Choosy reef fish select cleaner fish that provide high-quality service

Reef fish that actively visit cleaner fish to have parasites and dead or infected tissue removed face two potential problems: they might have to wait while cleaners inspect other clients, and cleaners might feed on healthy body tissue, a behaviour that is referred to as cheating. Individuals of some ‘client’ species have large home ranges, which cover several cleaning stations, while others have small territories or home ranges with access to only one cleaning station. The former can thus choose between cleaners, while the latter cannot. We investigated whether clients with large home ranges change cleaning partners to outplay cleaners against each other to achieve (1) priority of access over clients with no choice at cleaning stations and (2) control over cheating by cleaners. We followed individuals of longnosed parrotfish, Hipposcarus harid, for up to 120 min in their natural environment and noted their interactions with cleaner wrasses, Labroides dimidiatus. Individuals were likely to return to the same cleaning station if the previous interaction had ended without conflict but changed cleaners for the next inspection if they had been either cheated or ignored, at least if the time between two consecutive visits was short. The overall attractiveness of a cleaning station seemed to be largely independent of service quality, which appeared to be similar at all stations. This is the first empirical evidence that the option to change partners is used as a control mechanism to stabilize cooperative behaviour.