Published online 29 September 2008 | Nature | doi:10.1038/news.2008.1139

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Piscine predators distracted from hunting during their trip to the spa.

A cleaner wrasse gives the larger Oriental Sweetlips (Plectorhinchus vittatus) a full service. WaterFrame / Alamy

Only humans and a handful of other primates will attempt to make peace between warring third parties. But now there is a new diplomat on the block: the cleaner wrasse.

Cleaner wrasse (Labroides dimidiatus) feed on the parasites that attach themselves to the outsides of other fish, and even throw in a calming massage with their pectoral fins. In return for the services, client fish don't attack their cleaners and return regularly to their territories to supply them with more food.

The cleaner-fish scenario is a textbook example of mutualism, in which each species benefits, but ecologist Karen Cheney at the University of Queensland, Australia, suspected that there was more to it. Cheney had seen little evidence of aggression between reef-fish clients at cleaning stations and few instances of predation. Fish also often stayed inside cleaner territories long after the cleaning was over, suggesting that the territories could be functioning as a safe haven.

“It's like when the rough types from the street enter the barber shop, they stop punching each other and behave.” Frans de Waal

Emory University

To test the theory, Cheney and her colleagues set up three aquaria, each of which contained a combination of carnivorous and herbivorous cleaner fish clients. In the first aquarium, they introduced a cleaner fish. In the second, they used a control fish that was neither predator nor prey to see whether the presence of a third type of fish changed the social dynamics between the herbivores and the carnivores. The third aquarium contained no extra fish. The researchers closely monitored behaviour — recording chasing, predation and cleaning.

The team report in Behavioral Ecology1 that in the second and third aquaria, the herbivores were chased 7-10 times in 30 minutes, and three prey species were eaten. But in the aquarium containing the cleaner wrasse, there were just 2-3 chases in 30 minutes and no fish were eaten.

Calm down

"This nicely illustrates how direct interactions between individuals can have carry-over effects on others, creating 'safe zones' for prey," say evolutionary biologist Dan Blumstein at the University of California, Los Angeles.

But why should predators cease hunting in the presence of a cleaner fish? "Why not catch prey while you're waiting?" asks zoologist Frans de Waal at Emory University in Atlanta, Georgia. De Waal thinks that the predators might be going into a different 'mode' when they enter the cleaning area and focus less on catching prey because of their altered state. "Animals often go into a foraging mode, social mode or travel mode instead of trying to do everything at once. Perhaps being in the presence of a cleaner makes them drop the hunting mode," de Waal says.

"It's like when the rough types from the street enter the barber shop, they stop punching each other and behave. The barber provides a calming activity, like the cleaner fish does."

The study also showed that the longer a predator was cleaned, the less likely it was to chase nearby prey once the cleaning ended, revealing that cleaner wrasse have the ability to directly alter their clients' behaviour. "But we will only know for certain that they are intentionally mediating between species when we measure whether they increase the amount that they massage predators when large numbers of prey are nearby," Cheney says.