Effects of heat stress on the pectoral skeletal muscle of the Antarctic icefish Chionodraco hamatus and the red-blooded Trematomus bernacchii

Antarctic teleosts are extreme stenotherm species living at temperatures near to sea water freezing point (−1.9 °C), with annual fluctuations of less than 1 °C. Increasing ocean warming exposes all species to stressful conditions, including decreased oxygen availability. Therefore, the impact of heat stress on morpho-functional traits of Antarctic fishes is receiving intense attention, particularly concerning species without haemoglobin (Hb). In this study, were analyzed the effects of acute heat stress on the pectoral muscle of two Antarctic teleosts, the red-blooded Trematomus bernacchii and the Hb-less Chionodraco hamatus, on morphology, apoptosis and expression of molecules involved in vascular motility and heat shock response. After heat stress, we observed structural changes in the pectoral muscle of both species. In particular, those of icefish appear more vascularised with larger vessels. By immunolocalisation we evidenced species-specific differences in expression and localisation of endothelial nitric oxide synthase (eNOS), serotonin (5HT), vesicular acetylcholine transporter (VAch) and Heat Shock Protein 90 (HSP90) that, after exposure to 4 °C, in icefish seem to be up-regulated. Moreover, using the TUNEL technique, we found more apoptotic nuclei in the icefish after heat stress. Besides showing species-specific responses of pectoral muscle to heat stress in the two Antarctic teleosts, our results suggest a greater vulnerability to warming of Hb-less fish species.