Terrain following and applications:Caenorhabditis elegansswims along the floor using a bump and undulate strategy

Abstract

Nematodes such as Caenorhabditis elegans are heavier than water. When submerged in water, they settle to the bottom surface. Observations reveal that the animals do not lie flat on the bottom surface, but remain substantially suspended above the surface through continuous collisions with the surface, while maintaining their swimming gaits. Consequently, the swimming animals follow the bottom surface topography. When the bottom surface is inclined, the animals swim up or down along the incline. As the magnitude of the gravitational force can be easily estimated, this behaviour provides a convenient means to estimate the animal's propulsive thrust. The animals' tendency to follow the surface topography provides a means to control the swimmers' trajectories and direction of motion, which we demonstrate with a saw tooth-like ratchet that biases the animals to swim in a selected direction. The animals can also serve as surface topography probes since their residence time as a function of position provides information on surface features. Finally, we take advantage of surface following to construct a simple motility-based sorter that can sort animals based on genotype and state of health. IntroductionMotility assays for nematodes, such as Caenorhabditis elegans, often monitor, from above, the motion of animals suspended in aqueous solutions. In most cases, the animals are observed to swim. Caenorhabditis elegans is, however, heavier than water [1] and sediments to the bottom. Although nematodes' sedimentation per se has not been investigated extensively, nematologists have known for a long time that nematodes sediment in a gravitational field and have taken advantage of this phenomenon to isolate animals (i.e. in the Baermann funnel method) [2]. Nematode settling is also used extensively in various assay preparations [3].That gravitational forces play a significant role in nematodes' hydrodynamics is hardly surprising. To demonstrate that gravitational forces impact nematodes' swimming trajectories, we carry out a simple scaling analysis. Fluid mechanicians define the gravity parameter G ¼ ðr a À r l Þ=r l  ga 2 =nU, representing the ratio of the gravitational body force ðr a À r l Þga 2 L and the viscous force mUaL=a. In the above, r a and r l are, respectively, the density of the animal and the suspending liquid, L is the length of the animal, g is gravitational acceleration, a is the animal's radius, m is the suspending liquid's viscosity, v ¼ m/r l , and U is the animal's velocity. When an adult C. elegans is suspended in water, ðr a À r l Þ=r l 0:07 [1]. Adult C. elegans has a radius a 40 mm and length L 1 mm. The liquid kinematic viscosity n 10 26 m 2 s 21 and the adult animal's velocity U 200 mm s 21. G is of order 1, indicating that gravitational forces are as important as propulsive forces and significantly impact the animal's swimming trajectory.What happens to the animal once it settles to the bottom? One might naively assume that the animal lies flat on the bottom surface. If this were the case, the anima...

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