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During their diet vertical migration in the Gullmarsfjord (Swedish west coast) Nordic krill Meganyctiphanes norvegica made excursions into regions of severe hypoxia, >70 m depth, during 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 ([lactate] HL ) under conditions of acutely declining pO 2 only increased below 4 to 6 kPa. During anoxia, no krill survived >1 h and accumulation of L-lactate was at its most pronounced. Handling stress had little effect on [lactate] HL . Field observations (January 1998) showed that during the day krill resided at a depth of 65 to 85 m (pO 2 = 3 to 10 kPa). Net cages were stocked with krill, trawled from 60 to 70 m depth at dusk, and then kept overnight at 40, 70 and 90 m depth (pO 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 [lactate] HL no greater than that of normoxic laboratory individuals (3.04 ± 1.05 mmol l -1 ), survivors at 70 m showed elevated concentrations (9.91 ± 1.68 mmol 1 -1 ). Furthermore, newly trawled krill (at dusk) had [lactate] HL = 7.18 ± 2.72 mmol 1 -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 pO 2 is below that required to maintain aerobic metabolism. Utilization of anaerobiosis seems to be critical to the ability to enter such O 2 poor water although the krill are close to the limits of their physiological capacity at such times.

Possessing a poor anaerobic capacity does not prevent the diel vertical migration of Nordic krill Meganyctiphanes norvegica into hypoxic waters