During their diel vertical migration in the Gullmarsfjord (Swedish west coast) Nordic krill Meganyctiphanes norvegica made excursions into regions of severe hypoxia, >7O m depth, dunng the day. Consequently, we investigated the capacity for anaerobic metabolism by M. norvegica and the extent to which they utilize this capacity in the field. L-lactate was the main end-product of anaerobic metabolism. The concentration of L-lactate in the haemolymph ([la~tate],,~) under conditions of acutely declining p 0 2 only increased below 4 to 6 kPa. Dunng anoxia, no krill survived > 1 h and accumulation of L-lactate was at its most pronounced. Handling stress had little effect on [lactateJHL. Field observations (January 1998) showed that during the day krill resided at a depth of 65 to 85 m (PO, = 3 to 10 kPa). Net cages were stocked with krill, trawled from 60 to ?O m depth at dusk, and then kept overnight at 40, 70 and 90 m depth ( p 0 2 >14, 6.1 and 1.8 kPa respectively). This resulted in krill mortality of 7, 70, and 100% respectively. While individuals caged at 40 m showed [1actatelHL no greater than that of normoxic laboratory individiials (3.04 i 1.05 mmol 1-I). survivors at 70 m showed elevated concentrations (9.91 I 1.68 rnmol 1"). Furthermore, newly trawled knll (at dusk) had [ l a~t a t e ]~, = 7.18 t 2.72 mmol 1-', indicating that, like the caged individuals at that depth, they too had resorted to anaerobic metabolism. This study has shown that while anaerobic metabolism is not well developed in M.norvegica, individuals do enter bodies of water where the p 0 2 is below that required to maintain aerobic metabolism. Utilization of anaerobiosis seems to be cntical to the ability to enter such O2 poor water, although the krill are close to the limits of their physiological capacity at such times.
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