43.9 Thursday, Jan. 5 Peristaltic burrowing in beach sands by the polychaete Thoracophelia mucronata DORGAN, K.M.*; ROUSE, G.; Scripps Institution of Oceanography; Scripps Institution of Oceanography firstname.lastname@example.org
Recent work has shown that a range of polychaete annelids extend burrows through muddy sediments by fracture. Beach sands, however, are non-cohesive granular materials with different mechanics than elastic muds. The opheliid, Thoracophelia mucronata, burrows by peristalsis in well-sorted beach sands, and differs anatomically from other opheliids that use lateral sinusoidal movements. Kinematic analysis of T. mucronata burrowing in cryolite, a clear mineral with similar refractive index to seawater, shows direct peristaltic waves ending with forward progression of the head and expansion of the anterior. Using chaetae stained with a fluorescent chitin dye as a body markers, we show that the body expands radially against the sand, and forward movement occurs in narrow, undilated segments. Both in cryolite and in spherical glass beads, movement of grains more than a few grain diameters from the worm was not observed during peristalsis, although worms occasionally dilate the entire anterior of the body and displace grains radially. No regions of fluidization were observed. This indicates that the worm is using normal rather than shear forces to locally rearrange grains, and that friction is minimal. Analysis of burrowing behavior helps explain the functions of several morphological characteristics - the anterior septa allows the worm to apply higher forces near the front where most of the grain repacking occurs, and a glandular ridge extending from the tenth segment is similar in size to the sand grains and likely anchors the worm in place during peristalsis. Burrowing behavior is kinematically similar to peristaltic burrowing in muds in that anterior expansions apply forces perpendicular to the direction of locomotion, but the mechanical response of the sand is very different.