Why are clownfishes not stung by sea anemones?

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Abstract
The clownfish Amphiprion clarkii is able to live unharmed amongst the tentacles of the sea anemone Stichodactyla haddoni . The latter has a powerful stinging response and would be capable of capturing any non-symbiotic fish that entered the tentacles. The presence of clownfish affects the anemone’s behaviour but does not impair its stinging ability, suggesting that a general inhibitory effect mediated by the anemone’s nervous system is not involved. A. clarkii achieves protection from stinging by means of its external mucus layer. This layer appears to be three to four times thicker than that of related fishes that do not inhabit anemones and consists largely of glycoprotein containing neutral polysac­charide. The mucus of A. clarkii remains inert after exposure to extreme denaturing conditions, suggesting that it does not contain specific nematocyte inhibitors or excitatory substances that are masked chemically; its inert nature probably results from a lack of those stimulatory compounds that are present in the mucus of non-symbiotic fishes. A. clarkii may become covered in a small amount of anemone mucus as a result of contact with its host; this is perhaps beneficial in that it increases the thickness of the inert layer covering the fish, but there is no evidence that A. clarkii is primarily dependent upon anemone mucus for protection.