Abstract

C. elegans is a small (1mm long) worm living in the soil, and extensively studied as a model organism for molecular biology and neuroscience. It relies on undulatory locomotion to move through its natural habitat: a wave of muscular contraction propagates from the head to the tail, thus pushing the worm forward. Two examples of locomotion have been described in the laboratory. The worm crawls on the surface of agar gels, where capillary forces constrain it to a bi-dimensional plane. In water the worm swims, with a Reynolds number close to 1. These two locomotion modes are characterized by distinct body shapes and bending frequencies. It remains unclear whether these two modes are the modulation of a single locomotion mechanism or rather the expression of different patterns of neural excitation.

To investigate the transition from crawling to swimming, we designed an original experimental set-up. A worm, immersed in a liquid, is gradually confined between an agar gel and a glass plate. We are thus able to observe a continuous evolution of the locomotion gait from free swimming to crawling as the degree of confinement is increased. The evolution of the measured bending frequency, wavelength and body curvature of the worm displays no discontinuity in the wild-type nematode as the confinement is varied. Swimming and crawling thus simply result from the modulation of a single gait. Additional experiments, where a crawling worm is suddenly released by lifting up the glass plate indicate that this modulation is smooth and does not require an adaptation time when the environment changes. The use of mutants provides some additional clues about the neural and molecular mechanisms behind this modulation: the regulation of the locomotion in response to changes in the mechanical environment might rely on the integration of external signaling as well as proprioception.