Intelligence has degrees that begins with any abstraction and ends with arbitrary conscious theorizing and decision making (and a metric of how effective this ability is).
But isn't that what one of the studied fungi did? To make a decision to grow into the direction where it found food the last time instead of every direction like it did before, it needs to have some sort of model of its environment in its memory, and needs to be able to make decisions based on that model.
chemoreception
> They are so smart that they find the shortest paths between nodes arranged as Japanese cities
survivor bias
Even unicellular ones has to do it, that is what a complex adaptive system that wants to sustain its life-state has to do. But none of this is symbolic processing the way humans are capable of. Reducing intelligence to mere adaptivity simply makes it a useless term.
https://www.youtube.com/watch?v=LaD7AwPC5Pw&ab_channel=Sesam...
https://www.youtube.com/watch?v=vtb2EKGBNr8&ab_channel=Sesam...
All multicellular living things seem to do some information processing. We would not agree to call a fungus intelligent, but would agree to call a human intelligent.
OK, from human we can descend to an ape, a dog, a crow, a sparrow, a crocodile, a goldfish, a flatworm, a jellyfish. Jellyfish don't have a dedicated nervous system and probably can't be called "intelligent". But can you determine where in the sequence there is a cliff, an abrupt enough change that delimits "intelligence"? I have trouble doing that in a meaningful, non-arbitrary way.