The orange clownfish (Amphiprion percula) also known as percula clownfish and clown anemonefish, is widely known as a popular aquarium fish. Like other clownfishes (also known as anemonefishes), it often lives in association with sea anemones.
The anemones have a highly toxic defence mechanism which is powerful enough to rapidly kill any other small fish.
The clownfishes symbiotically live amongst the anemone tentacles, semi-permanently, with no apparent damage. There is currently no agreed explanation as to how they do this.
There are several hypotheses : including the theory that the fish are protected by a mucous layer, or that they have 'acquired immunity' of some kind.
Further info at Wikipedia