Metabolic and enzymatic adaptations in northern krill, Meganyctiphanes norvegica, and Antarctic krill, Euphausia superba

The Antarctic krill, Euphausia superba, is restricted to the Antarctic Ocea n. The northern krill, Meganyctiphanes norvegica, is extremely widely distr ibuted from the arctic North Atlantic to the warm Mediterranean. Respiratio n 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, k rill showed higher rates in April than in September. indicating reactions t o the short but intensive productive season. Krill can passively benefit fr om enhancements of overall metabolism when ascending into upper. warmer wat er strata during their pronounced diel vertical migration. Michaelis-Menten constants (K-m) of citrate synthase (CS) were compared. In terms of respir ation and enzyme regulation, krill from the Ligurian Sea stand apart: tempe rature and nutrition appear to be of different influence. relatable to gene tic differentiation in the species. In contrast, K-m of CS in E. superba is temperature independent, highlighting the species stenothermal physiology. A basal level of activity of digestive enzymes ensures immediate utilizati on of patchy food sources. Specific induction, including that of chitinases , indicating omnivory in both species, underlines krill's exceptional capac ity to adapt to highly variable trophic environments. Processes of moult, g rowth, and reproduction are locally and seasonally adjusted.