These Boots Are Made for Walking: Sex-Specific Physiological and Metabolomic Strategies Reflect Male-Skewed Vulnerability to Ocean Warming in a Keystone Amphipod.

Sex-specific variation can critically shape species' physiological responses to environmental change, with potentially strong implications during reproduction. Yet this source of variation is often overlooked. To help address this paucity, we investigated fitness and whole-organism traits and metabolomic profiles to elevated temperature in males and females of Gammarus locusta, a keystone intertidal amphipod inhabiting thermally dynamic coastal environments. Individuals were gradually acclimated to four ecologically relevant temperatures (16°C, 21°C, 26°C and 31°C) and maintained at these conditions for 21 d under controlled laboratory settings. Survival declined at 26°C and 31°C in both sexes, and females exhibited consistently higher upper thermal limits, broader thermal safety margins, but lower thermal acclimation capacity compared to males. Whilst reproductive output was lower in elevated temperatures (26°C and 31°C), juveniles' body length increased, indicating a "quality-over-quantity" reproduction strategy. At the metabolomic level, females exhibited greater plasticity, with enhanced pathways linked to energy metabolism, amino acid biosynthesis, and cellular stress defence, suggesting adaptive activation rather than heightened vulnerability, consistent with their higher thermal tolerance and survival. Elevated temperatures also impaired amphipods' energetic status, with both glucose and ATP:ADP ratio decreasing, particularly at 31°C. Overall, we show that sex-specific metabolomic strategies define different sex thermal tolerance levels in G. locusta. As ocean warming intensifies, males' greater vulnerability could skew population sex ratios toward females. This imbalance may limit mating opportunities and alter population operational sex ratio and therefore dynamics, if the number of males is insufficient to fertilise all females. Our study highlights the importance of considering sex-specific physiological responses by integrating them with life-history and fitness measures to build a mechanistic understanding of how thermal stress may shape reproductive dynamics and, ultimately, affect population viability. By linking cellular metabolism with organismal fitness, we provide the cross-scale insight needed to more accurately forecast marine invertebrates' responses to global change.