Overview: A fish displays an excellence in swimming on the basis of the equality of action and reaction against the water in which it is swimming. This is because it seems to modify the swimming manoeuvre in paired fin motion, body undulation, and other things in order to achieve optimal swimming energetics. The use of paired fins for stability and other behavioural control is one of the notable instances, as suggested by several researchers (e.g., Webb, 1991; Wilga & Lauder, 1999; Drucker & Lauder, 2003; and others). Those manoeuvres vary depending on the interspecific difference in morphology (Webb, 1984), and also seem to vary depending on allometric variations in morphology, most of which are related to fish length (Yanase & Arimoto, in press); and between fish species with and without a swim bladder (Webb & Weith, 1994); as well as between bony fish and cartilage.
The sand flathead (Platycephalus bassensis C.) studied by Yanase and Arimoto (in press) was hydrostatically stable in roll, but unstable in pitch. Moreover, this negatively buoyant species was using its body and pelvic fins as effective hydrofoils to compensate for a lack of lift. However, the results of the study still leave some questions unanswered. For instance, the centre of mass (CM) for P. bassensis was posterior to the centre of buoyancy (CB). As swimming speed increases, the hydrodynamic lift acting on the fish body increases, the centre of pressure (CP) will be shifted to a position anterior to the CB, and hydrostatic equilibrium will eventually be lost unless the fish can somehow generate downward force anteriorly to the position where the CP for the lifting force is located. One implication derived from stability analysis is that P. bassensis may strategically use pectoral fins to enhance stability. The results also provide with ecologically and evolutionally important implications that this trait might be common in bentic fishes if it gives them an advantage in their vital daily activities such as prey capture and predator avoidance although the ability of migratory swimming may be compromised instead.
It is known that the propulsive efficiency in undulatory movement is affected by the ratio of the propulsive wave velocity (V) to the swimming velocity (U). Unfortunately, only a few data for the U/V ratios have been recorded across a wide range of swimming speeds. In one notable exception, Videler & Hess (1978) measured the U/V ratios using a carangiform swimmer, mackerel (Scomber scombrus L.), and a subcarangiform swimmer, saithe (Pollachius virens L.). Their data surprisingly includes the U/V ratios nearly or equivalent to 1.0, however such a swimming manoeuvre may practically be impossible at a Reynolds number in several hundred thousands or up to nearly a million (e.g., 4-6 LTs-1 for a fish of 40 cm LT) unless the transition from laminar to turbulent flow in boundary layer is postponed or a separation of the boundary-layer flow is avoided to the extent of a relatively higher Reynolds number.
Hypothesis: The swimming performance of fish is a product of the maximised thrust and minimised cost of locomotion (Blake, 2004). The importance of stability control and manipulation of turbulent structure (vortices) in fishes with aquatic locomotion is greater than compared to that in animals with terrestrial locomotion. Even small-scale turbulent disturbance, if the displacement vector of a water particle is strong enough to cause disorder in the synchrony of undulatory kinematics, can result in the increased cost of locomotion. Moreover, relative inability of a fish to efficiently move itself forward has a significant impact directly and indirectly on fitness components including survival, reproduction, and growth.
I would like to put forward the hypothesis that to enhance the efficiency of undulatory locomotion fish may somehow exploit the hydrodynamic information encoded by the mechanosensory lateral line system in combination with the vestibular system for stability. Fish may develop sensorimotor strategies predicted by extroceptive feedforward control, from ambient water, for which mechanosensory lateral line system is highly responsible. The lateral line lies on the skin (superficial neuromasts) or just beneath it in a fluid-filled canal (canal neuromasts). The canal is open to the external environment via an array of openings called pores. The lateral line organs are sensitive to 'near-field' water displacements, and form an acoustico-lateralis system together with the inner ear (Roberts & Russell, 1972). To investigate hypothetis, I have devoted my attention mainly to the hydrodynamic stimuli perceived by the array of neuromasts in the trunk canals. The stimuli include subtle gradients in hydrodynamic pressure due to the flow-velocity gradient near the body surface. Subtle gradients in hydrodynamic pressure can cause motion of the ciliary bundle of hair cells deflected relative to that on the adjacent canal neuromasts. The detection of those ambient disturbances may enable fish to precede undulatory motor output with some modes of modulation, for example altering swimming kinematics or possibly generating longitudinal (streamwise) vortices in the area where flow-separation would otherwise occur. To understand the mechanism of sensorimotor control, the precise determination of local shear stress by the direct measurement of flow-velocity profile near the body surface can be of greater help.
Long-term Goal
The outcomes of the proposed research will be directed towards the overall goal of discovering evolutionary convergence in functional and morphological adaptation of fishes in relation to locomotory performance.
References
| Blake, R. W. (2004). Fish functional design and swimming performance. Journal of Fish Biology 65, 1193-1222. | |
| Drucker E. G. & Lauder G. V. (2003). Function of pectoral fins in rainbow trout: behavioural repertoire and hydrodynamic forces. | |
| Journal of Experimental Biology 206, 813-826. | |
| Roberts, B. L. & Russell, I. J. (1972). The activity of lateral-line efferent neurones in stationary and swimming dog fish. Journal of | |
| Experimental Biology 57, 435-448. | |
| Videler, J. J. & Hess, F. (1984). Fast continuous swimming of two pelagic predators, saithe (Pollachius virens) and mackerel | |
| (Scomber scombrus): a kinematic analysis. Journal of Experimental Biology 109, 209-228. | |
| Webb, P. W. (1971). The swimming energetics of trout. II. Oxygen consumption and swimming efficiency. Journal of | |
| Experimental Biology 55, 521-540. | |
| Webb, P. W. (1984). Form and function in fish swimming. Scientific American 251, 58-68. | |
| Webb, P. W. (1991). Composition and mechanics of routine swimming of rainbow trout, Oncorhynchus mykiss. Canadian | |
| Journal of Fisheries and aquatic sciences 48, 583-590. | |
| Webb, P. W. & Weihs, D. (1994). Hydrostatic stability of fish with swim bladders, not all fish are unstable. Canadian Journal of | |
| Zoology 72, 1149-1154 | |
| Wilga, C. D. & Lauder, G. V. (1999). Locomotion in sturgeon: function of the pectoral fins. Journal of Experimental Biology 202, | |
| 2413-2432. | |
| Yanase, K. & Arimoto, T. (in press). A hydro-mechanical approach to the scaling of swimming performance in the sand flathead | |
| Platycephalus bassensis: effects of changes in morphological features based on fish size. Journal of Fish Biology. | |
 |