Virtually all living beings on our planet exhibit daily and seasonal rhythms. These rhythms are generated by endogenous clocks, which allow organisms, including humans to synchronize daily and seasonal life cycle functions with rhythmic changes of their environment However, our current molecular understanding of biological rhythms and clocks is mostly restricted to land model species. By contrast, we know very little about the endogenous clocks of marine organisms, and how they interact with environmental cycles. This is particularly true for marine ecological keystone species like Antarctic krill (Euphausia superba), endemic to the Southern Ocean, a high latitude region which is characterized by extreme environmental changes across seasons (day length, light intensity, food availability). These polar regions are experiencing the fastest warming on the planet, with environmental alterations and ecosystem shifts, resulting in profound changes in trophic interactions and nutrient/energy fluxes. These finely tuned interactions, which have evolved over millions of years, are likely to going out of phase by the rapid climate change in these regions. Therefore, our overarching objective is to identify how regular environmental cues (day/night cycle, photoperiod) generate molecular rhythmic oscillations that allow polar marine organisms like Antarctic krill to anticipate the rhythmic changes in their environment, and to pre-adjust their life cycle functions accordingly. To achieve this, we aim to further investigate the involvement of endogenous clocks into central life cycle functions in Antarctic krill, by use of seasonal behavioural experiments along with gene expression analysis of clock and metabolic marker genes. In addition, we aim to characterize the location and anatomy of the master circadian clock in the brain of E. superba by use of fluorescent in-situ hybridization and immunocytochemical studies to understand the molecular and neuronal mechanisms that underly the endogenous clock. Finally, we will experimentally manipulate the circadian clock to reveal to which extent the endogenous rhythm and the changing environment determine the behavior and physiology of Antarctic krill. We thus hope to get further insights into the mechanisms that underly krill’s adaptation to extreme environmental conditions and into its plasticity towards ongoing changes in the Southern Ocean ecosystem.