Manuscript prepared for the Dahlem Workshop on Genetic and Cultural Evolution of Cooperation Biological markets: the ubiquitous influence of partner choice on the dynamics of cleaner fish-client reef fish interactions

The applicability of biological market theory with its emphasis on partner choice is explored using the interactions between the cleaner wrasse Labroides dimidiatus and its ‘client’ reef fish as a model system of mutualism. The cleaners have small territories, which the majority of reef fish species actively visit to invite inspection of their surface, gills and mouth. Clients benefit from the removal of parasites while cleaners benefit from the access to a food source. Some client species (choosy clients) have large home ranges that cover several cleaning stations while other clients have small ranges and have access to one cleaning station only (resident clients). Field observations, field manipulations and laboratory experiments revealed that whether or not a client has choice options influences several aspects of both cleaner and client behaviour. Cleaners give choosy clients priority of access. Choosy clients switch partners if cheated by a cleaner (=cleaner feeds on mucus/scales), while resident clients punish cheats. Cleaners and resident clients, but not choosy clients, build up relationships before normal cleaning interactions take place. Cleaners are particularly cooperative if choosy clients are bystanders of an interaction but less so when resident clients are bystanders. When it comes to the frequency of cheating by the cleaner fish, however, partner choice options are overrun by client control mechanisms: predatory clients are far less often cheated than non-predatory clients, irrespective of choice options. Future research should focus more on empirical testing of game theory, and this new information should be used to formulate deductive models.