Swimming C. elegans in a wet granular medium.

C. elegans is a round worm that lives in wet soil and moves by undulating its body in a sinusoidal wave. We perform experiments on wild-type C. elegans swimming in a granular medium composed of 98 m glass particles and aqueous fluid. Figure 1 shows that a worm swims faster in a granular medium V 0.46 mm /s than in a fluid without particles V 0.37 mm /s . C. elegans in a granular medium undulates at a lower frequency and with the same amplitude. We measured the Strouhal number St= fA /V , a ratio of lateral to forward velocity, to characterize the surrogate efficiency of locomotion. Swimming motion in a granular medium, which more closely simulates its natural environment, is more efficient St 0.5 than swimming in a fluid St 1.5 . FIG. 1. a A wild-type C. elegans swims in a wet granular medium and b in a fluid. CHAOS 18, 041106 2008