ABSTRACT
Many of the most abundant aquatic invertebrates display metachronal swimming by sequentially beating closely spaced flexible appendages. Common biophysical mechanisms like appendage spatial asymmetry and phase drive the success and performance of this locomotor mode, which is generally explained by the need to maximize thrust production. However, the potential role of these mechanisms in drag reduction, another important contributor to the overall swimming performance, has yet to be evaluated. We present a comprehensive overview of the morphological, functional, and physical mechanisms promoting drag reduction during metachronal swimming by exploring appendage differential bending and leg grouping (coalescence). We performed μ-CT and in-vivo velocimetry measurements of shrimp (Palaemonetes vulgaris) to design a five-legged robotic metachronal analog. This test platform enabled simultaneous flow and force measurements to quantify the thrust and drag forces produced by flexible and stiff pleopods (legs) beating independently or coalescing. We tested the hypothesis that coalescence and bending effectively reduce drag during the recovery stroke (RS). The curved cross-section of the pleopods enables passive asymmetrical bending during the RS to reduce their coefficient of drag by up to 75.8% relative to stiff pleopods. Bending promotes physical interactions facilitating the coalescence of three pleopods at any time during the RS to reduce drag such that the mean net thrust produced during coalescence is increased by 30.2%. These improvements are explained by the production of a weaker wake compared with stiff and non-coalescing pleopods. Our results describe fundamental biological and physical components of metachronal propulsion that may aid the development of novel bio-inspired underwater vehicles.
Summary statement Shrimp swimming legs bend nearly horizontally and cluster together during metachronal propulsion to reduce drag and improve the overall swimming performance.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
Competing interests: The authors declare no competing interests.
Funding: This project was supported by funding to M.M.W. from the National Aeronautics and Space Administration (NASA) Rhode Island Established Program to Stimulate Competitive Research (EPSCoR) Seed Grant.
Data Availability Statement: The shrimp swimming and morphological data and metadata files reported in this investigation have been deposited in the Wilhelmus Lab Brown digital repository (BDR). CAD files and custom Matlab scripts developed to compute kinematics and fluid flow data have also been made available.
Minor edits were performed on the original manuscript to improve clarity and conciseness. The results and their interpretation remain unchanged.
