Mitochondria: at the heart of fish thermal plasticity

Cardiac mitochondria in fish are remarkably plastic, with the capacity to change in volume and density, membrane composition and fluidity, aerobic capacity and reactive oxygen species production. This flexibility allows the highly aerobic fish heart to cope with temperature variations, thus enhancing animal fitness and survival, a feature that is particularly important for species living under narrow thermal regimes. This Review explores the remodelling of fish cardiac mitochondria in response to temperature variation, focusing on their role in supporting heart function. It highlights the mechanisms underlying these adaptations, the impact of acute and chronic thermal stress, and specific responses to environmental challenges with reference to the pressure imposed by rapid and extreme thermal events. The unique adaptations of Antarctic fish to cold environments are used to exemplify extreme evolutionary events, with distinct mitochondrial morpho-functional features facilitating survival under frigid conditions. Throughout the Review, attention is given to mitochondrial membrane dynamics, oxidative phosphorylation and the production of reactive oxygen species. Despite growing research effort, species-specific phenotypic and genotypic mechanisms that sustain the response of fish cardiac mitochondria to fluctuating temperatures are not fully understood. Further research in this area is important not only in terms of basic knowledge but also to understand how global climate change shapes the energetics of fish cardiac performance, influencing animal resilience.