Physical constraints on sand crab burrows: Mechanical properties of wet sand explain the size and spatial distributions of burrows on beaches

The diameter and vertical depth of sand crab tunnels in sandy beaches are usually restricted to a few centimeters scale and several tens of centimeters, respectively. We designed a study to determine what physical factors restrict tunnel diameter and predict the maximum attainable tunnel diameter and depth. We collected field data on the size and spatial distributions of ghost crab (Ocypode spp.) burrows on two sandy beaches (Kawage Beach in Tsu, Mie Prefecture, Japan and Sakieda Beach in Ishigaki, Okinawa Prefecture, Japan), where O. ceratophthalma dominants the ghost crab fauna. We measured burrow depths and distance from shoreline in concert with water content of sandy beaches. To explain our observed distributions of crab burrows in the field, we performed experiments in a lab microcosm, comprising a horizontal tunnel through wet sand. We measured the static stability of tunnel structures in relation to water content and two strengths computed from loading force exerted on the sand overlying the tunnels. By comparing field and experimental data, we found that crabs construct their burrows in appropriately wet zones (wet enough to provide sufficient cohesion of the sand grains in tunnel walls to prevent collapse) and that tunnel diameters and depths are sufficiently small to prevent deformation and collapse of their tunnels.

沙滩下沙蟹洞穴的直径与垂直深度通常分别处于厘米级与数十厘米量级。本研究旨在探明限制沙蟹洞穴尺寸的物理因素，并预测洞穴可达到的最大直径与深度。我们于日本三重县津市川边海滩（Kawage Beach）以及冲绳县石垣市崎田海滩（Sakieda Beach）这两处沙滩，采集了鬼蟹（Ocypode spp.）洞穴的尺寸与空间分布数据，该区域的鬼蟹类群以角眼沙蟹（Ocypode ceratophthalma, O. ceratophthalma）为优势种。我们同步测定了洞穴深度、洞穴至海岸线的距离，以及沙滩的含水量。为解释野外观测到的沙蟹洞穴分布格局，我们搭建了实验室微型模拟系统，该系统包含一条贯穿湿沙的水平隧道。我们针对隧道结构的静态稳定性进行了测定，分析其与含水量的相关性，并基于隧道上方覆沙所受荷载力计算得到两项强度指标。通过对比野外与实验数据，我们发现：沙蟹会选择含水量适宜的区域开凿洞穴——该区域的含水量足以使洞壁沙粒获得足够黏结力，避免洞壁坍塌；同时洞穴的直径与深度均被控制在合理范围内，足以防止结构变形与坍塌。