Abstract
An abundance of swimming animals have converged upon a common swimming strategy using multiple propulsors coordinated as metachronal waves. The shared kinematics suggest that even morphologically and systematically diverse animals use similar fluid dynamic relationships to generate swimming thrust. We quantified the kinematics and hydrodynamics of a diverse group of small swimming animals who use multiple propulsors, e.g. limbs or ctenes, which move with antiplectic metachronal waves to generate thrust. Here we show that even at these relatively small scales the bending movements of limbs and ctenes conform to the patterns observed for much larger swimming animals. We show that, like other swimming animals, the propulsors of these metachronal swimmers rely on generating negative pressure along their surfaces to generate forward thrust (i.e., suction thrust). Relying on negative pressure, as opposed to high pushing pressure, facilitates metachronal waves and enables these swimmers to exploit readily produced hydrodynamic structures. Understanding the role of negative pressure fields in metachronal swimmers may provide clues about the hydrodynamic traits shared by swimming and flying animals.
Highlights
An abundance of swimming animals have converged upon a common swimming strategy using multiple propulsors coordinated as metachronal waves
In addition to sharing wave kinematics, the propulsors of all the species examined bent during the power stroke, in that they did not differ in how much they bent (Fig. 2a,c; bending angle = 27° ± 6.1; ANOVA, df = 5, 12, fstat = 0.71, p = 0.6) or where they bent along the propulsor (Fig. 2b,c; percent distance along propulsor = 59.2% ± 4.5; ANOVA, df = 5, 12, fstat = 0.73, p = 0.6)
The example image of the Pleurobrachia ctene bending is at 18.5° and it bent at a location 55.1% along the propulsor (Fig. 2c)
Summary
An abundance of swimming animals have converged upon a common swimming strategy using multiple propulsors coordinated as metachronal waves. We quantified the kinematics and hydrodynamics of a diverse group of small swimming animals who use multiple propulsors, e.g. limbs or ctenes, which move with antiplectic metachronal waves to generate thrust. The shape and kinematics of the propulsor must be controlled so that the propulsor maximizes the drag it generates during the power stroke and minimizes its drag during the recovery While this may appear fundamentally simple, the hydrodynamics of this process can be quite complex and may account for the observed diversity in the shape and kinematics of paddles among swimming animals. Understanding drag-based paddling is further complicated for animals with multiple propulsor units These designs are very common among swimming arthropods—which have sets of swimming legs—and polychaete worms—which have many parapodia—and ctenophores—which have rows of fused cilia called ctenes (Fig. 1)
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