  Understanding principles of neurolocomotion requires the synthesis of neural
activity, sensory feedback, and biomechanics. The nematode \textit{C. elegans}
is an ideal model organism for studying locomotion in an integrated
neuromechanical setting because its neural circuit has a well-characterized
modular structure and its undulatory forward swimming gait adapts to the
surrounding fluid with a shorter wavelength in higher viscosity environments.
This adaptive behavior emerges from the neural modules interacting through a
combination of mechanical forces, neuronal coupling, and sensory feedback
mechanisms. However, the relative contributions of these coupling modes to gait
adaptation are not understood. The model consists of repeated neuromechanical
modules that are coupled through the mechanics of the body, short-range
proprioception, and gap-junctions. The model captures the experimentally
observed gait adaptation over a wide range of mechanical parameters, provided
that the muscle response to input from the nervous system is faster than the
body response to changes in internal and external forces. The modularity of the
model allows the use of the theory of weakly coupled oscillators to identify
the relative roles of body mechanics, gap-junctional coupling, and
proprioceptive coupling in coordinating the undulatory gait. The analysis shows
that the wavelength of body undulations is set by the relative strengths of
these three coupling forms. In a low-viscosity fluid environment, the
competition between gap-junctions and proprioception produces a long wavelength
undulation, which is only achieved in the model with sufficiently strong
gap-junctional coupling.The experimentally observed decrease in wavelength in
response to increasing fluid viscosity is the result of an increase in the
relative strength of mechanical coupling, which promotes a short wavelength.
