Worms may look simple, but the way they move through the world is anything but. In nature, nematodes such as Caenorhabditis elegans live inside soil, decaying plant matter, and other crowded environments filled with obstacles. These spaces are three-dimensional, irregular, and mechanically complex. Yet most laboratory studies of worm movement observe them either swimming freely in liquid or crawling on flat surfaces, conditions that capture only a narrow slice of what worms actually experience.
A recent study at NCBS found how physical confinement affects the way worms move. The research team built a new experimental system that allows them to directly watch worms moving inside a three-dimensional environment whose mechanical properties can be precisely controlled.
The team created a transparent material made of soft hydrogel particles suspended in liquid. At rest, this material behaves like a soft solid, but when a force is applied, it briefly flows. By adjusting how closely the particles were packed, the researchers could control how confined the worms felt, without changing the chemical surroundings. This allowed them to study the effects of physical confinement alone.
When worms were placed inside this 3D matrix, their movement changed. In loosely packed conditions, the worms moved much like they do in liquid, swinging their bodies from side to side. As the space became more confined, their movement gradually shifted to a crawl-like pattern, similar to how worms move on solid surfaces. The worms did not simply slow down as the environment became tighter. Instead, their forward speed increased at first, reached a maximum, and only decreased when the environment became too confining.
“At first, we expected confinement to always make movement harder,” says Sreepadmanabh, who co-led the study with Saheli Dey. “But instead, we saw that a certain amount of confinement actually helps the worms move more efficiently”
As confinement increased, the speed at which waves travelled along the worm’s body became better matched to its forward motion. This reduced unnecessary sideways movement and allowed more of the worm’s effort to be converted into forward progress.
The worm did not plan this switch willingly. Even worms lacking soft touch sensory neurons showed the same shift in movement. This indicated that the behaviour did not depend on the nervous system making a decision, but instead emerged from the physical interaction between the worm’s body and its environment.
“This shows that the environment itself can shape behaviour,” explains Dr Tapomoy Bhattacharjee, the principal investigator of the study. “The worm does not need to actively sense or choose how to move. The physics of the surroundings naturally guides it into a more efficient mode.”
To support their experiments, the team developed a theoretical model describing how undulating bodies move through materials that resist motion. The model successfully predicted the observed changes in speed and movement pattern, reinforcing the idea that the transition between swimming-like and crawling-like motion is driven by physical principles.
“Our findings show that movement is not controlled by biology alone. Instead, it arises from an interaction between an organism’s body and the physical properties of its environment. By studying movement in realistic 3D settings, this work helps bridge the gap between laboratory experiments and the conditions organisms face in the real world,” says Dr Bhattacharjee.
Full link to the study: https://journals.aps.org/prxlife/abstract/10.1103/sdhy-5g9n
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