Agonistic behaviours between conspecifics are key modulators of social status, with conflicts usually arising from competition for resources. Recent research has shown that CO₂ levels can affect the behaviour of marine organisms due to the impairment of neurotransmitter function, specifically the GABAergic system. Since this neurochemical pathway has been consistently associated with aggression management, rising CO₂ levels in the ocean (followed by a concomitant pH reduction) may directly influence the outcome of agonistic interactions and change conspecifics’ behaviour. Additionally, future ocean warming may amplify such physiological and behavioural challenges. Within this context, here we present, for the first time, the potential synergistic effects of ocean warming (+3^oC) and acidification (∆pH=0.4) on the intraspecific agonistic behaviour of temperate and tropical marine invertebrates (shrimps Palaemon elegans and Lysmata amboinensis) and vertebrates (fish Salarias fasciatus and Salaria pavo). After 30 days of acclimation, individuals were allowed to interact with conspecifics in an inclusive set of behavioural and metabolic assays designed to access shifts in aggression levels and clarify the underlying mechanisms. Besides describing how these environmental factors will differently affected distinct taxonomic groups in terms of their behavioural and physiological ecology, we also discuss how these future conditions may shape the fitness and dynamics of temperate and tropical populations.