Disentangling the Functional Roles of Morphology and Motion in the Swimming of Fish

Abstract

In fishes the shape of the body and the swimming mode generally are correlated. Slender-bodied fishes such as eels, lampreys, and many sharks tend to swim in the anguilliform mode, in which much of the body undulates at high amplitude. Fishes with broad tails and a narrow caudal peduncle, in contrast, tend to swim in the carangiform mode, in which the tail undulates at high amplitude. Such fishes also tend to have different wake structures. Carangiform swimmers generally produce two staggered vortices per tail beat and a strong downstream jet, while anguilliform swimmers produce a more complex wake, containing at least two pairs of vortices per tail beat and relatively little downstream flow. Are these differences a result of the different swimming modes or of the different body shapes, or both? Disentangling the functional roles requires a multipronged approach, using experiments on live fishes as well as computational simulations and physical models. We present experimental results from swimming eels (anguilliform), bluegill sunfish (carangiform), and rainbow trout (subcarangiform) that demonstrate differences in the wakes and in swimming performance. The swimming of mackerel and lamprey was also simulated computationally with realistic body shapes and both swimming modes: the normal carangiform mackerel and anguilliform lamprey, then an anguilliform mackerel and carangiform lamprey. The gross structure of simulated wakes (single versus double vortex row) depended strongly on Strouhal number, while body shape influenced the complexity of the vortex row, and the swimming mode had the weakest effect. Performance was affected even by small differences in the wakes: both experimental and computational results indicate that anguilliform swimmers are more efficient at lower swimming speeds, while carangiform swimmers are more efficient at high speed. At high Reynolds number, the lamprey-shaped swimmer produced a more complex wake than the mackerel-shaped swimmer, similar to the experimental results. Finally, we show results from a simple physical model of a flapping fin, using fins of different flexural stiffness. When actuated in the same way, fins of different stiffnesses propel themselves at different speeds with different kinematics. Future experimental and computational work will need to consider the mechanisms underlying production of the anguilliform and carangiform swimming modes, because anguilliform swimmers tend to be less stiff, in general, than are carangiform swimmers.

Locomotor benefits of being a slender and slick sand-swimmer

“…The prediction of locomotor performance in relation to features of body morphology (body shape, skin properties and limb number) is a topic of interest from bio/neuromechanical (Aristotle, 350 BCE; Dayton et al, 2005;Gans, 1975;Mosauer, 1932;Russell, 1917;Tytell et al, 2010;Wainwright and Reilly, 1994) and robotic perspectives (Brooks, 1992;Lauder et al, 2007;Maladen et al, 2011a;Maladen et al, 2011c;Pfeifer et al, 2007;Yim et al, 2007). From an evolutionary perspective, it is important to quantitatively address questions associated with putative selective pressures for locomotor apparatus and bauplans in different environments (Lauder, 1981;Lauder and Drucker, 2004;Walker and Westneat, 2000).…”

Undulating fins produce off-axis thrust and flow structures

“…Third, a wake with a single row of diagonally distorted and linked vortex tubes implies an undulating ribbon fin swimmer. It is important to note that the transition from a single row wake to a bifurcated double row wake is due mostly to the differences in St exhibited by anguilliform and carangiform swimmers, and not simply to differences in kinematics (Tytell et al, 2010), leading us to predict that if a ribbon fin fish swam at a higher St, the wake would begin to bifurcate.…”

The hydrodynamic function of shark skin and two biomimetic applications

“…Because sharks are self-propelled deforming bodies, thrust and drag forces are hard to decouple (Anderson et al, 2001;Schultz and Webb, 2002;Tytell, 2007;Tytell et al, 2010), which makes it difficult to isolate drag forces alone during normal free-swimming locomotion to assess the effect of surface ornamentation. In order to investigate the possible drag-reducing properties of surface ornamentation such as shark skin denticles or various biomimetic products (or whether surface structures might possibly enhance thrust), it is necessary to use a study system that permits (1) the use of self-propelling bodies possessing different surface ornamentations, where thrust and drag forces are naturally balanced throughout an undulatory cycle, (2) accurate measurement of self-propelled swimming (SPS) speed so that the swimming performance of different surfaces can be compared statistically, (3) the imposition of different motion programs so that the effect of moving the ornamented surfaces in different manners can be assessed, and (4) various experimental manipulations of surface structure to test directly the hypothesis that it is the surface ornamentation alone that causes drag reduction and hence increased swimming speed.…”