Contrasting metabolic responses to increasing temperature in four mediterranean echinoderms

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.