Fish line up at cleaning stations and wait their turn to be groomed

Small cleaner wrasses set up 'stations' on reefs where much bigger fish, even predators, queue up and hold still to have parasites and dead skin picked off their bodies and even out of their mouths and gills. The predators get free grooming; the tiny cleaners get a meal and safety in a relationship so reliable that removing cleaner fish from a reef measurably shrinks the number of other fish species living there.

— Alexandra S. Grutter, Cleaning Symbiosis Among Marine Fish — Advances in Marine Biology, 2002