  1. Injects the protoenzymatic mix and activator in its prey.
  2. Waits until the pre-digestion takes place.
  3. Injects the enzyme antidote.
  4. Consume the pre-digested smoothie.

Taking the dead-ends out we can cross out trying to feed on mobile preys since they can go somewhere the creature connot feed safely. Even if the prey is safely immobilized (killed or paralysed) the intent is to harvest as much as possible so a cocoon is needed to serve as a vessel to hold the pre-digested matter.

The other problem is how would such a creature produce material for the cocoon if the material must be resilient to the digestion in the first place...

The workaround might be a creature that hunts, immobilizes and carries the prey to its safe habitat where is some seling hollow where the prey is put and injected the enzymatic mix.

The huge drawback of such a feeding system is its vultereability a time inefficiency compared to the stomach-wielding creatures.

  • The habitat can be invaded or damaged and the precious lunch is spoiled. With stomach, you protect your lunch for being spoiled or taken away from you (involuntarily).
  • All the time carrying the prey home is wasted by not hunting another prey or not resting. With stomach you start digesting at the time you devour your lunch.
  • There must be reasonably sized reservoirs for the enzymatic precursor and activator and the production must be either precisely controlled or they serve a dead mass during the hunting. With a stomach you can have the unexpected luch waiting inside a bit and produce all proto-enzymes, activators and deactivators at need.

There are real-life examples of stomachless creatures. BUT... Those are some insect species adults and they don't have any digestive tract at all. The sole purpose of this final stage is to feed on reserves stockpiled in larvae stages, mate and die.

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  • $\begingroup$ Seahorses are stomachless, but tbf they feed on tiny zooplankton. $\endgroup$ Commented Jan 23 at 13:45
  • $\begingroup$ How do they process the food to grow on it? $\endgroup$
    – Crowley
    Commented Jan 24 at 11:16
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All the previous answers are very good answers but still haven’t quite provided a satisfying answer we all can stomach. Which I believe I can provide by using the established common thread to tie it all together. We all know stomachs use enzymes and pepsin’s to digest proteins and foods. A species living solely on liquids would have little to no use for this as there would be nothing to break down. When a species no longer has a biological need for something, evolution can be very efficient in making the item permanently disappear. Which we can see demonstrated here on our little 3rd rock from the sun in that lover of a fish with the constant pucker known as a carp. Along with the carp, the lungfish and the chimera make up the Axis of missing digestive powers, as they all have gone under the evolutionary knife and had their stomachs removed. Last but not ever least we have the poster child of animals that shouldn’t exist and the biggest argument that the higher powers of creation are 420 friendly, the platypus. Adding to its already long and distinguished list of attribute oddities, the platypus doesn’t have a stomach, either. Now that we have the magic of genome mapping, it’s recently been discovered all species with evolutionary stomach loss also had a complete loss of the genes responsible for pepsin and acid digestion. After analyzing all the good information brought to our attention by each contributor to this answer, I feel confident in saying “No!” A creature living solely upon a liquid diet would still have a stomach. Albeit one that is strictly for show and just waiting to fall off the fossil record. Dependent on its stage of evolution. But I can also emphatically say “yes!” A liquid sucking and drinking creature would not have a stomach, if it is further along its evolutionary cycle. I’d also like to add, in a world of personalities just trying to blend in, the unique bravery and self-confidence we should all strive for is that of the platypus. A warm blooded, web-footed, egg-laying, duckbilled, semi-aquatic, stomachless, evolutionary titan.

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    $\begingroup$ " A species living solely on liquids would have little to no use for this as there would be nothing to break down."? All mammals feed on only liquid for some time after they are born, and they still need enzymes to digest that liquid. $\endgroup$
    – L.Dutch
    Commented Jan 21 at 15:36
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Yes. There are people who don't have a stomach or don't use a stomach and are able to get adequate and appropriate nutrition and be healthy. Think of people who are tube fed - you can be tube fed with the formula going directly into the stomach (G tube), or you be fed with the food going into the intestines, generally the jejunum (J tube).

If you do G feeds, you can use basically any type of formula, or blend up regular food into liquid consistency and use a pump or syringe to deliver the food to the stomach, where it then undergoes some digestion and proceeds through the rest of the GI tract normally.

If you do J feeds, formulas are best because they're more broken down; doing a feed with blended up food doesn't work as well because it hasn't been digested by the stomach and the intestines can't break things down like the stomach can. The easiest formulas for J feeds are elemental formulas, where the contents is broken down to the basics - the formula is composed of amino acid, some sort of lipid (fat), some form of sugar/carb (often dextrose), and then vitamins and minerals and any other necessary supplements. All that is mixed with water and fed straight into the intestines, where it's easily absorbed because it's already broken down into the individual elements. That would be analogous to creature who drink a substance or substances that provide them with the necessary nutrients they need. I'm not sure if the aliens are making this liquid or if they're feeding from animals or plants that contain liquids with broken down nutrients. Or, like some animals, inject the prey with a substance to liquify the inside of the animal to create the liquid they drink for nutrition. But it's definitely possible to survive and be healthy if getting nutrition only via the intestines.

Plus, you're creating these creatures. You can create creature with specialized sections of the intestines that absorb only certain nutrients at that section, that have a section where water is absorbed, etc. Human large and small intestines do different jobs, so alien intestines could too. Herbivore versus carnivore vs omnivore intestines are all different because they are specialized to get the nutrients they need from meat, plants, or both. Aliens could similarly have intestines that are specialized to best absorb the specific nutrients they need, based on their diet.

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