Evaluating the hypoxic tolerance of two maturity stages of Antarctic krill (Euphausia superba) at its range edge

The South Georgia region of the Southern Ocean represents the northernmost range edge for Antarctic krill. Of concern is the extent to which rapid warming of surface water temperatures and reduced oxygen contents around this region might challenge the physiological tolerance of krill, particularly the later maturity stages. Hypoxia is generally considered to be less than 30 to 20% of air saturation, hereafter as threshold hypoxia, while less than 10% of air saturation would qualify as severe hypoxia. These levels are unlikely to occur in the Southern Ocean but might happen in the middle of dense krill swarms. We investigated gene expression and biochemical markers related to aerobic metabolism, antioxidant defence, and heat-shock response under 6-h threshold (4 kPa; TH) and 1-h severe (0.6 kPa; SH) hypoxia exposure, to understand how hypoxia might alter respiratory and biochemical pathways in adult and subadult krill. After 6-h TH, subadults induced expression of citrate synthase (CS), and mitochondrial superoxide dismutase (also after 1-h SH) over normoxic expression levels. The maturity stages responded differently in glutathione peroxidase (1-h SH; lower in subadults and higher in adults), and CS (6-h TH; higher in subadults and lower in adults) activities as for the oxidative damage marker to lipids (6-h TH; lower in subadults and higher in adults). Subadults had a greater capacity than adults to deal with hypoxic conditions. This may be a strategy allowing them to exist in larger swarms to reduce predation pressure before reaching reproductive condition

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