Fish line up at 'cleaning stations' and let a shrimp pick parasites from their gills

Cleaner shrimp and cleaner wrasse fish run grooming stations on reefs, where client fish, some large enough to swallow the cleaner whole, line up and wait their turn to be picked clean of parasites and dead skin. Clients signal they want cleaning with specific postures, and cleaners even enter the mouths and gill chambers of predatory fish without getting eaten. It's one of the clearest examples of mutualism between species with wildly unequal power over the interaction.

— Conrad Limbaugh, Cleaning Symbiosis — Scientific American, 1961