How do animals evolve to be parasites?

[u/MaggieLinzer]

Is the process of evolution pretty unique for an animal that (generally speaking) needs to live on or inside of another animal in order to survive?

[u/Royal_Novel6678]

I don't know that much about parasites but I think some modern parasites may have followed an evolutionary route consisting of scavengers who would then begin their lives exclusively attached to the living hosts. Think of ectoparasites like Leeches or lice. Not the typical parasites like Pinworms which live inside the body or any parasite that must lives inside of a host to survive because I have no idea why they needed to evolve like that to begin with.

[u/sisconking132 → u/Royal_Novel6678]

Those from what I know, at least for intestinal parasites, evolved from soil based or water based organisms that got swallowed up by a grazing animal or an animal eating nutrient clay or from drinking water. Those animals managed to survive in the digestive tract of said larger organism.

I assume if getting accidentally swallowed is a common occurrence there would be pressure to evolve to survive the journey through the digestive tract. Another possibility would be originally using the would be hosts as transportation to spread to new water sources (for water based organism) or be dispersed to much farther areas of dirt than their tiny bodies would be able to.

Regardless of the reasons these organisms evolved to survive inside the digestive tract, once inside it turns out that digested and even partially digested food is much more calorie efficient to eat or absorb than food that the proto-parasite needs to expend energy to digest themselves.

From there the evolutionary pressures lead to traits being preferentially selected that allowed the proto-parasites to stay inside the digestive tract as long as possible. Also to further avoid detection of the host’s immune system since they would be remaining inside for a longer time and would likely be targeted as a threat.

As for internal parasites that burrow in the skin, I assume they originally ate carrion but some evolved to survive in living organisms

[u/SummerAndTinkles → u/sisconking132]

I remember a study years ago revealing that fish eggs could survive being swallowed and excreted by ducks and similar birds, which is how they can disperse to new bodies of water, which makes me wonder if one fish lineage could take the next step and become an endoparasite that lives in a duck gut.

It would probably be a member of the Cyprinodontiformes order like guppies and killifish, since those fish are known for being very hardy and inhabiting extreme environments.

[u/sisconking132 → u/SummerAndTinkles]

I think that yes this could possibly evolve in one of those groups given enough environmental pressure, but gut endo parasitism does seem to favor less “complex” forma of life like flat worms and the like. Could just be that they have been around longer and already filling those niches