Beschreibung: Marine hypoxia has become one of the major concerns of the world, as oceanic dead zones continue expanding horizontally and vertically, a phenomenon primarily caused by global warming and anthropogenic eutrophication. As consequence, drastic changes in community structures, predator-prey relationships (i.e. uncoupling) and/or habitat compression are expected followed by severe impacts on food-webs, ecosystems and fisheries. Moreover, habitat compression is aggravated by the synergistic effects of climate change, as elevated temperature and PCO2 will narrow the habitat from above. The jumbo squid, Dosidicus gigas, undergoes diel vertical migrations into oxygen minimum zones (OMZs) off the Eastern Tropical Pacific, where he plays an important ecological role both as predator and prey. In fact, this species can easily remove more than 4 million tons of food per year from the pelagic food web and is an important component in the diets of birds, fishes, and mammals. Besides its ecological role, the jumbo squid also plays an important economically role being target of the world’s largest cephalopod fishing industry with around 14% of world’s total squid catch and landings estimated at 818,000 tons in 2006. However, the main problem that arises with hypoxia is a reduced gradient that drives O2 uptake via diffusion pathways. At some point, the critical O2 partial pressure (Pcrit), the reduced diffusion gradient cannot support the metabolic demand fully aerobically, and has to be supplemented by anaerobic pathways and/or compensated by a reduction in metabolic rate. Commonly, aquatic animals respond to hypoxia by first attempting to maintain O2 delivery, as aerobic metabolism is much more efficient, followed by conserving energy expenditure and reducing energy turn over and finally by enhancing energetic efficiency of those metabolic processes that remain and derive energy from anaerobic sources. A further problem that vertical migrators of OMZs have to face is the elevated production of radical oxygen species (ROS) during the reoxygenation phase while ascending, as non-neutralized ROS formation can damage biological macromolecules (i.e. lipids, proteins and DNA) resulting in severe functional alterations in cells and tissues. To determine the cost and benefits of such diel vertical migrations, I investigated biochemical and physiological mechanisms in juvenile D. gigas off the Gulf of California with a focus on ventilation, locomotion, metabolism and antioxidant defense. The respiratory regulation in D. gigas was unpredictably high and is mirrored in maximized oxygen extraction efficiencies (EO2) at early (EH, 〈 160 min, 1 kPa O2) and late hypoxia (LH, 〉 180 min, 1 kPa O2). EO2 at EH was maximum 82% and achieved via (1) deep-breathing mechanism with more powerful contractions and an enlarged inflation period, and (2) reduction in the relaxed mantle diameter to favor diffusion. At LH, EO2 was still 40%, despite all other ventilatory mechanisms were drastically reduced, probably by using the collar-flap system (uncoupling of locomotory and ventilatory mechanisms) and a further reduction in the relaxed mantle diameter. Moreover, the drastic change in locomotion between EH and LH (onset of lethargy) was accompanied by a switch in the energy source of anaerobic pathways. At EH, anaerobic energy equivalents (AEE) primarily arrived via rapid energy reserve depletion (ATP, phospho-L-arginine), and, under LH, was mainly obtained via fermentative pathways (mainly octopine). As octopine formation simultaneously creates protons, intracellular acidosis and acid-base disturbances under progressing hypoxia are expected, which might negatively impact squid’s energy household and expenditures from locomotion towards more important cellular processes (i.e. ion regulation). Energy reserve depletion might even trigger lethargic behavior to conserve energy and extend hypoxia residence time. At EH, in contrast, deep-breathing behavior enabled D. gigas to pass the same amount of water through the mantle cavity per period of time and thereby could maintain a stable ventilatory volume per min, which explains its high level of activity observed under such extreme conditions. Moreover, D. gigas suppressed its metabolism (45-60%) at severe hypoxia (below Pcrit), as the reduction in O2 consumption rate (70-80%) could not be compensated by an upregulation in anaerobic energy production (70%). Cephalopods primarily feed on proteins and their glycogen storage potential is low (〈 0.4% of body weight). Therefore anaerobic protein degradation came into focus as strategy in hypoxia tolerant species. Yet, total protein concentration in muscle tissue of D. gigas did not vary significantly under severe hypoxia, but the reduced protein expression of heat shock protein 90 (Hsp90) and α- actinin indicates that, at least under progressing hypoxia, jumbo squids might degrade specific muscle proteins anaerobically. Moreover, the lower α-actinin expression at LH might be related to a decreased protection via the Hsp90 chaperon machinery resulting in increased ubiquitination and subsequent degradation. Therefore, the ubiquitin-proteasome system seems to play an important role in hypoxia tolerance, but further investigations are necessary to discover its full potential and pathways. Antioxidant enzyme activities in D. gigas were generally low and in the range of other squid species, but malondialdehyde concentrations (indicative of cellular damage) did not significantly change between normoxic and hypoxic conditions, demonstrating an efficient antioxidant defense system. Moreover, superoxide dismutase and catalase activities were enhanced under normoxia that seem to constitute an integrated stress response at shallower depths by buffering increased ROS formation, and, in addition, might even be a strategy to cope with the reoxygenation/recovery process. Moreover, heat shock protein 70 concentration was significantly increased under severe hypoxia (1 kPa O2), which may constitute a preparation for the reoxygenation phase during squid’s upward migration. Accordingly, the present thesis demonstrates that D. gigas evolved a variety of adaptive mechanisms and strategies to cope with hypoxia and the imposed challenges of diel vertical migrations. D. gigas might even actively descent into OMZs to suppress metabolism and escape from high metabolic demands at surface waters. Especially the high O2 uptake capacity and respiratory regulation were surprising taking into account cephalopods physiological and anatomical restraints. Therefore, D. gigas seems well-adapted to hypoxic conditions and might even out-compete less hypoxia tolerant species under hypoxia expansion, but the synergistic impacts of climate change, in turn, might endanger its survival.