Difference between symbionts and parasites

[Spathi]

TL;DR Summary

I suggest to discuss, how arbitrary is the line between the parasizes and symbionts; can it be that parasites often help the host a little, while symbionts do harm a little.

It is generally accepted that organisms that help the host are symbionts, and those that harm are parasites.
I suggest to discuss, how arbitrary is the line between them; can it be that parasites often help the host a little, while symbionts do harm a little.
If this assumption is incorrect, it can be formulated as the Matthew effect in biology: parasites become increasingly evil parasites, while symbionts become increasingly good symbionts.
Here are my examples:

1) The cleaner fish helps the big fish by eating parasites in its mouth; but sometimes cleaner fish can cheat a little:

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

2) Many people suffer from toxoplasmosis. This disease makes mice love cats, and maybe people too; some write that because of the toxoplasmosis, the Egyptians worshiped cats:

https://news.emory.edu/stories/2016/07/esc_loving_cats/campus.html

If this is so, why is the human body somehow reluctant to fight toxoplasma - perhaps because it is not only harmful but also beneficial?

3) Here it is written that there is an effective way to disinfect wounds - with green bot fly maggots:

https://www.acpjournals.org/doi/10.7326/aimcc.2023.0693

 

[BillTre]

I would agree that the distinction between parasites and symbionts can get blurred.
Biology is messy in this way.

Naming things can create problems. Who's to say that there are only two states of these kind of relationships.
One could argue that since there are both measures of gain or loss for each of the participants (host and infringer) in the relationship, that it is no longer a one dimensional (good:symbiont::bad:parasite) interaction space.
It involves at least a dimension for each participant.
This leads to the idea of a mushy middle ground group (un-named) of situations with different coordinates in relationship state world for each participant (axis).
In biological reality, there would be many axes to consider.

I think the best way to think about these kind of complex (multidimensional) relationships is to (when possible) figure out how they arose and understand how its complex structures were generated during its history.

A descriptive approach (naming based on some set of characters) without this kind of historical understanding will be confusing when dealing with such complex structures as organisms and their relationships.
Dobzhansky: “Nothing makes sense in biology except in the light of evolution”.

 

[smoky]

It is actually NOT accepted the symbionts help an organism in the scientific literature, though this use has permeated pop science. Symbiotic technically means closely closely entwined, so could include parasites. Beneficial organisms are known as mutualists. A beneficial organism that lives closely with you (eg. good bacteria gut, algae in coral) are symbiotic mutualists. Whether its overall positive or negative depends on its effect on fitness - do you survive and lead more offspring with or without the symbiont?

For a two species interaction, mutualists = +/+, predator or pathogen with prey is +/–, and commensalist is +/o (think lichen growing on tree bark)

It is also recognized that mutualist are rarely purely beneficial. The good may come with a price, such as giving away metabolic food like sugar to sustain your partner. Mutualists may even become deleterious under the right circumstances.