Mucus coat protects against stings: clownfish
 

The skin of clownfish is protected from sea anemone stings by a coating of mucus.

   
  "The mystery behind the clownfish and sea anemone relationship is how the clownfish avoids being stung and killed by its host anemone. Of the numerous theories that have been presented over the years to explain this relationship, the focus is now on a layer of mucus that coats the clownfish. 'The fish are not immune to being stung,' said [Daphne] Fautin. 'But their mucus coat protects them. The debate is the source of the mucus.' One theory holds that the fish produce the mucus themselves and that it contains chemicals that prevent the anemone nematocysts from stinging as they do other fish in the sea. The other theory is that the clownfish rub themselves against the anemone tentacles in elaborate dances, smearing anemone mucus over themselves. This coating tricks the anemone into confusing the fish for itself. 'There is evidence for both,' said Fautin. 'And since there is a wide variety of anemone hosts, and 28 species of fish, I am convinced these views present two ends of a spectrum, and a combination is probably true for many.'" (Roach 2003)
  Learn more about this functional adaptation.