The fluid dynamic "heartbeat": tracking coral physiological collapse through observing cilia and oxygen dynamics

A coral colony's resilience hinges on a complex symbiosis. This holobiont masks real-time tracking of the coral host's physiological collapse and its 'time of death', as other metrics reflect the colony's slower decline. Sessile reef corals actively generate local fluid motion via surface cilia, critical for vital mass transfer. We characterised cilia-driven fluid mechanics and oxygen dynamics in six Scleractinian species under thermal stress. Our approach combined high-speed imaging, Particle Image Velocimetry (PIV), and SensPIV—an optical technique coupling flow visualisation with oxygen-sensitive nanoparticles. With increasing temperature, we observed pronounced localised deoxygenation at the tissue surface, coinciding with a disruption of cilia beating frequencies and their associated vortical structures and near-surface turbulence. These changes significantly reshaped holobiont oxygen dynamics. Our results provide the first multi-species quantification of thermal effects on coral-generated microscale flows and oxygen gradients. Crucially, these fluid dynamic changes serve as a novel, real-time indicator of the coral host's physiological collapse, a 'fluid dynamic heartbeat' allowing us to infer individual host mortality at higher temporal resolution. This work underscores biologically driven hydrodynamics' vital importance, highlighting how thermal perturbations compromise these systems, offering a new lens to pinpoint critical mortality events at the host level.