This is a frustrating response to my comment. I am aware that symbiosis is universal. That's not what I'm talking about. I am talking about the specific and highly unusual behavior of crawling inside of a much larger animal's mouth and trusting it not to eat you. Cleaner fish are highly intelligent[1] and it appears that this intelligence is necessary for their niche:

- picking a good location for a cleaning station requires long-term planning and real strategic judgment;

- deciding which hosts to accept is a complex skill requiring some sort of rudimentary theory of mind + long-term development of social ties;

- like crows, cleaner fish are jerks who constantly try to screw each other over, so there is something of a cognition arms race.

I will add that the wrasse family of cleaner fish use rocks to smash open shellfish (i.e. they are tool-users), and they have very complex group strategies for raising their young. In fact I'm not convinced that wrasse evolved to be cleaner fish at all: they are natural scavengers and scum-suckers, perhaps cleaning stations are a form of cultural technology.

I would be extremely surprised if any of this was true for cone ants. I suspect that is more hard-wired, perhaps a local subspecies stumbled into a genetic fluke, and as you say due to game theory it is a local optimum this population has settled on. If this behavior were common like it is in vertebrates, we probably would have seen it earlier. But who knows? 20 years ago I would have thought "fish have a form of culture" is too ridiculous an idea to consider.

[1] Seriously: https://journals.library.columbia.edu/index.php/cusj/blog/vi... https://www.nature.com/articles/s41598-025-25837-0

Scientists discover "cleaner ants" that groom giant ants in Arizona desert