Acute temperature effects on cilia beating increase coral deoxygenation.

Cilia-induced vortical flows are critical for regulating oxygen (O2) and metabolite exchange across coral-water interfaces. While this active ventilation affects the coral tissue microenvironment, its role in thermal stress remains poorly understood. Using high-speed imaging of cilia beating, particle image velocimetry with O2-sensitive nanoparticles, and a mechanistic transport model, we quantified how ciliary dynamics in the reef-building coral Porites lutea respond to acute warming in darkness. Moderate warming (~35°C) enhanced ciliary activity and advective transport yet paradoxically thickened the concentration boundary layer with O2-depleted water, exposing tissues to transient hypoxia. At higher temperatures, ventilation failed to meet rising metabolic demands and anoxic regions expanded rapidly. Above ~37°C, ciliary coordination collapsed and vortical flows dissipated, shifting transport to a diffusion-limited regime accelerating coral mortality. These results identify ciliary beating as a key regulator of thermal tolerance and early indicator of critical physiological tipping points for reef-building corals in a warming, deoxygenating ocean.