intertidal animals. They are readily available and they survive well in the laboratory.
Likely the results are in most respects applicable to deep-sea fauna. Burrowing
through sand and mud, especially stiff mud, has been assumed to be extremely energy
demanding. However, that is a subject of ongoing research, and for several reasons it
is still neither established nor refuted. In particular, it is difficult to distinguish
metabolic energy required for digging (with associated inefficiencies) from that
required by digestion and basal respiration. The mechanics of burrowing, specifically
moving forward through sediment, have been reviewed by Dorgan et al. (2006). They
divide the subject according to sediment type: moving in sand and progressing
through mud work differently.
(^) Sand, both on a wet beach and subtidally, tends to be a stack of grains held in place
by gravity, each grain resting at a few points or edges on the grains below and to the
sides. The packing usually does not minimize the empty space filled with water
between grains. Find a wet beach (nearly flat, fine sand, drained at ebb tide to the
sand surface, but with interstitial spaces filled with water) and stand on it. Wiggle
your feet and you will disturb the stacking, liquefying the fluid–granular mix as the
grains find less space-filling arrangements. You will sink. When you step aside, you
will leave a depression holding water that will then drain down into the beach. Sand
dwellers use this liquefaction to move. Because the magnitude of the gravitational
vector sustaining the stacking is much less effective against horizontal disturbance,
much less force is required to liquefy the sediment when burrowing across the beach
than down into it.
(^) A worm moving horizontally pushes against the sand ahead, anchoring its length by
extending setae into the sand alongside or by hydraulically expanding some
specialized segments. Pushing against the sand ahead is by hydraulic extension, a
process akin to squeezing a sausage-shaped balloon with both hands. Circular muscles
contract, the body narrows and extends forward. Some worms, such as the polychaete
Nephtys, have an eversible proboscis that pops forward under such increased pressure
to push against the sand ahead. It is then drawn back by longitudinal muscles. In
addition, lateral body oscillations can liquefy the sediment all along the body,
enabling motion like swimming through a dense but fluid medium. Sand bottoms are
mostly shallow enough to be repeatedly restacked by wave action, so that animals
moving through them do not permanently convert the bottom to optimum packing of
the grains, stopping their movement employing liquefaction. Motion in sand,
especially moving to just below the surface, like mole crabs (Emerita) repositioning
on a beach, can also involve simply digging and then moving into the hole.
Liquefaction helps cover the animal’s downward retreat. Animals with access to the
water above can enhance liquefaction by pumping water into the sand ahead.
(^) Deep-sea mud (and off-channel estuarine deposits) are at the opposite end of the
particle-size spectrum with interstitial spaces smaller than even meiofauna. Moreover,