Date Published: May 1, 2019
Publisher: Public Library of Science
Author(s): Ayuko Shinoda, Shin-ichi Fujiwara, Hirofumi Niiya, Hiroaki Katsuragi, Geerat J. Vermeij.
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.
Mechanical properties of tunnels in soil have long been studied by civil engineers [1–5]. The focus of such studies has been to develop safe methods for reinforcing and stabilizing tunnels to prevent them from collapsing. The collapse of voids in cohesive granular matter is aso thought to be responsible for pit structures on comet surfaces [6, 7]. Both the minimum pit size on comets and the maximum size of crab burrows on sandy beaches have been estimated by determining the void stability of a cohesive granular layer .
We performed a set of experiments to characterize the mechanical properties of wet granular matter by constructing a microcosm with an artificial tunnel structure, in which we used beach sand to encase the tunnels. In similar experiments, we had used spherical glass beads to focus on the intrinsic physical attributes of wet granular matter [8, 22]. In contrast, we conducted this experiment with natural sand to compare the experimental results directly with naturally constructed ghost crab burrows. By comparing experimental results with the field data, we can determine limitations on the mechanical stability of crab burrows.
Ghost crab burrow tunnels vary within a narrow range of diameters and depths even though the water content of the beach sands through which they burrow varies widely in space. The spatial distribution of crab burrows is related to the distribution in water content of beach sands. Based on our lab experiments, we identified how water content in sands is related to tunnel stability. We also found that tunnels are deformed in a variety of ways, mainly dependent on the initial diameter of tunnels. We defined two types of tunnel strengths, based on effective yield and maximum strengths, which relate to the effective and maximum force that can be applied to sand overlying a tunnel when it deforms by the overburden weight.