UCL Psychology and Language Sciences


Cooperation in the cleaner fish-client mutualism

How and why do fish help one another out?

Tropical filsh

A cleaner fish pair servicing a giant potato cod.

The bluestreak cleaner wrasse, Labroides dimidiatus, can be found on coral reefs throughout the Indo-Pacific. These little fish live in small groups on cleaning stations, and offer a cleaning service to other reef fish by eating the surface parasites that live on the clients’ skin. All is not as harmonious as is appears, however. Clients want cleaners to eat the ectoparasites but cleaners prefer to feed on mucus and scales – which is harmful and painful for the client. The cleaner-fish client mutualism is therefore a wonderful model system for exploring how conflicts of interest are resolved and for identifying the mechanisms that support cooperation in real-world interactions.

I collaborate with Prof. Redouan Bshary to investigate the rules that  cleaner wrasse follow when providing a service to clients. The cleaner fish system is highly complementary to my work on humans since punishment and partner choice are thought to be key drivers of cooperative behaviour in both systems. Plus, working on cleaner fish means you get to visit Lizard Island Research Station.

Our research on this system has produced some of the following key findings:

  • Third-party punishment can yield individual benefits to punishers: Third-party punishment occurs when one individual intervenes to punish a cheat on behalf of a victim. In humans, it has been repeatedly suggested that this behaviour can only emerge via group-level selection. We show experimentally that cleaner fish invest in third-party punishment: male cleaners punish their female partners if the female bites a client visiting the cleaning station. However, this punishment is directly beneficial to the male cleaner: females are more cooperative after they are punished, and males have more opportunities to forage on joint clients as a consequence.
  • Punishment fits the crime in fish: Not only do male cleaners punish their female partners when the latter cheat, but they also adjust the severity of the punishment according to the female’s crime. Specifically, we show experimentally that male punishment is more severe when females cheat an attractive (i.e. large) client – and also when the female is closer in size to the male (because females who out-grow their male partners change sex in this species).
  • Familiarity breeds contempt in cleaner fish: We show experimentally that female cleaner fish are more cooperative in pairwise client inspections when they are paired with an unfamiliar male compared to when they are paired with a familiar male.
  • Cleaner fish don’t care about ‘fairness’: We explored experimentally whether cleaner fish show inequity aversion – the tendency to sacrifice own resources (or to work less hard) when a partner receives a better payoff. They don’t.