Diel effects on the expression of heat shock protein genes (hsp70 and hsp90) in early developmental stages of the Black Southern cod Patagonotothen tessellata (Richardson, 1845).

As larvae, fish are more sensitive to stress factors. Therefore, understanding the cellular defence mechanisms during the early stage of life can help infer variations in the type of response depending on their development and the influence of rhythmic cycles in their activity. We aimed to analyse the influence of diel variation on the expression of stress response-related genes (hsp70 and hsp90) in Patagonothen tessellata's embryos and yolk sac larvae under an experimental heat shock treatment. Significantly higher expression levels of the hsp70 and hsp90 genes in P. tessellata yolk sac larvae were observed when the heat shock treatment was performed at nighttime than at daytime, indicating the species' diurnal behaviour. In addition, higher expression levels of the hsp70 and hsp90 genes were observed in yolk sac larvae compared to embryos, highlighting ontogenetic differences in the species' response to thermal stress. This variation may be attributed to differences in the allocation of endogenous energy reserves during early development. Furthermore, increased gene expression levels were detected 1 h after the heat shock, suggesting a rapid activation of this gene system early in life, once P. tessellata hatched. In order to elucidate the capability of early life stages of fish to cope with future scenarios related to climate change, daily rhythms must also be considered to ensure accurate physiological responses.