Ciliary flow and morphology shape mass transport at the surface and within gastrovascular cavities of black corals.

Sessile marine organisms rely on morphology to mediate interactions with their environment, yet the microscale drivers of morphological diversification remain incompletely resolved. Black corals, with their high morphological repertoire and global ecological importance, provide an ideal taxon to explore these drivers. Using high-speed videography, microscale particle image velocimetry, oxygen-sensitive sensPIV, and microsensors, we quantified how colony morphology influences hydrodynamics on polyp to colony scales, and how these flow environments regulate mass transport and exchange at tissue surfaces. We document beating epidermal cilia in black corals and reveal that species deploy distinct morphology-cilia combinations to meet metabolic needs, revealing a trade-off between enhancing exchange surface area and maintaining effective ventilation. By visualizing cilia-driven circulation within gastrovascular cavities, we demonstrate strong internal connectivity shaped by skeletal spines, again revealing how morphology governs ciliary function. This form-function feedback highlights ciliary dynamics as a key but previously overlooked driver of performance, habitat specialization, and diversification in sessile life.