Sometimes the anemone makes a meal of its victims. But a chosen few creatures such as anemonefish, also often called clownfish, nestle within the tentacles without being harmed by their toxin-filled capsules, called nematocysts. The key to that ability lies in the fish's protective coating, the source of which has turned out to be a complex and sometimes baffling scientific puzzle. Anemonefish are not the only creatures to reside unharmed within the killers' reach. And spider crabs and shrimp live hidden from predators beneath the blanketlike folds of the anemone's base.
Many crabs even carry around small sea anemones as defensive weapons.
To find the answer, researchers have focused on anemonefish, which are among anemones' most brilliant and colorful guests. Each theory has supporters, and each may be true, depending upon the species of fish and anemone in question. Anemonefish depend on anemones for survival. The fish dive within the hosts' tentacles when threatened and seldom venture a great distance from that refuge. The fish's mucus is thought to be such an effective defense because it is especially thick and lacks substances that trigger nematocysts.