A fish called a cleaner wrasse spends its time cleaning parasites, dead scales, and other tissue off the bodies of other fish. Cleaner wrasse reside and set up shop in areas known as cleaning stations. They're sort of the spas of the sea.
Other fish actually line up for the service the wrasse provides! A lot of the wrasse's clientele are much bigger fish. These fish present themselves as passive and ready for cleaning by remaining still, spreading their fins and gills, and opening their mouths.
The cleaning is a somewhat invasive process. The wrasse not only cleans the outside of the fishes' bodies, but cleans inside the gills and mouth too.
What does the wrasse get out of it? Why, a good parasite meal, that's what.
Luckily, their larger clients almost never harm them. In fact, the cleaning station is one of the few places where diverse fish congregate without showing aggression.
It's not yet certain whether the parasites pose any danger to the fish, or even need to be removed. It does seem, at least, that the presence of cleaner wrasse leads to reef fish diversity.