The Orange clownfish - Amphiprion percula - (also known as the Percula clownfish and Clown anemonefish), is widely-known as a popular aquarium fish. Like other clownfishes (also known as Anemonefishes), it often lives in close association with sea anemones.

The anemones have a highly toxic defence mechanism which is powerful enough to rapidly kill any other small fish which come into contact with their tentacles.

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 an 'acquired immunity' of some kind.

For decades, it has been hypothesized that anemonefishes are able to live within the stinging tentacles of host sea anemone species because the chemical composition of their mucus layer inhibits or lacks the trigger for firing host nematocysts. However, there is very little molecular evidence for this, beyond suggestions that glycans in the mucus could be key.

Source : Biorxiv, feb 2024

Further info at Wikipedia