Heatwaves beneath the ice: organ-specific stress responses in Trematomus bernacchii, an Antarctic sentinel species.

Over the past few decades, the climate has undergone significant transformations, with increasing temperatures emerging as one of the most critical threats to marine ecosystems. Even the Southern Ocean and the Antarctic continent are not safe from these changes, and the increase in water temperatures is becoming a growing concern for the scientific community. This study focuses on gene expression in the liver and spleen, key organs in immune defence and metabolic regulation. The expression levels of antioxidant-related genes (prdx3, prdx5, cat, sod1, sod2, gpx1, gpx3, gpx4) along with two immune-related genes (trl2 and trl9) were analysed to assess how both systems respond to thermal stress and how they interact. Adult specimens were collected from Terra Nova Bay (Ross Sea) and exposed to a temperature gradient ranging from 0°C to +3°C; the results were compared with those of a control group maintained at 0°C. The liver showed an early and dynamic response: gpxs, sod2, prdxs and tlrs were upregulated at +1°C and +2°C but downregulated at +3°C, suggesting early mitochondrial antioxidant and immune activation. In contrast, the spleen remained largely unresponsive at mild stress levels, with marked activation of gpx1, sods, prdxs and tlrs only at +3°C. Catalase remained unresponsive to thermal stress. These results indicate a delayed but coordinated response, possibly reflecting an energy-conserving strategy under escalating stress. This research offers valuable insights into the coordinated responses of the antioxidant and immune systems of Antarctic fish to global warming, helping to address both the plasticity and vulnerability of the Antarctic ecosystem.