Emergent metachronal waves using tension-driven, fluid-structure interaction models of tomopterid parapodia

Metachronal Coordination of Multiple Appendages for Swimming and Pumping

As a strategy for creating fluid flow, metachronal motion is widespread across sizes and species, including a broad array of morphologies, length scales, and coordination patterns. Because of this great diversity, it has not generally been viewed holistically: the study of metachrony for swimming and pumping has historically been taxonomically siloed, in spite of many commonalities between seemingly disparate organisms. The goal of the present symposium was to bring together individuals from different backgrounds, all of whom have made substantial individual contributions to our understanding of the fluid dynamics of metachronal motion. Because these problems share a common physical-mathematical basis, intentionally connecting this community is likely to yield future collaborations and significant scientific discovery. Here, we briefly introduce the concept of metachronal motion, present the benefits of creating a research network based on the common aspects of metachrony across biological systems, and outline the contributions to the symposium.

Metachronal Swimming with Flexible Legs: A Kinematics Analysis of the Midwater Polychaete Tomopteris

Aquatic animals have developed a wide array of adaptations specific to life underwater, many of which are related to moving in the water column. Different swimming methods have emerged, such as lift-based flapping, drag-based body undulations, and paddling. Patterns occur across scales and taxa, where animals with analogous body features use similar locomotory methods. Metachronal paddling is one such wide-spread propulsion mechanism, occurring in taxa as diverse as ctenophores, crustaceans, and polychaetes. Sequential movement of multiple, near identical appendages allows for steady swimming through phase-offsets between adjacent propulsors. The soft-bodied, holopelagic polychaete Tomopteris has two rows of segmental appendages (parapodia) positioned on opposite sides along its flexible body that move in a metachronal pattern. The outer one-third of their elongate parapodia consist of two paddle-like pinnules that can be spread or, when contracted, fold together to change the effective width of the appendage. Along with metachronal paddling, tomopterid bodies undulate laterally, and by using high speed video and numerical modeling, we seek to understand how these two behaviors combine to generate effective swimming. We collected animals using deep-diving remotely operated vehicles, and recorded video data in shore- and ship-based imaging laboratories. Kinematics were analyzed using landmark tracking of features in the video data. We determined that parapodia are actively moved to generate thrust and pinnules are actively spread and contracted to create differences in drag between power and recovery strokes. At the same time, the body wave increases the parapodium stroke angle and extends the parapodia into undisturbed water adjacent to the body, enhancing thrust. Based on kinematics measurements used as input to a 1 D numerical model of drag-based swimming, we found that spreading of the pinnules during the power stroke provides a significant contribution to propulsion, similar to the contribution provided by the body wave. We conclude that tomopterids combine two different propulsive modes, which are enabled by their flexible body plan. This makes their anatomy and kinematics of interest not only for biologists, but also for soft materials and robotics engineers.