Sea anemones are animals that look like flowers, with dozens of brightly colored tentacles that wave about in the water. The tentacles deliver a deadly poisonous sting-and when an unfortunate fish blunders into them, it’s usually killed and eaten by the anemone. Sea anemones secret a poison that kills fish if they come near them. Clownfish is the only fish not eaten by sea anemones. They live right among the tentacles without being harmed. Sea anemones and clownfish are partners in a relationship that benefits both animals. The clownfish cleans away debris from among the anemone's tentacles. And this debris is often food for the clownfish. In addition, the relationship gives the fish protection: Few predators will risk the anemone's sting to pursue them.
How does the clownfish avoid being stung? Before it tries to
swim among anemone's tentacles, the clownfish will brush lightly against the
anemone. It does this repeatedly, quickly swimming away ach time. Scientists
think that by doing this, the fish builds up an immunity to the anemone's
poison. But the fish must remember which anemone to go back to --if it swims into the wrong one, it won't be
immune. (Grolier Book of Knowledge Encyclopedia)