Hydrodynamic and ecological functions of the parietal gaps in Porifera: insights from Computational Fluid Dynamics analysis

The interaction between organismal morphology and the environment constitutes a central theme in biology and ecology, providing foundational insights into how natural selection shapes the evolutionary trajectories of species. However, previous functional morphological studies on sponges have predominantly focused on their overall shapes, leaving the functional significance of other structural features underexplored. Parietal gaps—the conspicuous, large pores on the body wall of thin-walled sponges—are likely an adaptive trait for deep-water settings, as suggested by their bathymetric distribution. The origin of this characteristic can be traced back to the Cambrian Stage 3. Nevertheless, despite being documented in both fossils and extant sponges, the specific function of parietal gaps remains speculative and debated, lacking objective quantitative validation. This gap in understanding the morpho-functional principles has constrained paleontologists' ability to interpret organism-environment interactions and to leverage the morphological features for paleoenvironmental reconstructions. In this study, Computational Fluid Dynamics (CFD) was employed to explore the function of parietal gaps, using the hexactinellid sponge Euplectella aspergillum as a model. Comparative hydrodynamic models were developed to reflect the in vivo physiological condition of E. aspergillum, enabling simulations and analyses of flow field characteristics more accurately under conditions of three Reynolds numbers (Re = 437.5, 875 and 1750). The results demonstrate that parietal gaps significantly enhance passive flow and generate consistent upward currents within the atrium, thereby supporting essential physiological processes of the sponges, such as oxygen uptake, waste removal, and gamete exchange. With the presence of parietal gaps, although the time-averaged drag coefficient of the model is slightly increased, the amplitude of drag fluctuations is markedly reduced. Collectively, this contributes to the stabilization of the mechanical environment, reduction of vortex-induced vibrations, and mitigation of substrate erosion risks. The improved water exchange and mechanical stabilization conferred by the parietal gaps may offer critical advantages in low-oxygen, deep-water environments, effectively supporting the sponge's physiological functions. In contrast, in oxygen-rich, shallow-water habitats, these benefits become less pronounced; the presence of parietal gaps may even impair filter-feeding efficiency, potentially resulting in adverse effects. This study not only reveals the underlying mechanisms of parietal gaps as an adaptive feature to deep-water environments but also highlights the potential of CFD as a tool for investigating the morphological functions of deep-sea and fossil organisms.