Edited by David A. Weitz, Harvard University, Cambridge, MA, and approved October 31, 2019 (received for review July 1, 2019)
The physical principles of locomotion inside sediment beds at the bottom of water bodies are poorly understood because of the remoteness and opacity of the medium. We show that the common oligochaete Lumbriculus variegatus moves faster in sediments compared with water, while using a similar combination of elongation–contraction and transverse undulatory strokes, exploiting a higher drag anisotropy of its body. Tracking the worm inside transparent sediments, we show that its speed can be captured by a linear combination of calculated speeds with resistive-force theory and an anchor model of peristaltic motion. We then demonstrate that these locomotion strategies can be observed in composting worms and can be used to move effectively in media with a wide range of yield strengths.
We investigate the dynamics of Lumbriculus variegatus in water-saturated sediment beds to understand limbless locomotion in the benthic zone found at the bottom of lakes and oceans. These slender aquatic worms are observed to perform elongation–contraction and transverse undulatory strokes in both water-saturated sediments and water. Greater drag anisotropy in the sediment medium is observed to boost the burrowing speed of the worm compared to swimming in water with the same stroke using drag-assisted propulsion. We capture the observed speeds by combining the calculated forms based on resistive-force theory of undulatory motion in viscous fluids and a dynamic anchor model of peristaltic motion in the sediments. Peristalsis is found to be effective for burrowing in noncohesive sediments which fill in rapidly behind the moving body inside the sediment bed. Whereas the undulatory stroke is found to be effective in water and in shallow sediment layers where anchoring is not possible to achieve peristaltic motion. We show that such dual strokes occur as well in the earthworm Eisenia fetida which inhabits moist sediments that are prone to flooding. Our analysis in terms of the rheology of the medium shows that the dual strokes are exploited by organisms to negotiate sediment beds that may be packed heterogeneously and can be used by active intruders to move effectively from a fluid through the loose bed surface layer which fluidizes easily to the well-consolidated bed below.
Author contributions: A.K. and B.R. designed research, performed research, analyzed data, and wrote the paper.
The authors declare no competing interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at https://www.pnas.org/lookup/suppl/doi:10.1073/pnas.1911317116/-/DCSupplemental.
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