Breana Marie Riordan , Ludovic Dutoit, Tania King, Luciano Beheregaray, Neil Gemmell, Anthony Hickey, Sheri Johnson

Understanding species capacities to adjust to shifting thermal environments is crucial amidst current climate-mediated ocean warming. Fish populations displaying high thermal plasticity can undergo molecular, metabolic, and mitochondrial modifications in response to heat stress. Under the context of heat stress, such acclimation provides a means to maintain normal biological functions through alteration of thermal performance and provides a model to dictate which species will persist when this stress becomes prolonged. Here we combine measures of mitochondrial physiology (using a novel fluorescent technique) and gene expression analyses to investigate thermal resilience and acclimation capacity of two closely related endemic triplefin species, the intertidal common triplefin (Forsterygion lapillum) and the estuarine triplefin (F. nigripenne). Triplefins are an ideal evolutionary model to explore the molecular basis of thermal resilience. Both species evolved in thermally variable environments and are thus predicted to display resistance to heat stress. We observed enhanced mitochondrial function at higher temperatures, although only ATP production was significantly enhanced for both species. Different gene expression profiles were detected between warm acclimated and control fish, with high interspecific variation in acclimatory responses across brain transcriptomes. Differential gene expression and gene ontology highlighted an induction of stress response pathways and oxidoreductase activity in warm acclimated tissues, alongside a rearrangement of metabolic functions facilitating increased carbohydrate metabolism. Our findings indicate thermal acclimation potential in both species, with plasticity in mitochondrial performance enhancing upper thermal tolerance and transcriptional evidence of thermal compensation and homeostatic adjustments under warming conditions. Overall, these results demonstrate robust mechanisms of resilience in coastal fish species that have evolved under climatic variable conditions and provide a new methodological approach for future thermal studies.