The most common types of sea anemones found in coral reefs are those that are hosts to the clownfish, or the anemone fish.
Sea anemones are coelenterates, and their tentacles have numerous stinging cells called nematocysts.
The answer lies in the mucus covering that these clownfish have. The mucus layer protects the clownfish from the stinging cells from the sea anemones. The nematocysts found on the sea anemones are usually not big enough to cause any injuries, as they are not able to penetrate the skin. However, there are some that may cause quite a reaction from victims. The usual cause of injuries involving sea anemones would be contact with open wounds, including scratches, abrasions, and cuts.