Are there lower bounds to complexity for hosts and how are these described or modeled? In other words, one would expect an organism to have a certain amount of "surplus tissue" in order for parasites to be able to feed on it, at least without killing the host too soon for the parasites to thrive. For example, humans have surplus blood (we can draw a lot of blood and survive), making it possible to live with a few leeches, ticks, etc. Is this supposition really true? That is, do we know that "very simple" organisms cannot have parasites? If so, how do we characterize what is "too simple to be a host" and does it have anything to do with having "surplus tissue"? (I assume "surplus" is not the term used in parasitology because I haven't seen it in the books...)

  • $\begingroup$ yes it most at least have cells otherwise it does not qualify as a host. $\endgroup$
    – John
    Jul 20 at 14:03

"Complexity" is a difficult term to operate with in biology; most people that use it don't have an idea of how they'd like to operationally define it, and without an operational definition it isn't meaningful.

Phages are effectively parasites of bacteria - one could bicker that a virus can't be a parasite because it's not "alive", but I think this is just a semantic argument about what it means to be alive, and many of the definitions that exclude viruses are problematic because they those same terms exclude animal parasites.

Not all parasites are of the "feed on the host just a little bit" variety, either. Parasitoid wasps, for example, typically consume the entire target host, sometimes paralyzing it at the same time an egg is laid. Cordyceps is another fun group of (mostly) parasites, whose targets include other fungi and insects.

The only limit seems to be that the host must somehow be of use to the parasite. It must also be complex enough to be considered a living organism in its own right: you can't have a parasite of something that's already dead, for example, since that has a different meaning. I think these meanings are rather tautological, rather than interesting, though, so I'd answer your title question to say "no, not in any meaningful, non-tautological way".

  • $\begingroup$ Thank you this is very interesting! I guess I asked the question a bit wrong: I meant specifically hosts that are multicell organisms and specifically parasites that feed on a host for an extended period (as opposed to consuming it or killing it entirely). In these restricted senses, one would expect (although I am asking because I'm not sure) that the host must have "surplus tissue" to be able to yield to the parasite without dying. I guess I am interested in this from an evolutionary perspective: do we see the entry of parasites as correlated to the development of "surplus tissue"? $\endgroup$
    – Antoshka
    Jul 21 at 15:18
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    $\begingroup$ @Antoshka I don't think you'll be able to do anything mathematical here in order to develop any "rule", just to look at examples, and biology is pretty good at coming up with exceptions to any rule you think you found. It's certainly not true that parasites of the type you describe only feed on "surplus" tissue: tick infestations can be fatal to deer, weakening them to the point they cannot recover. $\endgroup$
    – Bryan Krause
    Jul 21 at 15:28

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