Cleaner gobies can solve a biological market task when the correct cue is larger

Animal cognition is deeply influenced by interactions with the environment. A notable example of sophisticated cognition in the animal kingdom is described by the mutualistic relationship between cleaner fish and clients, where decision-making processes play a pivotal role in partner choice and fish survival. In this context, while extensive research has explored the cognition of the cleaner wrasse Labroides dimidiatus, its Caribbean counterparts, Elacatinus spp., have been comparatively inadequately evaluated. In this study, we used plexiglass plates as surrogates for clients and assessed the ability of cleaner gobies, Elacatinus oceanops, to solve a biological market task where prioritising an ephemeral food plate over a permanent one would double the food reward. We varied cue-based decision-making using both ecologically relevant cues (plate size and colour) and non-relevant ones (presentation side). Additionally, we tested their capacity for reversal learning, an indicator of complex cognitive abilities. Notably, cleaner gobies were able to solve the biological markets task when the distinguishing cue was a larger plate size. Given that these gobies tend to prioritise larger predatory clients in nature, our results align with their natural inclination. Furthermore, considering these gobies were bred in captivity and never experienced cleaning interactions in their lifetime, our data might suggest that their intrinsic cognitive abilities are shaped by evolutionary pressures rooted in their ecological roles. In essence, even in the absence of direct ecological interactions, innate cognitive abilities in cleaner gobies seem to be deeply influenced by evolutionary forces tied to their natural ecological functions. However, their inability to solve the same task involving other cues, may be influenced by factors such as captivity-reared fish or limitations in the experimental design. Further research, including wild individuals, is essential to elucidate the cognitive abilities of the studied species and its implications in the ecological context and evolutionary history.