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In animals, epithelial tissues are barriers against the external environment, providing protection against biological, chemical, and physical damage. Depending on the organism’s physiology and behavior, these tissues encounter different types of mechanical forces and need to provide a suitable adaptive response to ensure success. Therefore, understanding tissue mechanics in different contexts is an important research area. Here, we review recent tissue mechanics discoveries in three early divergent non-bilaterian systems—Trichoplax adhaerens, Hydra vulgaris, and Aurelia aurita. We highlight each animal’s simple body plan and biology and unique, rapid tissue remodeling phenomena that play a crucial role in its physiology. We also discuss the emergent large-scale mechanics in these systems that arise from small-scale phenomena. Finally, we emphasize the potential of these non-bilaterian animals to be model systems in a bottom-up approach for further investigation in tissue mechanics.

 

The freshwater copepodHesperodiaptomus shoshonewas 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 femaleH. shoshonewere separately exposed to four turbulence intensity levels plus a negative control (i.e., stagnant flow) in two vortex orientations relative to gravity.H. shoshonedemonstrates a mild behavioral change in the presence of a Burgers vortex that is dependent on both sex and vortex orientation.H. shoshoneswim 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 betweenH. shoshoneand marine copepods are potential factors that may explain the differences in how the species respond to a Burgers vortex. The setal array ofH. shoshonediffers from the setal array of the previously studied marine copepods, suggesting differences in sensory ability and response. The small pond habitat ofH. shoshoneis rarely or intermittently mixed, creating a different hydrodynamic landscape compared to the ocean, which may influence the copepod/turbulence interaction.H. shoshonehas few, if any, naturally occurring predators whereas marine copepods have many, which may influence the difference in response to microscale turbulence.

 

Metachronal propulsion is commonly seen in organisms with the caridoid facies body plan, that is, 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 which 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 five-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.

 

Synopsis 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. Finally, 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.