A visit with Bernie Boudreau first exposed me to the oblate spheroidal shape of bubbles in mud and the fact that they move upward by propagating cracks through this medium. Since 1977, when Kristian Fauchald and I taught a course on polychaetes at the Friday Harbor Laboratories, I have been using double-strength gelatin to demonstrate burrowing, but until that visit did not realize that the mechanical properties of mud were roughly similar over relevant time and space scales to those of 2X seawater-gelatin (Johnson et al. 2002). Indeed the shapes of bubbles in mud can be predicted from two simple parameters, Young's modulus and the critical stress intensity that is just sufficient to cause a crack to propagate. Predicted shapes are observed (Boudreau et al. 2005). I discussed the implications for burrowing costs with Kelly Dorgan, who was beginning her Ph.D. studies in my lab, and she ran with the problem (got cracking). Her innovation of photoelasticity to burrowing studies led to the conclusion that indeed burrowing is much less expensive energetically than had been thought (Dorgan et al. 2005, 2007).
Understanding the mechanism of burrowing brought immediate ability to re-interpret morphological structures as well as re-interpret the basic process of burrowing (Dorgan et al. 2006). Clams are wedges. Amphipods are half bubbles with legs. Soft-bodied burrowers typically make a lateral expansion to generate an axial crack, just like the ones you hate to see running through your windshield, and then simply move into the already-made crack. Because of the inherently discoidal shapes of cracks in mud, fast-burrowing polychaetes tend to have bilateral symmetry, whereas slower worms tend toward radial symmetry and those in between to have bilateral symmetry in anterior segments and radial symmetry in posterior segments. Burrowing in mud is radically different from burrowing in sand (Dorgan et al. 2006) and burrowing in sand may have evolved much earlier (Jumars et al. 2007).
Gelatin is not identical in properties to mud, however. We experimented with a variety of transparent polymer mixtures to approximate more closely the properties of sediments. Although we were not able to achieve a perfect match, we did find that nereids adopt shapes during burrowing that are predictable from the physical properties of the medium through which they burrow (Dorgan et al., in press ).
Whereas crack propagation by simple linear elastic fracture apparently explains the making of the crack in many taxa, sediments do creep over longer time scales (are viscoelastic). Quantifying how both crack propagation and subsequent creep contribute to bioturbation is a new goal in our laboratory so that the relevant parameterization can be incorporated into automaton models, extending those innovated by Bernie Boudreau. More generally, we are interested to know how mechanics of burrowing through crack propagation interacts with processes of particle selection and of bioturbation.
Johnson, B.D., Boudreau, B.P., Gardiner, B.S., and Maass, R.. 2002. Mechanical response of sediments to bubble growth. Marine Geology 187: 347363,
Jumars, P.A., K.M. Dorgan, L.M. Mayer, B.P. Boudreau and B.D. Johnson. 2007. Physical constraints on infaunal lifestyles: May the persistent and strong forces be with you. pp. 442-457 In: W. Miller, III, Ed. Trace Fossils: Concepts, Problems, Prospects. Elsevier, Amsterdam.
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