A sea anemone is basically a flower made of stinging tentacles. Touch one and most fish get a painful jolt. But clownfish swim happily through the stings, sleep tucked into them, and even hatch their eggs inside them.
The trick is a slimy mucus layer on the clownfish’s skin. When a clownfish moves into a new anemone, it slowly rubs against the tentacles, picking up a coating that fools the anemone into thinking the fish is part of itself. The stingers don’t fire.
Both partners benefit. The clownfish gets a fortress full of stinging arms that keep bigger predators away. The anemone gets a tiny housekeeper that drives off butterflyfish (which eat anemones), drops crumbs of food, and even fans water through the tentacles to give it fresher oxygen.
