Climate change has a negative impact on fish cognitive sophistication

Climate change poses a major threat to coral reef ecosystems worldwide. While much research focuses on the impacts of projected future climate change on coral reefs, current extreme weather events may be altering communities now. Here, we present the immediate negative effects of extreme weather disturbances on the cognitive sophistication of cleaner wrasse <i>Labroides dimidiatus</i> (hereafter ‘cleaners’). Cleaners are key organisms on coral reefs contributing to the maintenance of healthy fish communities through their removal of ectoparasites. Their ability to effectively interact with a range of client species requires sophisticated cognitive strategies, and thus, cleaners have become a textbook example of both the application of evolutionary game theory to cooperation and the ecological approach to cognition. We report a significant decline in the ability of cleaners to manage their own reputation and strategically prioritise clients, key components of their cognitive performance and the stability of this mutualism, following habitat destruction in 2014-2016. Individuals caught from a site heavily impacted by cyclones in 2014 and 2015 suffered this cognitive impairment when tested in 2016, compared to individuals tested in 2003 and 2010. Furthermore, cleaners from a site unaffected by cyclones, but heavily impacted by the 2016 el Niño event, showed similar performance impairments when tested only three months following the onset of coral bleaching compared to individuals from the same site tested two years prior coral bleaching. This implies that the reduced cognitive performance has ontogenetic causes. A disproportionally high (80%) reduction in cleaner densities emerged as potential explanation for the observed loss in sophistication. Given that extreme weather events, including cyclones and heat waves, are increasing in frequency, contemporary climate change may cause diverse and unexpected effects on species interactions, including the loss of more complex cognition.