Why don't big predatory fish eat cleaner fish, the small reef-dwellers who groom and eat parasites off of their customers? Because they have a distinctive uniform!
Cleaner fish "uniforms" attract clients with stripes
Cleaner fish tend to have a dark side stripe set apart by patches of blue and yellow; this highly conspicuous "uniform" serves as an advertisement to draw in potential clients. A team of researchers planted fake fish into a reef and found that, indeed, fish with stripes and blue colors drew in more "clients" ready to be groomed. Somehow, fish such as barracuda, damselfish, and surgeonfish all know that a certain pattern of stripes and colors indicates "BARBER SHOP" rather than "MEAL". It is likely that the cleaning behaviour evolved first, after which the "uniform" became associated with a positive grooming outcome for clients. The relationship is, of course, mutually beneficial; the cleaner fish get a meal out of it, and their clients swim away cleansed of surface parasites. However, remarkably, the positive value of a grooming interaction goes beyond the obvious fitness benefits.
Fish enjoy massages, too: the tactile stimulation of "cleaning" lowers stress in fish
A Nature Communications study published in 2011 found that the experience of physical stimulation alone reduces stress in surgeonfish (reef-dwellers). Cleanerfish, such as the wrasse Labroides dimidiatus, regularly touch their clients with their fins, so these researchers simulated a cleaning experience to isolate the effects of the "massage" without the fitness benefits. Indeed, the physical stimulation alone reduces stress in the coral reef surgeonfish Ctenochaetus striatus. In other words-- fish enjoy massages for their own sake.
Cleaner fish are expert at managing their client relationships
Amazingly, cleaner fish -- who sometimes have as many as 2,300 interactions with various clients in a single day -- recognize familiar clients and prefer to spend time with clients with whom they have already established a relationship.
Further, cleaner fish have different strategies depending on how hungry their client is: cleaner fish both massage and "dance" for customers who appear to be especially hungry. The tactile dancing behaviours appears to calm down potential predators to allow cleaner fish to handle "dangerous" visitors. From a paper published in Current Biology comes this intriguing conclusion: "cleaner fish tactically stimulate clients while swimming in an oscillating "dancing" manner (tactile dancing) more when exposed to hungry piscivorous clients than satiated ones, regardless of the client's parasite load."
And if a cleaner fish tries to nibble at a little skin or mucus instead of just ectoparasites? The client will take her business elsewhere.
Like police and nurses, cleaner fish on coral reefs wear uniforms to signal their "professions"—a tactic that also helps the fish avoid being eaten by their clients, a new study says. Several species of small reef fish are known to invite larger fish to stop by "cleaning stations," where the cleaners groom their customers and pick them free of parasites. The clients swim away spic-and-span, and the cleaners get an easy meal: "a classic example" of a mutually beneficial relationship, the researchers write.