« Street-side businesses like barbershops and restaurants that provide on-site services rely on [...] loyal patrons to sustain their livelihoods. In a competitive world, it is only by delivering a good product that they are likely to build their client base. [...] It isn’t much different on the reef.
Consider the cleaner-client symbiosis of fishes. It is one of the most complex and sophisticated social systems of any animals. The system works as follows. One or two cleanerfishes signal that they are open for business. They work at specific locations, and may use swimming postures and bright colors to enhance visibility (a fish’s version of the rotating red-white-blue cylinder outside a barbershop). Other fishes of various types congregate at the cleaning station, where they wait their turn to be serviced by the cleaners, [who] pick over the clients’ bodies, removing parasites, dead skin, algae [...]. Clients benefit by receiving a spa treatment, including parasite removal. Cleaners get fed.
So important is the cleaning station to reef fish communities that cleaners can have a major effect on the diversity of fish species on a reef. [R]esearchers [found] that many fish species—particularly those that migrate between reefs—choose reefs based on the presence of cleanerfishes. [...]
The relationship between a cleanerfish and his or her client is not random. It is built on trust, and cultivated over weeks or months. A social contract such as this requires that individual cleaners recognize their clients. With dozens of clients per cleaner, cleanerfishes maintain an impressive mental database of clientele. In choice experiments where a cleaner could choose to swim near one of two clients, the cleaner spent more time near a familiar one. [...]
Clients are not passive participants. Prospective clients watch the performances of cleaners before deciding whether to let a particular cleaner inspect them. By doing this, client fishes accumulate—I’m not making this up—an “image score” for a given cleaner. Think of it as a fish’s version of eBay seller ratings. Cleaners have reputations [...].
When it is their turn, [clients] approach the cleaning station and hover in place, spreading their fins to help cleaners reach all the nooks and crannies. Some open their mouths and gill covers to allow the usually much smaller cleaners to enter and exit. A cleaner will sometimes butt her snout against fins and gill covers to signal the client to spread them for inspection. [...]
While the cleaner is busy, the [client] keeps an eye out for any possible threats. If the cleaner happens to be in the [client]’s mouth when danger is approaching, the [client] snaps its mouth closed but leaves just enough room for the cleaner to escape and dart into a safe cranny in the reef. »
— Jonathan Balcombe, What a Fish Knows: The Inner Lives of Our Underwater Cousins
