Energy allocation and nutritional status during early development of two sub-Antarctic Southern cod species under environmental stress

Abstract

During early development, fish depend on maternally derived yolk reserves, making energy allocation crucial for growth, metabolism, and survival under variable environmental conditions. In this study, we experimentally evaluated the effects of temperature and salinity on energy content and nutritional condition in early developmental stages of Patagonotothen cornucola and P. tessellata, two sub-Antarctic notothenioid species present in the Beagle Channel. Late embryos and yolk sac larvae were exposed to four treatments: (A) 4 °C – 28 PSU, (B) 4 °C – 15 PSU, (C) 8 °C – 28 PSU, and (D) 8 °C – 15 PSU. Energy density (ED), individual energy content (ECi), and nutritional condition (standardised RNA/DNA index, sRD) were measured. In both species, ED and ECi were highest in embryos and decreased markedly after hatching, reflecting rapid depletion of yolk reserves. In contrast, sRD increased during development, indicating enhanced metabolic activity in larval stages. The effects of temperature and salinity on ED and sRD were stage-dependent, with no significant influence on ECi. Reduced salinity (15 PSU) was generally associated with higher ED and increased sRD at specific larval stages, particularly after hatching, suggesting lower energetic costs of osmoregulation. Overall, early development in both species is characterised by a shift from energy storage to metabolic activation after hatching. A moderate reduction in salinity, even at elevated temperatures, does not impose additional stress during endogenous feeding, suggesting no excessive diversion of energy toward homeostasis at specific stages. These findings suggest physiological flexibility in early life stages of P. cornucola and P. tesellata, potentially conferring them resilience to short-term environmental variability in sub-Antarctic ecosystems.