Spatio-temporal patterns of the incoming water flow in pulsating corals

Sessile marine organisms rely on water flows to enhance mass transfer and facilitate key physiological processes. Pulsating corals exhibit rhythmic pulsation of their tentacles to generate flow that removes excess oxygen produced during photosynthesis, thereby enhancing the process. The ejected flow is counterbalanced by an incoming flow directed toward the polyp, potentially delivering essential nutrients. This mechanism may be crucial for these corals, which rely on epidermal nutrient uptake rather than zooplankton predation. The characteristics of the incoming flow and its interactions with the coral tissue, where mass transfer occurs, are largely unknown. Here, we characterize the origin of new water approaching the polyp, the pathway it takes, where on the polyp and when during the pulsation period this interaction occurs. We used particle image velocimetry on single polyps to measure the flow field around the polyp and reconstruct the trajectories of the incoming water. We found that incoming water primarily originates from below the polyp. Eighty percent of the new water interacts with the polyp during the downward stroke of the tentacle motion, and 75% of the new water contacts the aboral face. We used a conservation of mass analysis to estimate the flow between the tentacles and found significant bidirectional flow through the gaps. Pulsation draws in ∼26,800 polyp volumes of new water daily, containing sufficient nitrogen to fully meet the polyp's estimated daily demand for growth. Increased nutrient uptake may explain the persistence of pulsation during the night when photosynthesis ceases.