Taken from google:
"Clownfish and Damselfish are the only species of fish which can avoid the stings of an anemone, which can be quite potent. The exact mechanism by which this is accomplished is the subject of debate, and there are several theories which may all be partly responsible. The details of these theories are complex, but they fall into two major categories. One theory is that their slime coating is based on sugar rather than proteins so anemones fail to recognize the fish as food and do not fire their nematocysts, or sting organelles. A similar theory is that the mucous coating mimicks the anemone's own coating, a theory that is bolstered by the fact that it takes several days for a Clownfish to adapt to a new species of anemone. There is no acclimation period when a Clownfish is moved to another anemone of the same species. It is interesting to note that not all anemones make suitable hosts--many can and will sting and eat Clownfish. Also, particular species of clownfish will only use particular species of host anemones in nature. In captivity, certain Clownfish species will adapt to certain other anemone species, but not many. Another likely possibility is that their unique movements, which are unlike any other fish, let the anemone know that they are not food. This theory is bolstered by the fact that juvenile Clownfish, which have no coating, will immediately seek refuge in any compatible anemone and will not be stung. Juvenile clownfish will not survive for long without the protection of an anemone, and few actually find one before being eaten. "
