  Free-living nematode Caenorhabditis elegans is a powerful genetic model,
essential for investigations ranging from behavior to neuroscience to aging,
and locomotion is a key observable used in these studies. However, despite the
fact that in its natural environment C. elegans moves in three-dimensional (3D)
complex media (decomposing organic matter and water), quantitative in
vestigations of its locomotion have been limited to two-dimensional (2D)
motion. Based on our recent quantitative analysis of 2D turning maneuvers
[Phys. Fluids 25, 081902 (2013)] we follow with the first quantitative
description of how C. elegans moves in 3D environments. We show that by
superposing body torsion and 2D undulations, a burrowing or swimming nematode
can rotate the undulation plane. A combination of these roll maneuvers and 2D
turns associated with variation of undulation-wave parameters allows the
nematode to explore 3D space. We apply our model to analyze 3D chemotaxis of
nematodes burrowing in a gel and swimming in water; we conclude that the
nematode can achieve efficient chemotaxis in different environments without
adjusting its sensory-motor response to chemical signals. Implications of our
findings for understanding of 3D neuromuscular control of nematode body are
discussed.
