It started out with a TV documentary I saw about them....then, searching the internet.
There's an intriguing documentary I saw about a fish that only eats the fruit from a certain tree that overhangs (fruit drops into the water) the waterway where it lives and eats nothing else. If the tree disappears, these fishes die. Now, more intrigue.
The tree can only re-seed itself after its fruit and seeds pass through the gut of this fish. The seeds will not germinate unless they are eaten and expelled by this particular fish. The tree disappears without the fish.
A perfect example of obligate symbiosis.
"Some symbiotic relationships are obligate, meaning that both symbionts entirely depend on each other for survival. For example, many lichens consist of fungal and photosynthetic symbionts that cannot live on their own.[4][9][10][11] Others are facultative, meaning that they can, but do not have to live with the other organism."
This knowledge is very entertaining for me.
Do you remember which fruit?
I think I remember reading something like that about avocados - that there was some now extinct megafauna that accounts for the size of the seed and their natural distribution, outside of their cultivation.