Abstract
We used an ecological niche theory framework integrating physiological and behavioural ecology to understand interspecific differentiation of juveniles of four Antarctic fishes: Trematomus bernacchii , Trematomus pennellii , Trematomus nicolai and Pagothenia borchgrevinki . We focused on basal metabolic traits and the exploration‐avoidance axis of behaviour, two key dimensions of species fitness and drivers of niche differentiation. Metabolic metrics included routine metabolic rate (RMR) to understand resting energy requirements and Fulton's condition factor (CF) as a proxy for energy stores; together, these metrics give a baseline understanding of the energetic status of a species. Behavioural assays included the novel object test (NOT) and the novel tank test (NTT) to assess exploratory activity and anxiety. We found relatively low interspecific differentiation in basal metabolic demands—RMR did not differ across species and only T. nicolai exhibited higher CF than the other species. This suggests that T. nicolai may have greater on‐board energy stores available to cope with environmental stressors, although more research is needed to confirm this. While we generally saw behavioural differentiation across species, we did see evidence of a possible conserved freezing strategy in response to novelty, where T. bernacchii , T. pennellii and P. borchgrevinki reduced their activity after being exposed to a novel object. In both the NOT and NTT, species showed distinct exploratory tendencies, with T. bernacchii and T. pennellii spending more time in risk‐associated zones, T. nicolai being more avoidant and P. borchgrevinki exhibiting cautious exploration. We also observed substantial intraspecific variability in physiological and behavioural traits, particularly in T. bernacchii , T. nicolai and P. borchgrevinki , suggesting the presence of divergent behavioural phenotypes within species. Examining behavioural syndromes in these fishes is an important next step as behavioural syndromes are thought to be linked to metabolic status, individual fitness and species and community resilience. Collectively, our findings suggest that while basal metabolic demands appear relatively conserved across species at the juvenile life stage, divergent behavioural strategies could be a critical driver of niche differentiation in Antarctic fish assemblages and in species‐specific performance under environmental threats such as climate change.