Metachronal motion is used across a wide range of organisms for a diverse set of functions. However, despite its ubiquity, analysis of this behavior has been difficult to generalize across systems. Here we provide an overview of known commonalities and differences between systems that use metachrony to generate fluid flow. We also discuss strategies for standardizing terminology and defining future investigative directions that are analogous to other established subfields of biomechanics. Lastly, we outline key challenges that are common to many metachronal systems, opportunities that have arisen due to the advent of new technology (both experimental and computational), and next steps for community development and collaboration across the nascent network of metachronal researchers.
Metachronal propulsion is commonly seen in organisms with the caridoid facies body plan, i.e. shrimp-like organisms, as they beat their pleopods in an adlocomotory sequence. These organisms exist across length scales ranging several orders of Reynolds number magnitude, from 10 to 104, during locomotion. Further, by altering their stroke kinematics, these organisms achieve three distinct swimming modes. To better understand the relationship between Reynolds number, stroke kinematics, and resulting swimming mode, Euphausia pacifica stroke kinematics were quantified using high-speed digital recordings and compared to the results for the larger E. superba. Euphausia pacifica consistently operate with a greater beat frequency and smaller stroke amplitude than E. superba for each swimming mode, suggesting that length scale may affect the kinematics needed to achieve similar swimming modes. To expand on this observation, these euphausiid data are used in combination with previously-published stroke kinematics from mysids and stomatopods to identify broad trends across swimming mode and length scale in metachrony. Principal component analysis (PCA) reveals trends in stroke kinematics and Reynolds number as well as the variation among taxonomic order. Overall, larger beat frequencies, stroke amplitudes, between-cycle phase lags, and Reynolds numbers are more representative of the fast forward swimming mode compared to the slower hovering mode. Additionally, each species has a unique combination of kinematics that result in metachrony, indicating that there are other factors, perhaps morphological, which affect the overall metachronal characteristics of an organism. Finally, uniform phase lag, in which the timing between power strokes of all pleopods is equal, in 5-paddle systems is achieved at different Reynolds numbers for different swimming modes, highlighting the importance of taking into consideration stroke kinematics, length scale, and the resulting swimming mode.
Previously documented metachrony in euphausiids focused on one, 5-paddle metachronal stroke, where contralateral pleopod pairs on the same abdominal segment beat in tandem with each other, propelling the animal forward. In contrast, the mysid shrimp Americamysis bahia’s pleopods on the same abdominal segment beat independently of each other, resulting in two, 5-paddle metachronal cycles running ipsilateral along the length of the body, 180° out of phase. The morphology, kinematics, and nondimensional measurements of efficiency are compared primarily to the one-cycle Euphausia superba to determine how the two-cycle approach alters the design and kinematics of metachrony. Pleopodal swimming in A. bahia results in only fast-forward swimming, with speeds greater than 2BL/s (body lengths per second), and can reach speeds up to 12BL/s, through a combination of increasing stroke amplitude, beat frequency, and changing their inter-limb phase lag. Trends with Strouhal number and advance ratio suggest that the kinematics of metachrony in A. bahia are favored to achieve large normalized swimming speeds.
The freshwater copepod Hesperodiaptomus shoshone was exposed to a Burgers vortex, a flow feature meant to mimic small‐scale, dissipative turbulent eddies found in the organism's environment, to determine how this copepod responds to microscale turbulent flow structures. Male and female H. shoshone were separately exposed to four turbulence intensity levels plus a negative control (i.e., stagnant flow) in two vortex orientations relative to gravity. H. shoshone demonstrates a mild behavioral change in the presence of a Burgers vortex that is dependent on both sex and vortex orientation. H. shoshone swim with fairly linear trajectories across all four levels of turbulence intensity, which is a notable difference from the swimming behavior of marine copepods exposed to the same flow feature. Variations in morphology, physical environment, or ecological niche between H. shoshone and marine copepods are potential factors that may explain the differences in how the species respond to a Burgers vortex. The setal array of H. shoshone differs from the setal array of the previously studied marine copepods, suggesting differences in sensory ability and response. The small pond habitat of H. shoshone is rarely or intermittently mixed, creating a different hydrodynamic landscape compared to the ocean, which may influence the copepod/turbulence interaction. H. shoshone has few, if any, naturally occurring predators whereas marine copepods have many, which may influence the difference in response to microscale turbulence.
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