Meeting Abstract
93.4 Monday, Jan. 6 14:15 Effect of moisture content on burrowing performance of ghost crabs SPRINGTHORPE, D*; NG, P; FULL, R.J.; Univ. of California, Berkeley; Univ. of California, Berkeley; Univ. of California, Berkeley dspringthorpe@berkeley.edu
Prior work demonstrated that ghost crabs (Ocypode quadrata) build semi-permanent burrows in a relatively uniform substrate using a complex behavioral suite that includes specialized postures, locomotion in confined environments and goal-directed manipulation of the substrate. However, ghost crabs burrowing in nature must contend with highly variable environments where physical characteristics can quickly be substantially altered. Here, we examine burrowing performance in our artificial beach environment in response to changes in gravimetric moisture content (GMC). We measured burrow extension rate and burrow architecture for crabs ranging in body size from 26-45mm in carapace width. Crabs were unable to burrow in dried sand and in sand with a GMC above 21%. For GMCs ranging from 3-12%, which represents a 9% increase in substrate density and an estimated 30% increase in elastic shear modulus, crabs dug J- or U-shaped burrows, consistent with both prior laboratory and field observations. Crabs burrowed between 50 and 75 cm in depth. Smaller crabs burrowed significantly faster than larger crabs. Crabs with a carapace width of 26 mm extended their burrows (0.9±0.5 cm/min ) approximately three times faster than crabs with a carapace width of 45 mm (0.3±0.2 cm/min). Normalized burrow extension rate (body lengths per minute) demonstrated a significant negative correlation with carapace width. Varying GMC from 3% to 12% produced no significant changes in burrow extension rate. Ghost crab burrowing is a flexible behavior capable of accommodating a range of environmental conditions. Further experiments to identify the compensatory mechanisms will offer new insight into mobile manipulation with simple appendages and inspire a new generation of robots capable of transforming their terrain.
