Temperature is a major stressor within marine environments, acting to modify the physiological and biochemical parameters of organisms. The capacity to adjust to shifting thermal conditions, such as those in the context of climate-mediated ocean warming, is crucial for the persistence of marine ectothermic species. Studies are increasingly showing that fish populations displaying high innate thermal plasticity can undergo molecular, metabolic, and structural remodelling at all biological levels of organisation in an integrated response against heat stress. Mitochondria are considered to play a pivotal role in determining the thermal tolerance limits across fish species. Additionally, most thermal plasticity in ectothermic species occurs through alterations to the gene expression and transcriptome of heat-exposed tissues. Current research implicates mitochondrial adjustments in maintaining aerobic respiration and scope under elevated temperatures in teleost species, alongside metabolic shifts in the activity of enzymes contributing to aerobic versus anaerobic respiratory pathways. Transcriptomic studies demonstrate distinct transcriptomic profiles for fish under different thermal environments. This thesis aimed to investigate the thermal resilience and capacity for thermal acclimation in South Island populations of two endemic triplefin species, the common triplefin (Forsterygion lapillum) and the estuarine triplefin (Forsterygion nigripenne), highlighting the roles mitochondria and differential gene expression play in thermal resilience. This study represents the first research into the thermal physiology of the estuarine triplefin and the first transcriptomic analyses conducted on either species.

The common and estuarine triplefins were exposed to four temperatures (10, 14, 18 and 22℃) for 4-weeks before using a novel fluorescent approach to assess the performance of membrane potential and ATP equilibrium, used as a proxy for maintenance of ATP production, in brain mitochondria under thermal ramping scenarios. Additionally, the enzyme activity of citrate synthase (CS) and lactate dehydrogenase (LDH) were measured as indicators of aerobic and anaerobic potential, respectively, in the acclimated fish. RNA-Seq analyses were then performed on brain and heart tissue from fish subject to the control (10℃) and 22℃ temperatures, to compare the gene expression profiles of fish from the two treatments. From this data, differential gene expression and gene ontology analyses were performed to reveal genes and gene pathways enriched in the warm acclimated compared to control fish. 

Results demonstrated that both the common and estuarine triplefin from warm acclimated treatments enhanced maintenance of mitochondrial function at higher temperatures compared to fish from cooler treatments, though only ATP production was significantly enhanced in both species. Enzyme activity levels decreased with acclimation temperature for the estuarine triplefin but were less consistent for the common triplefin. Transcriptomics revealed significantly different gene expression profiles between warm acclimated and control fish. Tissue-specific responses occurred in the brain and heart tissues, respectively, with a greater variation in acclimatory responses between the brain tissue of the two triplefin species. Differential gene expression and gene ontology highlighted an induction of stress response pathways, heat shock protein genes and oxidoreductase activity in warm acclimated tissues, alongside a rearrangement of metabolic functions facilitating increased carbohydrate metabolism.

In conclusion, the southern populations of both species display plasticity in mitochondrial performance, enhancing upper thermal tolerance. Furthermore, transcriptional alterations indicated that both species underwent thermal compensation and homeostatic adjustments under warming conditions. Overall, these results support strong resilience for these triplefin species against future warming.