Heatwaves beneath the ice: organ-specific physiological responses in the Antarctic emerald rockcod, Trematomus bernacchii (Boulenger, 1902)

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. Mounting evidence of marine heatwaves (MHWs) events in the Southern Ocean and Antarctic region has intensified scientific efforts, revealing that these fragile ecosystems may act as crucial early indicators of wider climate-driven changes in marine environments' biodiversity. This study explores the transcriptional responses of Trematomus bernacchii to thermal stress, focusing on the liver and spleen, key organs in immune defence and metabolic regulation. Adult specimens (total n = 30) were collected from Terra Nova Bay (Ross Sea) during the austral summer (late October 2022) and exposed to a temperature gradient, mimicking MHWs, ranging from 0 °C to +3 °C; a separate control group was maintained at 0 °C. Expression levels of antioxidant-related genes ( prdx3 , prdx5 , cat , sod1 , sod2 , gpx1 , gpx3 , gpx4 ) along with two innate immunity genes ( tlr2 and tlr9 ) were analysed in liver and spleen tissues from five biological replicates. 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 response 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. Cat remained unresponsive to thermal stress. These results indicate a delayed but coordinated response, suggesting an energy-conservation mechanism under increasing thermal stress. They underscore the limited plasticity of polar fish physiology and offer molecular insights into the vulnerability of Antarctic marine species in a warming climate. • T. bernacchii mounts distinct, tissue-specific responses to marine heatwaves stress. • The liver responds early to heat stress; the spleen activates only under extreme conditions. • Liver antioxidant defence weakens at +3 °C, revealing a critical tipping point. • Coordinated antioxidant and immune gene expression suggests organ crosstalk. • Limited plasticity in T. bernacchii raises concerns for survival under frequent warming.