Oh, the slave species thing is outdated information from the original study in the 90’s. To be fair, that study is still very very respected, it was just wrong about this specific thing.
Their anemones aren’t a unique species, they are only visually distinct because the crab farms them in a very specific way - making a fragment of a larger hawaiian anemone, or finding a newly spawned one. When they carry it around, they force the anemone to adapt to an entire new lifestyle. The crab’s nocturnal behavior slowly kills the anemone’s photosynthetic symbionts, from which it gets its color. After that, the crab can control how much food the anemone actually gets by using it to mop up food off the sea floor, and take excess out of its mouth. Since it doesnt have to reach for sunlight, or reach to catch food, this encourages dense, but short tentacles.
The shortcoming of this study was just that its very hard to get the anemone away from the crab without killing both, and its equally hard to get an anemone to bounce back from this state.
Due to their introduction into home aquaria, we now know any anemone they take will end up in this state, (bubble tips, aipistasia, haitian anemones, the list goes on) and we also know they are not bound to carrying anemone’s specifically. They’ve been seen collecting zoanthids, palythoa, and euphyllia (though euphyllians usually don’t survive).