Active host control of the internal O₂ microenvironment in reef-building corals

The gastrovascular cavity of corals plays a central role in internal circulation, digestion, reproduction, and symbiont acquisition. However, despite its importance, our understanding of the physic-chemical characteristics and dynamic properties of this internal microenvironment remains limited. Here, we employ high-resolution microsensor measurements to investigate the vertical distribution of O₂ within the coral gastrovascular cavity. By combining microsensor analysis with time-lapse imaging, we show that O₂ levels inside corals are strongly affected by slow, synchronized tissue movements, suggesting a mechanism of active ventilation through modulation of cavity volume and exchange with the surrounding seawater. Together with cilia beating, these movements reduce O₂ accumulation under light conditions and alleviate O₂ depletion in darkness, thereby stabilizing internal O₂ availability. These findings highlight a hitherto overlooked role of behavior in actively regulating the internal microenvironment and O₂ status of the coral holobiont and provide critical insight into the organism-environment interactions of reef-building corals.