Biology Department, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA.
Biology Department, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA
J Exp Biol. 2019 Aug 9;222(Pt 15):jeb192518. doi: 10.1242/jeb.192518.
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 Péclet 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 and with average volumetric flow rates of approximately 0.71 cm s 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 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.
异足珊瑚的动态脉动是珊瑚礁中观察到的最迷人现象之一。我们首次量化了这些软珊瑚触须附近的流动，这些珊瑚的主动脉动被认为可以将共生体的光合作用率提高一个数量级。这些珊瑚虫直径约为 1 厘米，脉动频率约为 0.5 至 1 Hz。因此，使用触须长度和脉动频率计算的基于频率的雷诺数约为 10，并随着距离珊瑚虫的距离迅速衰减。这就提出了一个问题，即这些珊瑚如何将流动的可逆性降到最低，并在每次脉动时引入新的流体体积。我们估计珊瑚产生的体流动的佩克莱数约为 100-1000，而触须刷毛之间的流动约为 10。这说明了在去除氧废物方面，平流输送的重要性。使用粒子图像测速法进行的流动测量表明，单个珊瑚虫产生一股具有正垂直速度的水流，其速度不会低于 0.1 cm s，平均体积流量约为 0.71 cm s。我们的结果表明，在径向方向上几乎持续有流向珊瑚虫的流动，只有约 3.3%的回流。3D 数值模拟揭示了在扩张过程中触须之间存在缓慢混合的区域。我们估计，通过触须刷毛的平均流量约为 0.03 cm s。向珊瑚虫近乎持续流动、触须刷毛之间缓慢混合以及随后将这一体积的流体喷射成向上的射流的组合，确保了珊瑚虫不断地用足够的时间来进行交换，以获取新的水。
