Cnidarian stinging cells fire in response to combined chemical and mechanical cues. Anemones do not sting themselves, because their own mucus lacks the chemical triggers that provoke discharge. Anemonefish exploit that same rule.
A fish introduced to a host performs a graded acclimation, brushing the tentacles briefly and retreating, extending contact over a period that ranges from under an hour to several days depending on species pairing. During this the fish's mucus coat changes, and the resulting layer suppresses nematocyst discharge. Experiments in which the coat is removed show the protection goes with it, and the fish must acclimate again.
What remains genuinely open is the source of the change. One long-standing hypothesis is that the fish acquires anemone-derived mucus during contact; another is that it alters its own mucus composition. Evidence has been reported for both, and the balance may differ across the roughly thirty anemonefish species and ten host anemones. The relationship itself is mutual: the fish gains shelter and the anemone gains defence against tentacle-nipping butterflyfish along with nutrients and improved water flow.