Ocean deoxygenation and warming disrupt cooperation in coral reef fish mutualisms

Abstract Climate change is intensifying ocean deoxygenation and warming, compounding stressors that threaten tropical marine ecosystems. On coral reefs, cleaner wrasses sustain fish health and community stability by removing ectoparasites from client species during cooperative interactions. However, the mutualism is inherently unstable, as cleaners prefer to feed on clients’ protective mucus—a behavior considered cheating. To maintain cooperation, clients use partner control strategies, while cleaners employ sophisticated social techniques to manage conflict and maximize food intake. In this study, cleaner wrasses (Labroides dimidiatus) and their clients (Naso elegans) were exposed for at least 33 days to projected climate scenarios of ocean deoxygenation (90% O2 sat), warming (+3 °C), and the combination of both stressors. We assessed the resilience of the mutualistic interaction through a cooperativeness task (feeding against preference test), measuring learning and cooperation, and through cleaning interactions. Deoxygenation and warming significantly reduced the proportion of time spent in cleaning interactions and led to the breakdown of behavioral strategies, including tactile stimulation and clients’ retaliation. Notably, dishonesty persisted at similar levels during natural interactions, likely due to altered client responses rather than maintained self-control. Clients under stress were more tolerant to dishonesty, allowing cheating to go unpunished. This is supported by the cooperativeness test, in which cleaners exposed to deoxygenation exhibited reduced cooperation and diminished ability to suppress preferred but non-cooperative behavior, suggesting impaired cognitive control. Our results reveal that climate stressors impair cognitive and behavioral mechanisms underpinning cleaning mutualisms, potentially destabilizing key trophic interactions on coral reefs under future ocean conditions.