It's International Trace Fossil Day (not really), so let's talk about bioturbation!
One of the perennial problems with animals burrowing into sediment has been the presumption that, energetically, it is way more costly than walking, running, swimming, or flying, especially when you are burrowing into cohesive sediment. As such, it has always been a little tricky to understand why any critter would be willing to expend so much energy in evolving into a benthic bioturbator. Additionally, part of the problem seems to have been that physically studying the energy use of a burrowing critter in situ is a bit tricky. As such, folks in the past used Newton's Third Law as an assumption, and always seemed to come up with burrowing as a very high cost (metabolically) biological activity.
However, some fairly recent studies by Dorgan et al. (2005) and Dorgan et al. (2007) have overturned some of these assumptions through novel and (to me, at least) insightful experimental design. To give away the punch line, burrowing is actually much easier than we had previously thought, since animals can use crack propagation to move through muddy sediments, expending much less energy than the previous models of whole-animal burrowing indicated.
First off, both of these papers point out that previous studies of animal burrowing always took place with the animal near a rigid wall (like the transparent edge of an aquarium); as such, the animal behavior exhibited was not, energetically, the same as moving through deformable sediment. In order to overcome this wall effect, the workers used a transparent gelatin as their muddy sediment analog, which has similar mechanical properties to marine muds. Also, and this is the slick part, gelatin is birefringent, meaning that the workers could look at it through polarized light and clearly track the deformation occurring around the critter AS IT HAPPENED. Nifty, huh? The picture below (and the caption) was seized from the Dorgan et al. (2005) paper.
The worm (Nereis virens) moved through the gelatin by exerting a dorsoventral force against the walls of its burrow, resulting in an oblate hemispherical crack; stresses are concentrated at the tip of this crack, and exceed the critical stress needed for the crack to propagate. In otherwords, the worm creates a wedge-driven fracture (like an axe being struck into a log), rather than actively excavating a vacuity in the sediment. The picture below, also from the Dorgan et al. (2005) paper, explains the whole process visually.
The results of this study match (or exceed) modeled critical intensity stress values for sediment, suggesting that crack propagation processes are a viable burrowing mechanism. It's a pretty slick study, I think, and shows how some of the benthos may do their burrowin' in the sediment.
Dorgan, K.M., Jumars, P.A., Johnson, B., Baudreau, B.P., and Landis, E., 2005, Burrow extension by crack propagation: Nature, v. 433, p. 475.
Dorgan, K.M., Arwade, S.R., and Jumars, P.A., 2007, Burrowing in marine muds by crack propagation: kinematics and forces: The Journal of Experimental Biology, v. 210, p. 4198 - 4212.