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Lichenized fungus (Lecanoromycetes)
Lichen are incredibly tough organisms often found clinging to bare rock --- at the end of the last ice age, lichens were some of the first organisms to colonize the barren bedrock left in the wake of the retreating glaciers. The secret to their resilience is surprising --- while people often mistake them for moss, lichen can't really be called "plants" at all. They are actually composed of two organisms functioning as a single, intermingled unit --- a fungus and a photosynthesizing organism (usually either a green algae or a cyanobacterium, aka the "photobiont"). The photobiont is surrounded by layers of fungal filaments that provide protection from the elements while still allowing light to penetrate --- the fungus then feeds off the sugars created from the resulting photosynthesis.
While this arrangement benefits both partners and makes lichens incredibly successful, it does seem like the photobionts are forced to give up an excessive amount of the energy that they make --- rather than a truly mutual relationship, a "protection racket" may be a more apt analogy. What's more, the photobionts can often be found living independently in the environment (alga are very successful on their own) while the fungi typically can't survive if removed from their partner.
The resulting dual-organism stretches our definition of a "species" and their relationship can only be hazily understood in human terms --- as always with nature, things aren't as straightforward as they seem.