A novel mechanism of mixing by pulsing corals

Abstract

The dynamic pulsation of 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, the active pulsations of which are thought to enhance their symbionts' photosynthetic rates by up to an order of magnitude. These polyps are approximately 1 cm in diameter and pulse at frequencies between approximately 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 with distance 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 the Pećlet number of the bulk flow generated by the coral as being on the order of 100-1000 whereas 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 −1 and with average volumetric flow rates of approximately 0.71 cm 3 s −1 . Our results show that there is nearly continual flow in the radial direction towards the polyp with only approximately 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 approximately 0.03 cm s −1 . 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.Each polyp was filmed with a single camera at 125 or 60 frames s −1 in a quiescent fluid using a Photron SA3 120K camera. Tentacles 2

Pulsing corals: A story of scale and mixing

“…This average behavior of a tentacle is what was used to enforce the prescribed motion of the immersed boundary. For more details on this analysis, see [27]. A coral pulsation cycle was divided into the 3 phases, see Figure 3 and below, 1.…”

Under the sea: Pulsing corals in ambient flow

“…The kinematics were captured by tracking positions along a single tentacle from 5 different coral polyps. These positions were then fit with polynomials and averaged to enforce the prescribed motion of the immersed boundary, as in [7]. A coral pulsation cycle was divided into the 3 phases, see Figure 3…”

“…Since both pulsing corals and upside down jellyfish reside in marine environments, the currents they produce are subject to perturbations from the ambient fluid motion in their environment. Previous and current studies have quantified the current produced by both the upside down jellyfish and pulsing coral in quiescent conditions [5], [6], [7], [8], as well as the upside down jellyfish with background flow [9]. The addition of ambient flow was shown to significantly affect the currents produced by the upside down jellyfish [9], suggesting that the bell and oral arm morphologies are advantageous for particle capture and feeding under a range of conditions.…”

High-speed two-color scanning volumetric laser-induced fluorescence

“…However, fluids flows are intrinsically three-dimensional, and hence to experimentally capture the full dynamics volumetric imaging is necessary. For example, planar imaging techniques have been widely applied to study the flow dynamics of animals (Katija et al (2015); Pepper et al (2015); Samson et al (2019)). In one study, volumetric PIV imaging was applied to study hydrodynamics of a shark tail (Flammang et al (2011)), leading to qualitatively different results than previous work using planar PIV on the wake structures Lauder (2002, 2004)).…”