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

Abstract

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.

Adaptive foraging strategy of an insular snake (Lycodon semicarinatus, Colubridae) feeding on patchily distributed nests of sea turtles

“…We were not able to dig deep because the hole collapsed quickly. The persistence of a tunnel made on sandy beaches changes depending on the water content of the sand that composes the tunnel, and the persistence increases as the water content increases except under saturated conditions (Shinoda et al, 2019). The water content changes depending on the shape and size of the sand: the water content increases as the sand grain size becomes finer (Mitarai & Nori, 2006).…”

System-Specific Complex Interactions Shape Soil Organic Carbon Distribution in Coastal Salt Marshes

“…To further assess if indirect effects could possibly shape the SOC distribution, we used structural equation modeling to take into account the indirect effects of abiotic soil variables and crab burrows. Based on previous studies [20,30,31,36], we tested for the indirect effects of soil properties (BD, pH and EC) on SOC through affecting plant performance (AGB) and crab burrowing, and the indirect effects of crab burrowing on SOC through affecting plant AGB. These effects were represented by the following three specific causal pathways, (1) soil properties → plant AGB → SOC, (2) soil bulk density (hardness for burrowing) → crab burrowing → SOC and (3) soil bulk density → crab burrowing → plant AGB → SOC.…”

Burrow morphology of ghost crab <i>Ocypode stimpsoni</i> on Ikarashi beach, Niigata, Japan

“…Ansell et al 1972;Seike 2008Seike , 2009Seike & Nara 2008). Among such animals, ghost crabs of the genus Ocypode (Decapoda, Ocypodidae) and their burrows are well studied taxonomically and ecologically because they actively move across and construct large burrows in sandy beaches (Barrass 1963;Utashiro & Horii 1965;Burrows & Hoyle 1973;Chakrabarti 1981;Robertson & William 1981;Chan et al 2006;Seike & Nara 2007;Trivedi & Vachhrajani 2016;Shinoda et al 2019).…”