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

During their diel vertical migration in the Gullmarsfjord (Swedish west coa st) 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 whi ch they utilize this capacity in the field. L-lactate was the main end-prod uct of anaerobic metabolism. The concentration of L-lactate in the haemolym ph ([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 o f L-lactate was at its most pronounced. Handling stress had little effect o n [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 we re 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 respectivel y). 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 normo xic laboratory individuals (3.04 +/- 1.05 mmol l(-1)), survivors at 70 m sh owed elevated concentrations (9.91 +/- 1.68 mmol l(-1)). Furthermore, newly trawled krill (at dusk) had [lactate](HL) = 7.18 +/- 2.72 mmol l(-1), indi cating that, like the caged individuals at that depth, they too had resorte d to anaerobic metabolism. This study has shown that while anaerobic metabo lism 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 ph ysiological capacity at such times.