Proc. Natl Acad. Sci USA http://doi.org/22g (2015)

Credit: © HEITI PAVES / ALAMY

What's the best way of dealing with an oncoming current? For worms, and other undulatory aquatic animals, the answer might be to meet it head on. There is evidence to suggest that these low-Reynolds-number swimmers undergo rheotaxis, orienting themselves with or against a flow — sometimes even overcoming chemical cues to do so. A study from Jinzhou Yuan and co-workers indicates that this may not be a conscious decision, and that rheotaxis could simply be a matter of hydrodynamics.

Yuan et al. performed experiments on swimming Caenorhabditis elegans (pictured) and noticed that near surfaces, the worms tended to orient themselves to face the flow. The team had previously identified interactions between the swimmer's flow field and the surface, which generated a torque that tilted the swimmer towards the surface. Experiments with touch-insensitive mutants confirmed that this was a hydrodynamic effect, independent of the animal's nervous system. Worms swimming to the surface were then subject to a velocity gradient: the fluid was stationary at the surface and so the animal's tail felt a stronger flow than its head, effecting a head-first orientation.