The collective motion of nematodes in a thin liquid layer

TL;DR

This study demonstrates that nematodes in a thin liquid layer exhibit collective motion driven by surface tension, with the capillary force acting as an attractive mechanism without affecting their individual movement performance.

Contribution

First experimental evidence of the 'Cheerios effect' acting on living organisms, showing capillary forces induce collective behavior without altering nematodes' kinematic performance.

Findings

Capillary forces cause nematodes to cluster in thin liquid layers.

Clustered nematodes do not show significant changes in movement dynamics.

Surface tension-driven attraction is stronger than muscular forces at close proximity.

Abstract

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.