  The effects of fluid viscosity on the kinematics of a small swimmer at low
Reynolds number are investigated in both experiments and in a simple model. The
swimmer is the nematode Caenorhabditis elegans, which is an undulating
roundworm approximately 1 mm long. Experiments show that the nematode maintains
a highly periodic swimming behavior as the fluid viscosity is varied from 1.0
mPa-s to 12 mPa-s. Surprisingly, the nematode's swimming speed (~0.35 mm/s) is
nearly insensitive to the range of fluid viscosities investigated here.
However, the nematode's beating frequency decreases to an asymptotic value
(~1.7 Hz) with increasing fluid viscosity. A simple model is used to estimate
the nematode's Young's modulus and tissue viscosity. Both material properties
increase with increasing fluid viscosity. It is proposed that the increase in
Young's modulus may be associated with muscle contraction in response to larger
mechanical loading while the increase in effective tissue viscosity may be
associated with the energy necessary to overcome increased fluid drag forces.
