A novel mechanism of mixing by pulsing corals

The dynamic pulsation of the xeniid corals is one of the most fascinating phenomena observed in coral reefs. We quantify for the first time the flow near the tentacles of these soft corals whose active pulsations are thought to enhance their symbionts’ photosynthetic rates by up to an order of magnitude. These polyps are about 1 cm in diameter and pulse at frequencies between about 0.5 and 1 Hz. As a result, the frequency-based Reynolds number calculated using the tentacle length and pulse frequency is on the order of 10 and rapidly decays as one moves away from the polyp. This introduces the question of how these corals minimize the reversibility of the flow and bring in new volumes of fluid during each pulse. We estimate that the Peclet number of the bulk flow generated by the coral as being on the order of 100-1000 while the flow between the bristles of the tentacles is on the order of 10. This illustrates the importance of advective transport in removing oxygen waste. Flow measurements using particle image velocimetry reveal that the individual polyps generate a jet of water with positive vertical velocities that do not go below 0.1 cm/s and with average volumetric flow rates of about 0.71 cm 3 /s. Our results show that there is nearly continual flow in the radial direction towards the polyp with only about 3.3% back flow. 3D numerical simulations uncover a region of slow mixing between the tentacles during expansion. We estimate that the average flow that moves through the bristles of the tentacles is about 0.03 cm/s. The combination of nearly continual flow towards the polyp, slow mixing between the bristles, and the subsequent ejection of this fluid volume into an upward jet ensures the polyp continually samples new water with sufficient time for exchange to occur.