The polychaete Nereis virens extends its burrow in muddy sediments by fracture. We use gelatin as a clear analog for muds, as it has similar mechanical responses to forces applied on the temporal and spatial scales of burrowers.
Dorsal view (Dorgan et al., 2007)
Lateral view (Dorgan et al., 2007)
Burrowing behavior depends on the mechanical properties of the sediment, specifically the ratio of fracture toughness to stiffness. Nereis virens burrowing in gelatin, with similar mechanical properties to sediment, extends its burrow by moving its head from side to side and driving itself forward like a wedge.
(Dorgan et al., 2008)
In a tougher gel, the worm makes its body thicker and blunter by expanding its body dorso-ventrally with its hydrostatic skeleton and by everting its pharynx. In contrast to the worm in gelatin, it does not move its head from side to side, a behavior that extends the crack laterally away from the worm and reduces the elastic force compressing the worm dorso-ventrally. With the lateral crack edges close to the body, the worm can apply greater stresses to the crack walls.
(Dorgan et al., 2008)
In a stiffer gel, the worm's body is thinner (in the dorso-ventral direction), and it alternates between moving its head from side to side (extending the lateral crack edge farther than the worm in gelatin) and everting its pharynx, which extends the crack out in front of the worm.
(Dorgan et al., 2008)
Even though the morphology is very different from that of Nereis, Cirriformia moorei burrows by the same mechanism. Instead of applying stress on the burrow walls by everting a pharyx, it expands the anterior of its body using peristalsis. Whereas Nereis moves its head from side to side to extend the burrow laterally, Cirriformia has a pointed head that it pushes forward, then expands the body laterally, extending the crack edges (Che and Dorgan, 2010a).
Cirriformia moorei also burrows backwards! The kinematics are very similar to forward locomotion (Che and Dorgan, 2010b).
The glycerid polychaete, Hemipodus simplex, has a proboscis that is much longer than the pharynx of Nereis. However, most of the stress is applied by the tip of the everted proboscis, which flares at full eversion. These focused forces are larger than needed to extend the burrow by fracture, and may be important in creating permanent burrow structures (Murphy and Dorgan, 2011).
Lateral view