Burrowing in marine muds by crack propagation: kinematics and forces

Abstract

The polychaete Nereis virens burrows through muddy sediments by exerting dorsoventral forces against the walls of its tongue-depressor-shaped burrow to extend an oblate hemispheroidal crack. Stress is concentrated at the crack tip, which extends when the stress intensity factor (K I ) exceeds the critical stress intensity factor (K Ic ). Relevant forces were measured in gelatin, an analog for elastic muds, by photoelastic stress analysis, and were 0.015±0.001·N (mean ± s.d.; N=5). Measured elastic moduli (E) for gelatin and sediment were used in finite element models to convert the forces in gelatin to those required in muds to maintain the same body shapes observed in gelatin. The force increases directly with increasing sediment stiffness, and is 0.16·N for measured sediment stiffness of E=2.7ϫ10 4 ·Pa. This measurement of forces exerted by burrowers is the first that explicitly considers the mechanical behavior of the sediment. Calculated stress intensity factors fall within the range of critical values for gelatin and exceed those for sediment, showing that crack propagation is a mechanically feasible mechanism of burrowing. The pharynx extends anteriorly as it everts, extending the crack tip only as far as the anterior of the worm, consistent with wedge-driven fracture and drawing obvious parallels between soft-bodied burrowers and more rigid, wedge-shaped burrowers (i.e. clams). Our results raise questions about the reputed high energetic cost of burrowing and emphasize the need for better understanding of sediment mechanics to quantify external energy expenditure during burrowing.Supplementary material available online at

Environmental Constraints on the Mechanics of Crawling and Burrowing Using Hydrostatic Skeletons

“…Static and kinetic frictional forces are smaller than normal forces, as indicated by friction coefficients, which are the ratios of frictional to normal forces under steady conditions, of less than one. Although the normal forces for burrowers are large, it seems likely that kinetic friction forces are smaller than those predicted from the kinetic friction coefficient because the animal is exerting force on the cohesive, elastic mud with the stationary part of the body while little force is exerted by the moving segments [40]. This is similar to mechanisms identified for crawlers to increase the static relative to kinetic friction, except burrowers have the added advantage of two surfaces against which to exert normal force rather than only one.…”

“…Nereis everts its pharynx, extending the throat region anteriorly out of the mouth like a balloon, to exert a dorsoventral force on the walls of the crack, and stress in the sediment is amplified at the crack tip. The worm drives itself forward like a wedge, extending the crack anteriorly [40]. Forward movement during burrowing is undulatory, although visualization of stresses around the burrowing worm show stress patterns in gelatin resulting from an anterior-traveling peristaltic wave [40].…”

“…The worm drives itself forward like a wedge, extending the crack anteriorly [40]. Forward movement during burrowing is undulatory, although visualization of stresses around the burrowing worm show stress patterns in gelatin resulting from an anterior-traveling peristaltic wave [40]. Quantitative kinematic comparisons to crawling or swimming nereids have not been conducted, but the constraints of lateral crack edges suggest that the amplitude of the undulatory wave is likely smaller for burrowing worms.…”

The mechanics of explosive dispersal and self-burial in the seeds of the filaree, Erodium cicutarium (Geraniaceae)

“…Farquhar and Zhao, 2006). Others have applied crack propagation theory to the burrowing of worms in sediment (Dorgan et al, 2005;Dorgan et al, 2007), in which case fracture with the minimum energetic cost is necessary.…”

Robotics: Science and Systems VI

“…Many desert organisms like scorpions, snakes, and lizards burrow and swim effectively in sand [33,34,35,36,37] to escape heat and predators, and hunt for prey [38,39]. It has been hypothesized that many of these animals have evolved morphological adaptations like marked body elongation and limb reduction to deal with deformable terrain [40,41].…”