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

Abstract

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.

Physiology and Metabolism of Northern Krill (Meganyctiphanes norvegica Sars)

“…Despite an earlier report which seemed to suggest that M. norvegica would not traverse a pycnocline into an area of low O 2 (Bergström and Strömberg, 1997), excursions into severely hypoxic bottom water (PO 2 ¼ 3-10 kPa at a depth of 65-85 m, roughly 15-50% O 2 saturation; the waters are hypoxic because of a delay, or sometimes cessation, in annual water renewal as a result of altered currents) in the Gullmarsfjord meant that krill during the day resided at such depths (Spicer et al, 1999). van den Thillart found that M. norvegica was able to regulate its O 2 uptake down to approx.…”

“…So in common with a number of other krill species M. norvegica displays some ability to maintain O 2 uptake in the face of declining O 2 tensions but the ability does not seem to be any more developed than that found in krill species which do not encounter hypoxia on a regular basis, if at all. Furthermore, M. norvegica is characterised by one of the poorest anaerobic capacities of any crustacean, surviving not more than 1 h in anoxia, and accumulating large concentrations of L-lactic acid quickly (Spicer et al, 1999). So reliance on a shift from aerobic Physiology and Metabolism of Northern Krill to anaerobic metabolism when exposed to low O 2 can barely be seen as an adaptation to migrating into hypoxic layers during DVM; indeed if krill are caged in deep hypoxic water for a greater period than they would normally reside there, they have extremely high levels of L-lactate if they survive, but survival itself is poor (Spicer et al, 1999).…”

“…Furthermore, M. norvegica is characterised by one of the poorest anaerobic capacities of any crustacean, surviving not more than 1 h in anoxia, and accumulating large concentrations of L-lactic acid quickly (Spicer et al, 1999). So reliance on a shift from aerobic Physiology and Metabolism of Northern Krill to anaerobic metabolism when exposed to low O 2 can barely be seen as an adaptation to migrating into hypoxic layers during DVM; indeed if krill are caged in deep hypoxic water for a greater period than they would normally reside there, they have extremely high levels of L-lactate if they survive, but survival itself is poor (Spicer et al, 1999). Given this poor anaerobic capacity, it would be interesting to know the functional significance (if any) of the low levels of polymorphism of the enzyme LDH (lactate dehydrogenaseresponsible for converting pyruvate to lactate) found in M. norvegica (Mulkiewicz et al, 2001).…”

“…Thus, they suggested that a reduction in temperature dramatically improves regulatory ability (linked to a reduction in overall metabolism) and could allow excursions into cold hypoxic water. If the hypoxic deep water below the pycnocline had been characterised by the same (high) temperature as found at the surface, then krill would have been unable to regulate their metabolism, and the poor anaerobic capacity of these animals (Spicer et al, 1999) would have been insufficient to permit residence in those waters for more than a few hours. The modest regulatory ability of M. norvegica when challenged by acutely declining PO 2 s was not present in the earliest life stages investigated (calyptopis III/early furcilia I) but began to be detectable in furcilia III (Spicer and Strömberg, 2003).…”

The molecular heterogeneity of hemocyanin: Its role in the adaptive plasticity of Crustacea

“…Recent studies have, in addition, revealed significant changes within the day. The Nordic krill Meganyctiphanes norvegica, like other Euphausiacea, adopts diel vertical migration moving from 100 m depth during the day to the surface layers during the night (Spicer et al, 1999). This organism, therefore, is cyclically exposed to environments that are characterized by different temperature, salinity, dissolved oxygen and food availability (Strömberg and Spicer, 2001).…”