ABSTRACT
Burrowing by benthic infauna mixes both sediment grains and interstitial fluids, affecting sedimentary redox conditions and determining fates of organic matter and pollutants. Explicit, quantitative analyses of material properties of sediments, however, have been applied only recently to understand mechanisms of burrowing. Muds are elastic solids that fracture under small tensile forces exerted by burrowers, and are dominated by adhesive forces between sediment grains and the surrounding mucopolymeric gel and (or)
by cohesion of this gel. By contrast, in clean sands behaving as granular materials, gravity is a much more significant force holding grains together than is adhesion or cohesion. Burrowers in muds have diverse structures that act as wedges to propagate cracks and elongate their burrows. In sands, increased rugosity on a small, and liquefaction on a larger scale, facilitate displacement of the grains that carry compressive forces along distinct force chains or arches. The classic dual-anchor system described for burrowers is reinterpreted as having several additional functions. The characteristic dilations or expansions function primarily as wedges that exert lateral tensile forces to propagate cracks forward, secondarily as double O-ring seals holding fluid pressure in the advancing burrow (maintaining tensile stresses needed to open a crack), and thirdly as anchors (to pull the shell along in bivalves in particular). Burrowing bivalves are wedges. In the case of burrowing gammarid amphipods, the dorsal exoskeleton mirrors the shape of half a sedimentary bubble and constitutes a wedge. A great many anatomical features of burrowers can now be understood analogously. The identification of the mechanisms of burrowing by crack propagation suggests that a substantial revision of the previously described feeding guilds of polychaetes is required.
