The Antarctic krill, Euphausia superba, is restricted to the Antarctic Ocean. The northern krill, Meganyctiphanes norvegica, is extremely widely distributed from the arctic North Atlantic to the warm Mediterranean. Respiration measurements showed no seasonal differences in rates determined in krill from the thermally stable Clyde Sea (Scotland) and the cooler but variable Danish Kattegat. In the warm Ligurian Sea, where temperatures are stable, krill showed higher rates in April than in September, indicating reactions to the short but intensive productive season. Krill can passively benefit from enhancements of overall metabolism when ascending into upper, warmer water strata during their pronounced diel vertical migration. Michaelis-Menten constants (Km) of citrate synthase (CS) were compared. In terms of respiration and enzyme regulation, krill from the Ligurian Sea stand apart: temperature and nutrition appear to be of different influence, relatable to genetic differentiation in the species. In contrast, Km of CS in E. superba is temperature independent, highlighting the species' stenothermal physiology. A basal level of activity of digestive enzymes ensures immediate utilization of patchy food sources. Specific induction, including that of chitinases, indicating omnivory in both species, underlines krill's exceptional capacity to adapt to highly variable trophic environments. Processes of moult, growth, and reproduction are locally and seasonally adjusted.