Modelling Growth and Form of the Scleractinian Coral Pocillopora verrucosa and the Influence of Hydrodynamics

The growth of scleractinian corals is strongly influenced by the effect of water motion. Corals are known to have a high level of phenotypic variation and exhibit a diverse range of growth forms, which often contain a high level of geometric complexity. Due to their complex shape, simulation models represent an important option to complement experimental studies of growth and flow. In this work, we analyzed the impact of flow on coral's morphology by an accretive growth model coupled with advection-diffusion equations. We performed simulations under no-flow and uni-directional flow setup with the Reynolds number constant. The relevant importance of diffusion to advection was investigated by varying the diffusion coefficient, rather than the flow speed in Péclet number. The flow and transport equations were coupled and solved using COMSOL Multiphysics. We then compared the simulated morphologies with a series of Computed Tomography (CT) scans of scleractinian corals Pocillopora verrucosa exposed to various flow conditions in the in situ controlled flume setup. As a result, we found a similar trend associated with the increasing Péclet for both simulated forms and in situ corals; that is uni-directional current tends to facilitate asymmetrical growth response resulting in colonies with branches predominantly developed in the upstream direction. A closer look at the morphological traits yielded an interesting property about colony symmetry and plasticity induced by uni-directional flow. Both simulated and in situ corals exhibit a tendency where the degree of symmetry decreases and compactification increases in conjunction with the augmented Péclet thus indicates the significant importance of hydrodynamics.水流运动对石珊瑚（scleractinian corals）的生长具有显著影响。这类珊瑚具备高度的表型变异，并呈现出多样的生长形态，且往往拥有复杂的几何结构。鉴于其形态结构的复杂性，模拟模型成为补充生长与水流相关实验研究的重要手段。本研究通过耦合平流-扩散方程（advection-diffusion equations）的增量生长模型，分析了水流对珊瑚形态的影响。我们在无水流与恒定雷诺数（Reynolds number）的单向水流两种工况下开展了模拟实验，通过改变扩散系数（而非流速）来调控佩克莱特数（Péclet number），以此探究扩散相对于平流的相对重要性。本研究采用COMSOL Multiphysics软件对水流与输运方程进行耦合求解。随后，我们将模拟得到的珊瑚形态，与一系列在原位可控水槽实验装置中、暴露于不同水流条件下的疣状蔷薇珊瑚（Pocillopora verrucosa）的计算机断层扫描（CT）影像进行了对比。研究结果显示，模拟珊瑚与原位珊瑚均呈现出随佩克莱特数升高的相似变化趋势：单向水流会促使珊瑚产生不对称生长响应，使得珊瑚群体的分枝主要朝向上游方向发育。对形态特征的进一步分析还揭示了单向水流诱导下珊瑚群体对称性与表型可塑性的一项有趣特性。无论是模拟珊瑚还是原位珊瑚，均表现出随佩克莱特数升高对称性降低、结构紧凑性提升的趋势，这一结果证实了水动力环境对珊瑚生长的重要调控作用。