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Ulcerai are odd, parasitic fish. They let large animals like dragons eat them, then they resume life inside their stomach, protected from the acid by mucus-coated bodies (scales, gills, eyes, everything) and breathing through either symbiotic bacteria (which produce oxygen inside their gills or swim bladder) or by tapping into the host's bloodstream with special fangs, whichever is more feasible. (This should go without saying, but they may also feed off what their host eats).

The thing is, they need to spread their progeny, right? There are two ways to do that: eggs in the vomit or eggs in the waste. Frequent vomiting has a nasty habit of killing things, so the latter is more likely. Now, there are two ways to do this:

  1. Water-Eggs come with crud, which is exposed to rain, which hatches the Ulcerai eggs and sends them slithering off to the nearest water source. This is the easiest way to do it, and the most likely/feasible method. Instead, Ulcerai use...

  2. Horror Mode-Using chemical injections, Ulcerai alter their host so that they tend to eat prey whole, then they (not one but many, working together) periodically save a swallowed prey creature, enabling it to pass through the digestive system. The survivor then walks away, unaware that its body is covered in eggs that it transfers to whatever it touches.

Modifying host behavior is perfectly feasible for parasites, but A) fish living inside a creature is pretty dang unlikely and B) fish who also periodically save swallowed prey so they can spread their eggs are even more unlikely. So my question is, How Feasible Are Ulcerai?

Specifications:

  1. Yes, I know the premise is outlandish, but that's kind of what this site is for. For this purpose, the best answer should explain the conditions under which Ulcerai could develop the desired traits above. If it is truly impossible, I'm willing to go the 'mad mages made this thing' route as a last resort, but only as a last resort.

  2. The best answer should also explain how Ulcerai could make it work, as this will be invaluable in suspending disbelief for these creatures.

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    $\begingroup$ it sound like this to me but iam not sure about the gill $\endgroup$
    – Li Jun
    Sep 9, 2021 at 4:30
  • $\begingroup$ So a garden variety intestinal parasite, just piscine? They have all sorts of varied and complex methods of spreading, but it’s usually eggs in the poop. $\endgroup$
    – Joe Bloggs
    Sep 9, 2021 at 7:49
  • $\begingroup$ After rereading the post: not your garden variety parasite. This fish is capable of safely guiding another creature through the digestive tract of a dragon??? $\endgroup$
    – Joe Bloggs
    Sep 9, 2021 at 8:07
  • $\begingroup$ @JoeBloggs: yep, that's kind of the idea, just a small school of fish instead of one. Any other questions? $\endgroup$
    – Alendyias
    Sep 9, 2021 at 22:16

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mucus-coated bodies (scales, gills, eyes, everything)

Where we're going, we won't need eyes. Or gills, for that matter.

breathing through either symbiotic bacteria (which produce oxygen inside their gills or swim bladder) or by tapping into the host's bloodstream with special fangs, whichever is more feasible.

There's a good reason why more conventional gastrointestinal parasites are anaerobic: there's not a whole lot of oxygen in there. It isn't like the atmosphere, where it just wafts around waiting to set things on fire, or like the sea where its just dissolved everywhere you might want to live.

That means that if you want your beasties to be aerobic metabolizers (and they don't have to be, you know) then some kind of weird vampirism might indeed do the job.

It'll be awkward though, as if they don't maintain a perfect seal around their bite point (or they swim away for a bit, for whatever reason) then the host's own digestive juices will damage the exposed tissues underneath causing an ulcer. If there are a lot of these things in there, or they move around a lot, the host is going to have feeding and fitness issues. You may or may not care.

I don't see how your symbiotic oxygenators could work, though.

There are two ways to do that: eggs in the vomit or eggs in the waste

Real world parasites have so many neat tricks. Consider a single-celled stage in the lifecycle that is released into the bloodstream, tunnels through the thin membrane in the lungs and is coughed up, for example. Trickier with a multicellular organism, but not impossible, especially if the host is particularly large.

periodically save a swallowed prey creature, enabling it to pass through the digestive system

Haha, no. Unless the organism is capable of surviving without oxygen, and in a very hostile chemical environment, its gonna get digested. The amount of effort required to build a "space suit" capable of preserving it through its journey probably exceeds the value of the thing as a vector for reproduction. Vomiting starts looking like a better option.

Consider though, there might be some kinds of animal in your world that would fit the bill. Weird, slimy, nasty things. The sort of thing that nothing in their right mind would eat. Unless, y'know, they had mind-altering parasites in them.

Your beasties might even be hyperparasites, using other dragon-digestive-tract resistent parasites to carry their eggs out of their host, perhaps.

I'm willing to go the 'mad mages made this thing' route as a last resort, but only as a last resort.

Thing is, there's no reason to live in the stomach, as opposed to further down in the digestive tract. Its the most hostile part of the system, with a hazardous, corrosive, oxygen-free chemical environment. You don't have to crawl that far further down to avoid all the acid and enzyme attacks, be well protected with a nice regulated environment, and get fairly regular meals of mostly-digested food so you don't even need to go through all the faff of digesting it yourself. Even just moving to the other side of the equivalent of the pyloric valve into whatever serves as a small intestine seems better... at least you can't get puked up then, if you're not holding on tight enough.

Possibly, the best excuse you might manage is to give your dragons a thriving ecosystem of parasites who are extremely hostile to each other, rendering the rest of the dragon very difficult to inhabit for your ulcerai. I'm not certain that living in the actual stomach is a valid response (as opposed to in the mouth, like Cymothoa exigua or, uh, just outside the other end like Placobdelloides jaegerskioeldi, also known as the "hippo ass leech") but you might just about be able to handwave it away.

It does make me feel very sorry for your dragons, though.

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