Abstract
Numerous worm and arthropod species form physically-connected aggregations in which interactions among individuals give rise to emergent macroscale dynamics and functionalities that enhance collective survival. In particular, some aquatic worms such as the California blackworm (Lumbriculus variegatus) entangle their bodies into dense blobs to shield themselves against external stressors and preserve moisture in dry conditions. Motivated by recent experiments revealing emergent locomotion in blackworm blobs, we investigate the collective worm dynamics by modeling each worm as a self-propelled Brownian polymer. Though our model is two-dimensional, compared to real three-dimensional worm blobs, we demonstrate how a simulated blob can collectively traverse temperature gradientsviathe coupling between the active motion and the environment. By performing a systematic parameter sweep over the strength of attractive forces between worms, and the magnitude of their directed self-propulsion, we obtain a rich phase diagram which reveals that effective collective locomotion emerges as a result of finely balancing a tradeoff between these two parameters. Our model brings the physics of active filaments into a new meso- and macroscale context and invites further theoretical investigation into the collective behavior of long, slender, semi-flexible organisms.
Highlights
Throughout the living world, interactions among individuals, and between individuals and the environment, give rise to emergent collective phenomena across scales: cell migration, flocking birds, schooling fish, and human crowds moving in unison [1,2,3,4]
While most examples of collective behavior occur in regimes without physical contact among individuals, many insect, arthropod, and worm species form dense aggregations, where constituent individuals are in constant physical contact with each other, for the purposes of survival, foraging, migration, and mating [5,6,7]
We focus on Lumbriculus variegatus, an aquatic worm known as the California blackworm or mudworm
Summary
Throughout the living world, interactions among individuals, and between individuals and the environment, give rise to emergent collective phenomena across scales: cell migration, flocking birds, schooling fish, and human crowds moving in unison [1,2,3,4]. It was asserted that a model of collective worm behavior would likely need to account for the self-propelled tangentially-driven motion of individual worms [23] Motivated by these experiments and insights on aggregations of blackworms and sludge worms [16, 17, 23], we pursue a theoretical model that captures the collective behavior of aquatic worms by linking together local rules governing interactions between individual worms with the emergent macroscale dynamics of the blob. The bending rigidity, activity, aspect ratio, and density of filaments define phases of flocking, spiraling, clustering, jamming, and nematic laning [28] Drawing upon these models, we model worms as twodimensional active Brownian polymers, driven by experimental observations of the behavior of single worms (Figure 1A), worm blobs (Figure 1B), and the collective locomotion of worm blobs in temperature gradients (Figure 1C; [17]). Number of monomers Equilibrium distance between monomers Interaction coefficient, single worm Spring constant Bending stiffness Self-propulsion force magnitude Interaction coefficient for blob (attraction parameter)
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