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.