This reverse fast food drive-up is beneficial to both, the Hawaiian Cleaner Wrasse getting nourishment and the host fish getting a free grooming and "delousing," so to speak. It is referred to as mutualism, a biological win-win situation. They'll even venture inside a moray eel's mouth for a little session of teeth cleaning, though it would be easy enough for the moray to make a meal of it. Seems that the moray and other host fish have figured out they are better served in the long run by a living, hard-working cleaner wrasse. At night these little fish, just around four inches in length, close up shop. Most small wrasses will snuggle into the sand for nighttime protection, but the cleaner wrasse sleeps in a cocoon of mucous (a behavior seen in one of their clients, the parrotfish), content with their excellent job security.
Hawaiian Cleaner Wrasse from Bertil Holmqvist on Vimeo.