Caribbean Cleaning Gobies Prefer Client Ectoparasites Over Mucus

If cooperation often involves investment, then what specific conditions prevent selection from acting on cheaters that do not invest? The mutualism between the Indo-Pacific cleaner wrasse <i>Labroides dimidiatus</i> and its reef fish clients has been a model system to study conflicts of interest and their resolution. These cleaners prefer client mucus over ectoparasites – that is, they prefer to cheat – but punishment and partner switching by clients enforce cooperative behaviour by cleaners. By contrast, clients of Caribbean cleaning gobies (<i>Elacatinus</i> spp.) do not to use punishment or partner switching. Here, we test the hypothesis that the behavioural differences between these two cleaner fish systems are caused by differences in cleaner foraging preferences. In foraging choice experiments, we offered broadstripe cleaning gobies <i>Elacatinus prochilos</i> client-derived parasitic isopods, client mucus and a control food item. The cleaning gobies significantly preferred ectoparasites over mucus or the control item, which contrasts with cleaner wrasses. We propose that the low level of cleaner–client conflict arising from cleaning goby foraging preferences explains the observed lack of strategic partner control behaviour in the clients of cleaning gobies.