In spite of such a bright colour and a defenceless transculent body, fish do not attack Flabellina (neither most of other nudibranchies). There is a special astonishing device, that cares about it: nematocysts (stinging cells) of the hydroid polyps, that the slug is feeding on do not get digested but move through the lever to the tips of papillae called cerata on it's back (among many other functions these are secondary gills), where they are stored forming likeness of a syringe loaded with venom, that stings if being touched by an enemy.
It was observed, that young inexperienced fish sometimes attempt upon a slug, but never repeat an encroachment for the second time.