Evolution of Interior Life

Question

"Interior Life? What's that?" Quite simply, it is life that lives inside something else. As far as I know, interior life is limited to microrganisms (bacteria like E. coli) or parasites (tapeworms, Toxoplasma, and so forth), and I have to wonder why.

Mucus inside the stomach (lining the stomach walls) prevents the acid there from eating its way out to the other organs, right? And there are species, like parrotfish, clownfish, and eels, that coat themselves in mucus for protection. Natural selection favors traits that enable greater survival, so why is it we don't have a fish that evolved to survive inside another being's stomach?

Coated in a layer of protective mucus, it'd be protected from predators (because what kind of predator would look there for food?) and have a free source of food that's already chewed and everything! Seriously, it'd just have to mutate so it produced stomach-wall-grade mucus from its skin, and if the aforesaid clownfish are any indication, it can happen. In fact, this thing might be a clownfish whose ancestors survived being eaten by their anemone!

Considering that species of early fish came out of the water and evolved into terrestrial organisms, and that air is trapped inside stomachs over time, air-breathing interior life may also be possible, though extremely unlikely. Extremophiles, like the bacteria that live in Yellowstone's geysers, hydrothermal tube worms, and the orange cave crocs, make it clear that this shouldn't be just a faint possibility.

So, in sum, my question is simple: How Would Interior Life Come To Pass?

Criteria for Best Answer:

1. The best answer will explain how interior life-specifically, complex organisms of higher orders than Annelida, like a fish or squid, or perhaps even some type or equivalent of hominid-could come into being, or how and in what conditions complex life could develop to live inside another organism's stomach or digestive tract. This organism may technically be parasitic, however if it is more complex than the above examples, it counts for this question.
2. While it certainly isn't necessary, I'd greatly appreciate it if answers first explain why life that lives inside other life is apparently relatively simple and limited in variety to microscopic organisms. A good example of what I'd be looking for would be cave fauna (like cave crickets), but adapted to the conditions of a stomach in particular, like acid, potentially low air, and so forth.

Answer 1

The ecstasy fish

Legends of the candiru have stalked the nightmares of men from prehistoric times. But as humans invaded their environment, the fish had to adapt and evolve. That opportunity came, at last, when a raid on a Brazilian drug lab dumped genetically engineered yeast into a tributary of the Amazon. With their skin flora supercharged with constructs producing potent enkephalin analogs, the once dreaded candiru became known as the very emissaries of pleasure. Every moment of contact with the fish is ecstasy, even where its spines anchor it in place. Biological fluids released in never-ending waves by its hosts permitted the fish to thrive and survive, while the host's respiration provided sufficient oxygen for the quiescent fish to survive.

From chance contactees to spiritual seekers, to urban adventurers and at last the cartel market, the fish spread rapidly. Viewing them as a method of neutralizing enemy troops or unwanted lower class populations, government authorities looked on the fish with bemusement and a certain smug self-satisfaction. But as the list of unauthorized genetic modifications grew, it was only a matter of time before the fish began to spread to unexpected places in the body ... and the world...

Answer 2

(I realize below example is morbid.. so @dog lovers please avoid reading below example, I'd like to explain how a genetic change in an exoparasetic insect, followed by adaptation and natural selection - sometimes inadvertedly accellerated by humans - could yield an endoparasite, living inside the body, and not deadly at first)

The digging dog flea

A behavioural change took place involving a common dog flea species, some 1200 years ago. As a result of a mutation, the flea developed the ability to survive with less oxigen, it can now extract all oxigen needed from blood. After being fertalized, a female flea can dig herself into the skin and reside under the skin, instead of being clamped on top of it, vulnerable for scratching and preditors. Eggs grow in the flea itself, like the Jigger flea does. Digging fleas need to make a cavity, which causes a painfull itch for the dog around its tail. When the eggs hatch, the female flea dies and the cavity would burst open, larvae emigrate with the dog's excrements. The excrements provide a place for coccoons and new fleas will infest other dogs.

The cavity is left to heal, the dog survives the parasite, which is going inside now.

Initially, digging variant did not replace the common dog flea. But for about 1000 years, there was peace: a fleas infested dogs, but nobody noticed some dog fleas actually went through dog skin and could escape hygiene measures ! Dogs experienced more itching, and while their owners started to take measures to help them get rid of fleas, the digging variant thrived: measures taken were the old, known measures against fleas, many of the old dog flea species went extinct, the digging flea survived the measures.

The dogs survived too.. but in the 18th century, certain (expensive) dog breeds had a problem ! Because of the persistent scratching, the fur of the dogs looked bad. Dogs are selectively bred by humans for esthetic and commercial reasons.. to help "improve their dog race", humans now started to selectively breed the symptoms away: only dogs that did not scratch all the time could be sold for good money.

During 1000 years, the digging flea had only caused an itch. It lived beneath the skin, with no grave consequences for the dog. The dog could easily survive the flea development cycle.

But humans did (again) not realize the consequences of their action: fighting the scratching symptom, modifying their dogs, allowed the deepest digging fleas to survive. These fleas would penetrate deeper into the flesh.. not causing itching and scratching. They survived.

After 1200 years and 3 evolutionary steps, 2 of which involved natural selection and adaptation, 21thC humans finally found out what had been going wrong. The eggs now hatched inside the animal. To prevent massive spread of the larvae, they could only cremate their dog.. in an attempt to extinguish the bugger..

https://en.wikipedia.org/wiki/Flea

https://en.wikipedia.org/wiki/Dog_flea

https://en.wikipedia.org/wiki/Tunga_penetrans

https://en.wikipedia.org/wiki/Trombicula

Answer 3

(...) complex organisms of higher orders than Annelida, like a fish or squid, or perhaps even some type or equivalent of hominid-could come into being, or how and in what conditions complex life could develop to live inside another organism's stomach or digestive tract.

Sea cucumbers are often home to small fish, who enter the cucumber by an orifice that is not the mouth. Sometimes the fish will also eat the cucumber's innards, which is why Lady Evolution bestowed upon some cucumbers nature's greatest weapon for self defense: anal teeth.

Most cucumbers are chill about fish though. I could leave a picture here of a pearlfish and a sea cucumber, but that is the kind of thing that once you see you can't unsee, so I will just leave a link to an image search which you can check if you want. The fish have their heads out, but they can go completely inside if they sense danger.

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Also, some shrimp live inside some sponges, but that is not as impressive as echinoderms with teeth in their assess.

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Oh, and did you know that some insects lay eggs inside human wounds? The larva develop under your skin.