| Summary: | The Southern Ocean surrounding the Western Antarctic Peninsula region is rapidly warming. Survival of members of the dominant suborder of Antarctic fishes, the Notothenioidei, will likely require thermal plasticity and adaptive capacity in key traits delimiting thermal tolerance. Herein, we have assessed the thermal plasticity of several cellular and biochemical pathways, many of which are known to be associated with thermal tolerance in notothenioids, including mitochondrial function, activities of aerobic and anaerobic enzymes, antioxidant defences,protein ubiquitination and degradation in cardiac, oxidative skeletal muscles and gill of Notothenia coriiceps warm acclimated to 4◦C for 22 days or 5◦C for 42 days. Levels of triacylglycerol (TAG) were measured in liver and oxidative and glycolytic skeletal muscles, and glycogen in liver and glycolytic muscle to assess changes in energy stores. Metabolic pathways displayed minimal thermal plasticity, yet antioxidant defences were lower in heart and oxidative skeletal muscles of warm-acclimated animals compared with animals held at ambient temperature. Despite higher metabolic rates at elevated temperature, energy storage depots of TAG and glycogen increase in liver and remain unchanged in muscle with warm acclimation. Overall, our studies reveal that N. coriiceps displays thermal plasticity in some key traits thatmay contribute to their survival as the Southern Ocean continues to warm. Excel Funding provided by: National Science Foundation Crossref Funder Registry ID: http://dx.doi.org/10.13039/100000001 Award Number: PLR-1341602 Funding provided by: National Science Foundation Crossref Funder Registry ID: http://dx.doi.org/10.13039/100000001 Award Number: PLR-1341663 Funding provided by: National Institutes of Health Crossref Funder Registry ID: http://dx.doi.org/10.13039/100000002 Award Number: 2P20GM103395
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