energy reserve and the reserve rapidly provides energy to meet any elevated metabolic cost of maintenance during moderate stress. However, individual fitness is reduced by moderate stress as additional homeostatic energy costs lead to energetic trade-offs and result in reduced activity, scope for growth, and/or reproductive output (Sokolova, 2013). For example, increased allocation of energy to maintenance diminishes energy available for other functions such as buffering fluctuating food availability or provisioning offspring (Sokolova, 2013). Moderate stress typically induces a transition from a state of cellular growth to one of cellular maintenance (Kassahn et al., 2009), which is reflected in sustained down-regulation of at 7.5-12.5 MPa in L. maja (Fig. 6). Microtubule tubulin polymers are essential cytoskeletal components affecting cell division in all eukaryotes, and down-regulation of will reduce microtubule filament formation, with a multiplicity of cascading cellular effects (Nogales, 2000). At the whole-organism level, compensatory mechanisms involve accelerated oxygen uptake to support increasing allocation of energy to maintenance (Willmer et al., 2005). Increasing allocation of energy to maintenance is evident in elevated and increasing oxygen consumption at 7.5-12.5 MPa in L. maja (Figs 4 and 5), and is also apparent in up-regulated cs expression (Fig. 6). Expression of the cs gene is correlated with mitochondrial citrate synthase activity, a key and rate-limiting constituent of the TCA cycle (Stager et al., 2015). Moderate stress is sustained at 7.5-12.5 MPa (Figs 5 and 6), without increased dependence on anaerobic respiration, at least at 7.5 MPa (Fig. 8). The transition to sustained moderate hyperbaric stress in L. maja precedes the 790 m bathymetric limit to the species’ known distribution (Fig. 9), and it is therefore evident that it is the metabolic cost associated with moderate hyperbaric stress that constrains L. maja’s bathymetric distribution. This interpretation advocates incorporating HP in a matrix of energy-limited biogeographic range in marine ectotherms, dependent on the energetic costs associated with tolerance of interacting suboptimal environmental conditions including temperature, dissolved oxygen,
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