Sex, genes and metabolism: exploring drivers of success through native and invasive populations of the European green crab.

The European green crab (Carcinus maenas) is a keystone predator in its native range and one of the world's most damaging invasive species. Although its invasion success has frequently been associated with exceptional physiological plasticity, comparatively few studies have simultaneously examined organismal energetics, biochemical regulation, physiological organization and mitochondrial genetic diversity across native and invasive populations. In this study crabs were sampled from native populations inhabiting cooler waters in France and warmer waters in Portugal, and from a long-established (warm-water) invasive population in Australia. Across 12 sites, we measured whole-animal respiration, gill-based biochemical markers (antioxidant activity, oxidative damage, acetylcholinesterase activity, and detoxification enzymes), hepatopancreas morphology, and genetic diversity using the mitochondrial cytochrome oxidase subunit I (COI) gene. Growth scaling exponents were conserved across all populations, indicating that invasion history did not alter ontogenetic growth trajectories, despite population-specific shifts in body condition. Native populations displayed clear physiological differentiation between regions: crabs from colder regions of France exhibited higher respiration rates and metabolic turnover than crabs from warmer waters (Portugal). In contrast, the invasive Australian population maintained organismal metabolic traits and body condition comparable to warmer native populations but exhibited markedly elevated antioxidant capacities and higher acetylcholinesterase activity while maintaining similar haplotype diversity to native populations. These findings suggest that invasion success in Australia is not driven by body condition or elevated metabolic demand, but rather by differences in biochemical regulation. Together, our results indicated that the success of invasive C. maenas population may rely on regulatory and metabolic flexibility rather than changes in baseline energy balance. By highlighting the interplay between physiology, sex, and genetic diversity, this work advances our understanding of how invasive populations persist under environmental stress and offers new perspectives for managing marine invaders in a changing ocean.