Clownfish
Depth: 20 m · Sunlight Zone
Clownfish live safely among the stinging tentacles of sea anemones because a protective layer of mucus makes them immune. In return, they keep the anemone clean.
For scientists
A protective layer of mucus makes the clownfish immune to the stinging cells of the anemone it lives in safely; in return it drives off predators and keeps the anemone clean. All clownfish are born male; when the dominant female of a group dies, the largest male turns into a female.
Source: Wikipedia
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