It's tough to be small: dependence of burrowing kinematics on body size

Burrowing marine infauna are morphologically diverse and range in size over several orders of magnitude. Whilst effects of ontogenetic and morphological differences on running, flying and swimming are relatively well understood, similar analyses of burrowing mechanics and kinematics are lacking. The polychaete Nereis virens Sars extends its burrow by fracture, using an eversible pharynx to exert force on the walls of the burrow. The resulting stress is amplified at the anterior tip of the burrow, which extends when the stress exceeds the fracture toughness of the material. Here we show that the polychaete Cirriformia moorei extends its burrow by a similar mechanism, but by using its hydrostatic skeleton rather than an eversible pharynx. Based on the dimensionless wedge number, which relates work of fracture to work to maintain body shape against the elasticity of sediment, we predicted that smaller worms would exhibit behaviors characteristic of tougher sediments and that scaling of kinematics would reflect decreasing difficulty in fracturing sediment with increasing body size. We found that smaller worms were relatively blunter and thicker, and had a greater variation of thickness than larger worms as they burrowed. Although these kinematic differences increase the stress amplification at the crack tip, smaller worms still generate lower stress intensity factors. The greater relative body thickness and shape changes of smaller worms are consistent with ontogenetic changes in forces exerted by earthworms, and are likely driven by the challenge of exerting enough stress to extend a crack with a small body size.