Meeting Abstract

37.1  Monday, Jan. 5  Worms as wedges: Effects of sediment mechanics on burrowing behavior DORGAN, Kelly M*; ARWADE, Sanjay R; JUMARS, Peter A; University of California, Berkeley; University of Massachusetts, Amherst; University of Maine kelly.dorgan@berkeley.edu

Marine muds are elastic solids through which animals move by propagating a crack-shaped burrow. Fracture mechanics depend on material stiffness as well as fracture toughness, so we prepared a range of transparent gels that varied in stiffness and fracture toughness to assess the dependence of burrowing behavior on these material properties. When the polychaete Nereis virens elongated its burrow, it altered its body shape and behavior across these gels in a manner consistent with predictions based on theory of stable, wedge-driven crack propagation. For wedge-driven fracture, the ratio of fracture toughness to stiffness of a material is more important than the absolute values; more work is required to crack a tough material, but a wedge does more work in displacing a stiff material. In materials with higher fracture toughnesses, worms everted their pharynges to become thicker and blunter wedges, as theory predicted. In stiff materials with low toughness, worms moved their heads from side to side to extend the crack edges laterally, relieving elastic forces in the sediment compressing them and allowing them to maintain body shape more easily. We introduce a dimensionless wedge number to characterize the relative importance of work required to fracture the sediment material and extend the burrow and work to maintain body shape against the elastic restoring force of the material. The mechanism of burrowing by crack propagation is utilized across a range of material properties found in natural muds, and variation in these properties strongly influences burrowing behaviors.