Funny Fish Face

Some species use cleaner fish to remove external parasites. The best known of these are the Bluestreak cleaner wrasses of the genus Labroides found on coral reefs in the Indian and Pacific Oceans. These small fish maintain so-called "cleaning stations" where other fish congregate and perform specific movements to attract the attention of the cleaners. Cleaning behaviors have been observed in a number of fish groups, including an interesting case between two cichlids of the same genus, Etroplus maculatus, the cleaner, and the much larger Etroplus suratensis.
Evolution
Fish do not represent a monophyletic group, and therefore the "evolution of fish" is not studied as a single event.