The Southern Ocean ecosystem is characterized by seasonal features such as varying daylight duration, food availability and sea ice coverage. Antarctic krill, Euphausia superba, are one of the most abundant species in the world and play a key role in this ecosystem. Krill have evolved seasonal rhythms to adapt to their seasonal environment, but how the synchronization process with environmental cues functions mechanistically is not fully understood. Within the scope of this thesis the effect of the photoperiod on the seasonal course of metabolic activity of Antarctic krill should be investigated. Krill were exposed to three different photoperiodic simulations each over a period of one year (half a year respectively). All other experimental conditions like food availability and water quality/temperature were constant. The (photoperiodic) simulations were 1) a natural photoperiodic course corresponding to light conditions at 66° S (LD), 2) constant darkness (DD) and 3) the natural course of the photoperiod shortened to half a year (LD 1/2). Under the simulation of the natural course of the photoperiod it could be shown that the activity and mRNA levels of metabolic key enzymes involved in the carbohydrate metabolism and the respiratory chain followed the simulated photoperiodic course, irrespective of the constant food availability. These observations are similar to the previous ones made in the field, where activities were high in summer months and low during winter months. Activities and mRNA of levels of HOAD (beta oxidation of fatty acids) showed a different pattern with increasing values (with a hold-up in winter) and a peak in spring. These observations are also reasonable in context with lipid dynamics observed in the field. Similar seasonal patterns for all enzymes were observed for animals kept under total darkness, suggesting that the metabolic activity is regulated endogenously. In this context the results from the shortened photoperiod LD 1/2 demonstrated an adaptation of the course of metabolic activity to the photoperiodic cycle. This leads to the conclusion that the photoperiod is a very important environmental zeitgeber for the regulation of the seasonal cycle of krill's metabolic activity. The courses of the metabolic activity observed in DD point to circannual rhythmicity and a putative circannual clock, which governs these rhythms. However, the involvement of krill's circadian clock in seasonal time-keeping cannot be excluded yet and further investigations need to focus on the role of the circadian clock in seasonal time-keeping.