Marine animals are increasingly threatened by a combination of reduced environmental oxygen (hypoxia) and chronically rising sea temperatures. Our current understanding of the effects of temperature on hypoxic performance is based predominantly upon studies of acute warming demonstrating reductions in hypoxic performance in numerous species. However, these studies do not take account of the possibility for altered performance via thermal acclimation. They are also largely focused at the organismal level with integration of different levels of biological organisation just beginning to appear. This thesis takes an integrative approach to investigate the effects of thermal acclimation on hypoxia thresholds using gammarid amphipods as models. The metabolic, biochemical and transcriptomic responses to hypoxia in isolation were first characterised in the brackishwater amphipod Gammarus chevreuxi. The consequences of warm acclimation for hypoxic performance were then identified and compared across closely-related gammarid species. Finally, the underpinning molecular mechanisms driving altered metabolic responses to hypoxia were investigated in the intertidal amphipod Echinogammarus marinus. This thesis demonstrates that, firstly, the integrated responses to hypoxia in isolation are complex and dependent upon the severity of hypoxia organisms experience. Secondly, thermal acclimation may enhance oxyregulatory capacity under hypoxia in some species, contrary to predictions from studies of acute warming, which may be associated with warm acclimated changes to metabolic rate. Thus, some species with a greater capacity for thermal acclimation may cope better with increasingly hypoxic environments. Intraspecific alterations to metabolic performance under declining oxygen tensions may be associated with widespread change at the molecular level. Warm acclimated E. marinus displayed a greater degree of hypometabolism under hypoxia compared to cold acclimated individuals, which may be associated with transcriptional changes occurring during the acclimation period associated with reductions in cellular energy demand. Thermal acclimation therefore induces changes at multiple levels of organisation which may prepare for hypoxia. Acclimation should be considered in any attempt to predict the consequences of future climate change driven hypoxia on marine species.

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