Oxygen availability modulates clownfish larvae responses to acute thermal stress in experimental settings

Ocean deoxygenation, driven by warming and eutrophication, poses an escalating threat to marine biodiversity. In tropical coastal ecosystems, diel cycles naturally produce substantial oxygen variability, with nighttime deoxygenation driven by community respiration and daytime oxygenation from intense photosynthesis, regularly reaching levels of supersaturation (>100% air saturation). Critically, these supersaturation peaks coincide with peak daily temperatures, raising the question of whether elevated oxygen levels can buffer organisms against acute thermal stress. To test this, we exposed newly hatched Amphiprion bicinctus larvae to three oxygen regimes-deoxygenation (approx. 4 mg l⁻¹), air saturation (approx. 6 mg l⁻¹) and supersaturation (approx. 12 mg l⁻¹)-under ramping temperatures (+0.5°C h⁻¹, 30-34°C). Larvae under deoxygenation exhibited the lowest lethal temperature for 50% mortality (LT₅₀= 30.2°C), while both air-saturated (31.3°C) and supersaturated (31.54°C) treatments improved thermal resistance. Mortality remained lower under supersaturation up to 31.5°C, with slower declines than in other treatments. Additionally, parental care increased daytime oxygen in egg masses by approximately 51%, enhancing oxygenation. These results show that oxygen supersaturation-naturally occurring during thermal peaks-can mitigate physiological stress during development. This highlights an underrecognized role of diel oxygen dynamics in climate resilience, with implications for the management of marine nurseries, fisheries and aquaculture under warming conditions.