the miniscule particles are consistently bound together in a matrix of mucopolymers,
forming an elastic, quasi-solid different in behavior from saturated sand. Organic
“matrix surrounds all particles, of whatever size” (Watling 1988). Dorgan et al. (2005,
2008) use clear gelatins of various penetrabilities to model mud, with the advantage
that benthonts moving through gel blocks can be seen and photographed. They have
used the polychaete Nereis as a model animal that can be induced to move forward in
gelatin by orienting them in vertical starter burrows. Nereis pushing through such
media force the propagation of cracks somewhat ahead of the anterior tip. The force
opening the crack, again from hydraulic expansion of the anterior end of the worm
(that is, the “prostomium”, the anterior sides and in Nereis a proboscis) is tensile,
pushing perpendicular to the direction of travel and causing the crack to expand in a
U-shape ahead. Stress (tensile force per unit area) on the sediment is amplified ahead
of the worm by the lever action of the worm on the elastic mud walls behind the
opening tip of the crack. Both “O-ring” body expansions and setae may serve as
anchors along the length of the worm to assist in pushing the anterior tip forward – a
very old notion of how worms, including earthworms, move. The news is the splitting
of the sediment well ahead of the prostomium. After establishing a crack, the worm
moves into it. Setae are drawn inward as segments elongate and are pulled forward.
An important aspect of the cracking process is that the burrow shape is flat and wide
(Fig. 14.16; Dorgan et al. 2005); thus many polychaetes when actually in the sediment
are strongly compressed in the dorso-ventral direction.
Fig. 14.16 Outlines of a burrow made by Nereis virens in a gelatin block
photographed with polarized light. (a) Anterior, end-on view of the crack with the
worm’s dorsal and ventral surfaces against the gelatin. (b) Dorsal view with the
outline of the burrow indicated by arrows.
(^) (From Dorgan et al. 2005.)
(^) Many other benthonts moving beneath the sediment surface also generate cracking.
Snails and clams both extend a narrow edge of their foot as a wedge anchored by the
shell in the burrow behind, then pull the shell into the crack. Dorgan et al. (2008)
provide a detailed mechanical description of movement in mucopolymer-bound mud
based on the stress-to-strain relationships of “visco-elastic” substances, basically a