Abstract
Marine ectotherms, organisms whose body temperature depends on their environment, often rely on physiological plasticity to withstand rapid temperature increases when behavioural adjustments are insufficient. Despite extensive research on thermal tolerance, gaps remain in understanding species- and population-level metabolic responses to acute thermal stress, particularly in rapidly warming regions like the Mediterranean Sea. This study assessed metabolic responses to acute warming in four echinoderm species with distinct thermal affinities but overlapping distributions in the Western Mediterranean: the sea urchins Arbacia lixula (subtropical) and Paracentrotus lividus (temperate-cold), and the brittle stars Ophiothrix sp. II (temperate) and Ophiocomina nigra (temperate-cold). Oxygen consumption, used as a proxy for Basal Metabolic Rate (BMR), was measured at sequential temperatures (16 °C, 20 °C, 23 °C, 26 °C), following a short acclimation period. Species exhibited divergent metabolic trajectories and thermal sensitivities (Q₁₀), reflecting their thermal affinities, local adaptations, and phenotypic plasticity. A. lixula and Ophiothrix sp. II displayed sharp BMR increases, indicating resilience but proximity to their upper thermal limits. In contrast, O. nigra maintained stable metabolic rates, suggesting broad physiological plasticity. P. lividus displayed population-level divergence: individuals with cooler-origin experienced metabolic suppression and severe thermal stress at 26 °C, whereas those with warmer-origin maintained higher metabolic activity. Overall, phenotypic plasticity emerged as a key short-term strategy to cope with acute warming. However, species with narrower thermal tolerance, such as P. lividus , might face long-term vulnerability under intensifying marine heatwaves. These results highlight the importance of integrating thermal history, plasticity, and genetic variation to accurately predict resilience to ocean warming.