  Many organisms live in confined fluidic environments such as the thin liquid
layers on the skin of host organisms or in partially- saturated soil. We
investigate the collective behaviour of nematodes in a thin liquid layer, which
was first observed by Gray and Lissmann, [J. Exp. Biol. 41, 135 (1964)]. We
show experimentally that nematodes confined by a thin liquid film come into
contact and only separate again after some intervention. We attribute this
collective motion to an attractive force between them arising from the surface
tension of the layer and show that for nearby nematodes this force is typically
stronger than the force that may be exerted by the nematodes' muscles. We
believe this to be the first demonstration of the "Cheerios effect" acting on a
living organism. However, we find that being grouped together does not
significantly alter the body stroke and kinematic performance of the nematode:
there are no statistically significant changes of the Strouhal number and the
ratio of amplitude to wavelength when aggregated. This result implies that
nematodes gain neither a mechanical advantage nor disadvantage by being grouped
together; the capillary force merely draws and keeps them together.
