Punishment can help to sustain cooperation where it would otherwise fail. ~ English zoologist Nichola Raihani
Cosmopolitan coral reefs make an irresistible place for ectoparasites to feed. Fortunately, blighted fish can get cleaned free of charge. Cleaner fish, such as the cleaner wrasse, work hard at keeping their clients unsullied.
Cleaning is competitive. Cleaner fish provide a higher-quality cleaning service when competitors are around.
An individual wrasse inspects as many as 2,300 fish a day, consuming up to 1,200 parasites from clientele. This sums to 7% of a wrasse’s body weight.
Each male cleaner wrasse holds a territory which encompasses several female breeding partners. For cleaning services, cleaner wrasse often work in pairs: a male and a female.
Although the service that cleaner wrasse provide is removing skin ectoparasites, they prefer to feed on client tissue. A client won’t sit still for that.
In order to receive a good cleaning service, clients require cleaners to cooperate by feeding against their preference. Clients achieve this either by avoiding cleaners they observe cheating other clients, avoiding cleaning stations where they have received a poor service in the past, or aggressively punishing cheating cleaners. ~ Nichola Raihani
Male wrasse keep their ladies in line; punishing them for cheating by aggressively chasing after them. The larger the client, the more esteemed. To females who cheat, male cleaners mete out a level of punishment commensurate with how valuable the client was.
Female wrasse may switch sex if they become as big as their partner. This is the real incentive for male wrasse to keep females in line, beyond the immediate problem of losing clients. A female that eats too well becomes a competitor.
In contrast to wrasse, Caribbean cleaner gobies do not change sex. Male gobies do not punish females that cheat.
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The service provided by cleaner fish is invaluable to clients. Fish that fail to go to the cleaners have 5 times as many ectoparasites as those that do.
Fish cleaning is not without hazard, and not just from snappy clients or greedy workmates. Cleaner shrimp form monogamous pairs that claim exclusive service areas. Competitors are eliminated, typically during the night while shedding old skin, when they are momentarily more vulnerable.