dc.description.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.
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