Pulsing corals: a story of scale and mixing

Abstract

Effective methods of fluid transport vary across scale. A commonly used dimensionless number for quantifying the effective scale of fluid transport is the frequency based Reynolds number, Ref, which gives the ratio of inertial to viscous forces in a fluid flow. What may work well for one Ref regime may not produce significant flows for another. These differences in scale have implications for many organisms, ranging from the mechanics of how organisms move through their fluid environment to how hearts pump at various stages in development. Some organisms, such as soft pulsing corals, actively contract their tentacles to generate mixing currents that enhance photosynthesis. Their unique morphology and the intermediate Ref regime at which they function, where both viscous and inertial forces are significant, make them a unique model organism for understanding fluid mixing. In this paper, 3D fluid-structure interaction simulations of a pulsing soft coral are used to quantify fluid transport and describe fluid mixing across a wide range of Ref. The results show that net transport is negligible for Ref < O(10¹), and continuous upward flow is produced for Ref ≥ O(10¹). Sustained net transport is necessary to bring in new fluid for sampling and to remove waste. As the Re is increased well above O(10¹), the slow region of mixing necessary for gas exchange between the tentacles is reduced. Since corals live at Ref between about 8 and 36, the flows they produce are defined by sustained net transport of fluid away from the coral in a continuous upward jet and a slow region of mixing between the tentacles necessary for gas exchange.